inflight calibration of the near multispectral imager

Icarus140, 66–91 (1999)

Article ID icar.1999.6118, available online at http://www.idealibrary.com on

Inflight Calibration of the NEAR Multispectral Imager1

Scott Murchie,∗ Mark Robinson,† S. Edward Hawkins III,∗ Ann Harch,‡ Paul Helfenstein,‡ Peter Thomas,‡ Keith Peacock,∗William Owen,§ Gene Heyler,∗ Patricia Murphy,∗ E. H. Darlington,∗ Allen Keeney,∗ Robert Gold,∗ Beth Clark,‡

Noam Izenberg,∗ James F. Bell III,‡William Merline,¶ and Joseph Veverka‡∗The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland 20723;†Department of Geological Sciences, Northwestern University, Evanston,

Illinois 60208;‡Center for Radiophysics and Space Research, Cornell University, Ithaca, New York 14853;§Jet Propulsion Laboratory,Pasadena, California 91109; and¶Southwest Research Institute, Boulder, Colorado 80302

E-mail: [email protected]

Received April 23, 1998; revised January 20, 1999

The multispectral imager on the Near-Earth Asteroid Rendezvousspacecraft has been subjected to a comprehensive series of inflighttests to validate its radiometric characteristics measured ongroundand to characterize instrument stability, pointing, geometric dis-tortion, coalignment with other instruments, and light-scatteringcharacteristics under flight conditions. The results of these tests,described herein, support the conversion of images of 253 Mathildeand 433 Eros into scientifically valid products with known geomet-ric and radiometric characteristics. Key results include stability ofdark current during cruise to within 1 data number; stability of theflat field to within the limits of inflight detectability; absolute radio-metric accuracy of ∼5%, with no evident systematic change withtime; validation of the focal length with an inflight measurement of166.85 mm, compared to 167.0± 0.2 mm derived onground; mea-surement of coalignment with the near-infrared spectrometer underflight conditions; and quantification of the intensity and distributionof scattered light. c© 1999 Academic Press

Key Words: asteroids; instrumentation; surfaces, asteroids;Mathilde; NEAR.

1. INTRODUCTION

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The Near Earth Asteroid Rendezvous mission, or NEARthe first of1 the Discovery Program missions intended to accoplish focused science objectives with spacecraft having shorvelopment times and low costs. Its mission is to rendezvousand orbit an asteroid, the S-type near-Earth asteroid 433(Chenget al.1997). The highly focused instrument complemeincludes a multispectral imager (MSI) and near-infrared specgraph (NIS) (Veverkaet al.1997a), an X-ray/γ -ray spectrometer(XGRS) (Trombkaet al. 1997), a magnetometer (Acu˜na et al.1997), a laser rangefinder (NLR) (Zuberet al.1997), and a radioscience experiment (Yeomanset al.1997). NEAR was launched

1 The U.S. Government’s right to retain a nonexclusive royalty-free licenin and to the copyright covering this paper, for government purposes, isknowledged.

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type asteroid, 253 Mathilde, on 27 June 1997 (Veverkaet al.1997b). On 23 January 1998 NEAR made a close encouwith Earth for a gravity assist to enable its encounter and oinsertion at Eros.

The MSI is the keystone to accomplishing NEAR’s scienobjectives. The design and function of the instrument arescribed in detail by Hawkinset al.(1997). Two major subassemblies make up MSI: a camera and a data processing unit, or Dwhich provides a digital interface to the spacecraft and supppower and the master timing to the camera. These two asblies are physically separated by about 100 mm and are locon the aft deck of the NEAR spacecraft, with the camera’s opaxis boresighted with NIS, NLR, and XGRS. A refractive optitelescope, a filter wheel, and a detector with its associatedtronics are all part of the camera. The camera uses a five-elerefractive optical design. Temperature of the optics is maintaat+20◦C using heaters. Seven narrow-band visible and ninfrared spectral filters permit multispectral imaging, and obroadband filter is used for optical navigation and faint obimaging (Table I). A one-time deployable cover, which protecthe optics from contamination during launch, was opened1 May 1996. The cover contains a fused silica port, whichduced the aperture from 18 to 5 cm2 and was intended to allowimaging in case the cover did not deploy successfully. A fistray light baffle prevents out-of-field sources≥36◦ from theoptical axis of the camera from directly illuminating the ouoptical element. A frame-transfer silicon charge-coupled de(CCD) converts the optical signal into an electrical one, whis then digitized to 12 bits in the focal plane detector (FPD)transferred to the DPU. Exposure times are controlled electically; there is no mechanical shutter. The image is exposedsome time (0–999 ms), transferred to a memory zone in 0.9held for 80 ms, and converted from analog to digital sigover 800 ms. The CCD is cooled passively to approxima−30◦C to minimize dark current. During flight, temperatures−28 to−32◦C have been maintained. MSI provides a fieldview of 537× 244 rectangular pixels, which view solid angl

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INFLIGHT CALIBRATION OF THE

TABLE IMSI Instrument Parameters

Camera headMass 3.7 kgPower 1.43 WWavelength coverage 450–1050 nm

DPUMass 4.0 kgPower 5.49 WData compression Lossless or 12-bit to 8-bit

conversion

of 162× 96µrad, yielding a total field of view encompassin2.95◦ × 2.26◦.

The four key measurement requirements for MSI at Eros(a) to determine the asteroid’s bulk properties (shape, volurotation); (b) to determine the morphology of the surface at smscales and constrain the history of surface processes; (c) tothe spectral properties and spectral heterogeneity of the surso that compositional properties and variations determinedNIS and XGRS can be extrapolated to smaller spatial scalescorrelated with geologic units; and (d) to determine the extence, sizes, spectral and morphologic characteristics, and oof any natural satellites. These science requirements are mecombination of instrumentation and measurement strategy.imaging before and during high orbital altitudes will establiboth the rotational pole of Eros and the shape of the asterFor determination of spectral properties, Eros will be mappin all spectral filters during approach and high orbit each tiresolution improves by a factor of two, at least to∼200 km al-titude at a resolution of∼30 m/pixel. Three-color imaging willcontinue into orbits at 100 and 50 km in radius. Later, frombits as low as 25 km in radius, highest priority will be giveto monochrome morphologic mapping at spatial resolutionshigh as 3–5 m/pixel.

The Discovery Program’s schedule constraints imposed aquirement of speed in instrument development. This was mepart, by simplifications in fabrication. For example, MSI is fixemounted to the aft deck along with NIS, XGRS, and NLR. Theinstruments’ boresights share a common orientation orthogoto the vector to the Sun, which places the asteroid in the fieof view during a typical orbit around Eros. This simplifies instrument design and coordination of observations, but preclua dedicated calibration target. The CCD used (a Thomson-CTH7866) provides the spatial resolution, responsivity, andnamic range necessary for MSI’s investigation, but its pixare rectangular and separated by nonresponsive “anti-bloomchannels” that carry excess current from saturated pixels tovent “bleeding” of saturation to adjacent pixels. Because ofrectangular pixels. MSI images require postprocessing forometric rectification. The anti-blooming channels create efftively subpixel “dead zones,” which are an issue both for
geometric character of the images and for radiometric measuments of small sources a few pixels or less in extent. Procedu
EAR MULTISPECTRAL IMAGER 67

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used to address the geometric and radiometric effects oanti-blooming channels are described in this paper.

The main objective of this paper is to describe the inflicalibrations during the cruise to Eros, to update results fromground tests, and to measure MSI attributes that cannot bedetermined onground. The results of onground calibrationsdescribed in detail by Hawkinset al. (1997). This paper summarizes the philosophy, strategy, and results of cruise calibratto date. Our aim is to provide the background necessary tomit further analysis of asteroid images released to the PlaneData System (PDS).

2. APPROACH

MSI testing and calibration were done using a three-tiestrategy of testing at the component level, testing ongrounthe instrument level, and retesting inflight. Onground instrumlevel tests were performed at the Optical Calibration Fac(OCF) at APL (Hawkinset al. 1997). The OCF consists of seeral large, linked, cryogenically cooled vacuum chambers,largest of which (the instrument chamber) contains a two-motion stage. Calibration sources can be placed at two locafor viewing, at the focus of a collimator or outside a fused silport window. Several sources can be located at the focus ocollimator, including a monochrometer, a point source, a relution target, and test targets. Either of two integrating sph(a large Spectralon-lined sphere or a small gold-lined sphcan be viewed through the port window or collimator. Resultcomponent-level tests and MSI radiometric, wavelength, andometric calibration in the OCF are described by Hawkinset al.(1997).

Inflight calibrations provide the link between onground cabrations and science measurements of asteroids. For radiomcalibration, this is done via multispectral imaging of the moand stellar sources. The Moon was imaged through all stral filters 3 days after launch, in February 1996 and agaiJanuary 1998. At both times highland regions were visible,their spectral properties are known from measurements oturned lunar soil samples. Lunar calibrations at these two timnearly 2 years apart, provided a test for both the accuracythe stability of MSI radiometric calibration. During intervenintimes and at Eros, imaging of Canopus is used to track anyvariations in responsivity, should they occur. Geometric calibtions, including both imager pointing and image distortion,carried out by imaging clusters of stars whose positions andgular separations are accurately known. Inflight calibrationsprovide the opportunity to measure some aspects of instrumperformance more accurately than is possible onground. Foample, the magnitude and distribution of scattered light cameasured in deep space using an airless-body source morerately than is possible in a terrestrial laboratory where reflectfrom the test apparatus and atmospheric scattering becomconcerns. Inflight calibration measurements, their purposes
re-restheir onground counterparts are summarized in Tables II and III.Additional image data were also acquired with MSI, as parts

68 MURCHIE ET AL.

hilde

TABLE IIMSI Cruise Images

Date Name Target MET range Objectives

21 Feb 1996 MSI-Moon 1 Undefined 233628–235161 Software validationMoon 256345–258201 Linearity, radiometric responsivity,

scattered light

24 Mar 1996 Hyakutake Undefined 3088771 Dark current monitoringHyakutake 3089211–3092472 Pointing accuracyUndefined 3093071–3093371 Dark current monitoring

29 April 1996 Canopus 1 Undefined 6172292–6172294 Dark current monitoringCanopus 6173014–6175602 Small-scale nonuniformity of CCD,

point-spread function, radiometricresponsivity with lens cover on

2 May 1996 Canopus 2 Undefined 6427889–6427891 Dark current monitoringCanopus 6428669–6430102 Point-spread function, radiometric

responsivity with cover offPraesepe 6431069–6434364 Geometric distortion, boresight in

spacecraft coordinate system

22 May 1996 — Undefined 8211989–8214390 Dark current monitoring26 June 1996 Low-Sun test Undefined 11225789–11225791 Dark current monitoring

kappa Velorum 11226449–11227289 Scattered light at 85◦ phase anglegamma-2 Velorum 11227709–11228549 Scattered light at 99◦ phase anglepi Puppis 11228969–11229809 Scattered light at 113◦ phase angleomicron-2 Canis Majoris 11230229–11231069 Scattered light at 126◦ phase anglebeta Canis Majoris 11231489–11232329 Scattered light at 135◦ phase anglexi Leporum 11232749–11233589 Scattered light at 141◦ phase angleUndefined 11234609–11234611 Dark current monitoring

15 Jan 1997 Shamtilly 2 Undefined 28722366–28722370 Dark current monitoringCanopus 28722786–28722966 Point-spread functionUndefined 28723932–28724086 Practice for Mathilde encounterPleiades 28726076–28726619 Geometric distortion, boresight in

spacecraft coordinate system

10 April 1997 — Scorpio 36197797–36200492 Validation of software upload23 April 1997 Shamtilly 3a Undefined 37398932–37399016 Practice for Mathilde encounter7 May 1997 Shamtilly 3b Vela 38417172–38435182 Practice for optical navigation to Mat21 May 1997 Shamtilly 4 Undefined 39645474–39646984 Practice for Mathilde encounter30 May 1997 Shamtilly 5 Undefined 40418575–40420085 Practice for Mathilde encounter25 June 1997 Opnav 1 Mathilde 42675049–42675079 Optical navigation to Mathilde

26 June 1997 Opnav 2 Mathilde 42693949–42693979 Optical navigation to MathildeOpnav 3 Mathilde 42719149–42719179 Optical navigation to MathildeOpnav 4 Mathilde 42740749–42740779 Optical navigation to MathildeOpnav 5 Mathilde 42761149–42761149 Optical navigation to Mathilde

27 June 1997 Opnav 6 Mathilde 42787549–42787549 Optical navigation to MathildeMathilde 42826048–42826184 High phase angle imaging

Flyby Mathilde 42826252–42827134 Moderate/low phase angle imagingMathilde 42827188–42827558 Satellite search

6 Aug 1997 Canopus 3 Undefined 52138903–52138906 Dark current monitoringCanopus 52139693–52141160 Point-spread function, monitoring of

radiometric responsivity

12 Oct 1997 Canopus 4 Undefined 56977304–56977306 Dark current monitoringCanopus 56978094–56979558 Point-spread function, monitoring of


7 Dec 1997 Canopus 5 Undefined 56977304–56977306 Dark current monitoringCanopus 56978094–56979558 Point-spread function, monitoring of


23 Jan 1998 MSI-Earth 1 Undefined 60948723–60948725 Dark current monitoringEarth 60950215–60950576 Monochrome image quality

23–26 Jan 1998 MSI-Earth 2 E Africa 60952323–60952560 Multispectral image quality,coordination of observations with NIS

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INFLIGHT CALIBRATION OF THE NEAR MULTISPECTRAL IMAGER 69

TABLE II—Continued

Date Name Target MET range Objectives

E Antarctica 60952548–60952560 Intercalibration with NISEarth Movie South pole 60953823–60991889, Scattered light, radiometric

61006125–61094849, responsivity, testing of automa61214205–61214369 exposure algorithm

23 Jan 1998 MSI-Moon 2 Moon 60996906–60998626 Infield scattered light, radiomeresponsivity

Moon (off-pointed) 60999405–61004206 Out of field stray light26 Jan 1998 MSI-NIS coalign Moon 61216605–61218705 Coalignment of MSI and NIS29 June 1998 — Canopus 77122028–77123902 Validation of software upload19 Aug 1998 — Canopus 78913009–78913744 Spacecraft roll tests for accura

kappa Velorum 78914089–78923809 star camera attitude informat

30 Sept 1998 — Undefined 82576008–82576008 Optical navigation test

TABLE IIITraceability of MSI Behavior Using Onground and Inflight Tests

Term of calibration equation Meaning Ground measurement Inflight measuremen

Darkx,y,t,T Dark current plus bias Darkened chamber Continuing: Monitoring odeep space, 1050-nm filte

Coeff Radiometric responsivity Integrating spheres Completed: MSI-Moon 1(assumes stability) and 2, Canopus 2–5

Continuing: Monitoringusing Canopus

Expt Exposure time (assumes linearity) Dependence of target DN Specific linearity teston exposure times at MSI-Moon 1

Attenc Attenuation of signal by Integrating spheres, cover Completed: Canopus 1 acover on and off 2, cover on and off

Flatx,y, f,c Flat-field response Integrating spheres Completed: low Sun test(assumes stability) subpixel nonuniformity

using slow scan acrossCanopus at Canopus 1

Respf,T Variation of radiometric Integrating spheres at None (temperatureresponsivity with temp. different temperature regulated)

Smearx,y,t Smear from frame transfer Field-filling and All images ofnon-field-filling targets extended sources

Additional propertiesGeometric distortion Pinhole pattern, Completed: Star

resolution target cluster imaging

Planned: Monitoring atEros using star images

Pointing, coalignment Measurement of optical Completed: Star clusterwith other instruments cubes with theodolite imaging, MSI-NIS

coalignment test at MoonPlanned: Monitoring at

Eros using Pleiades

Light scattering Pinhole and small Completed: in-fieldextended sources scattering at MSI-Moon

and Moon 2, out-of-fieldscattering at MSI-Earthand Moon 2

Point spread function Pinhole sources Continuing: Canopus

70 MURCHIE ET AL.

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of inflight testing of software uploads and during rehearsalsthe Mathilde encounter (Table II). File-naming conventionsimage data and time reckoning conventions are explaineAppendix A. (Time is reckoned in seconds of mission elaptime, or MET.)

3. RADIOMETRIC CALIBRATION

For radiometric calibration, the main objective of inflight caibration is solving for variables in the calibration equationconverting image data numbers (DNs) to physical units of rance, W m−2µm−1 sr−1. The calibration equation has the for

Radiancex,y, f,T,t,c

= {[DNx,y, f,T,t,c − Darkx,y,t,T ] − Smearx,y,t } •100

Flatx,y, f,c •Coeff •Respf,T •Attenf,c •Expt, (1)

where DNx,y, f,T,t,c is raw DN measured by the pixel in columx, row y, through filter f at exposure timet and temperatureT with the cover statusc open or closed. Darkx,y,t,T is the darklevel modeled for this pixel at exposure timet and tempera-ture T , derived from a short and long dark field exposuresthe appropriate temperature. Smearx,y,t is the scene-depende
readout smear for the pixel at exposure timet . Flatx,y, f,c is the
e,

pattern (Fig. 1) which, when subtracted from the data, has no

FIG. 1. (Top) MSI dark image acquired as part of Canopus observations on 29 April 1996. (Bottom) 5× enlargement of the upper left corner of the imagshowing fixed-pattern noise in the dark image as well as periodic noise. Contrast of both images has been highly enhanced to accentuate the∼6-DN variations in
dark levels between odd and even columns.
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flat field for filter f with the cover statusc open or closed.Coeff is the coefficient for converting dark-removed, flat fieland smear-corrected DN from filterf to radiance, for a baselinexposure time of 100 ms. Respf,T is the responsivity for thisfilter at temperatureT relative to the baseline, inflight operatintemperature (−29.6◦C). Attenf,c if appropriate, is the attenuation of incoming signal by the lens cover in filterf when thecover statusc is closed. Expt is exposure time in millisecondbetween 1 and 999 ms. (The constant 100 is the exposurein milliseconds for which the calibration coefficient Coeff isapplicable.) Each term in this equation is a matrix or vectocoefficients, and the equation itself assumes stable, predicinstrument behavior. All coefficients are listed in Appendix B

3.1. Dark Images

The signal level measured in deep space is the sum of tmajor components: (a) dark current from thermal electo(b) a bias of∼80 intentionally added to the output to prevethe occurrence of negative values which would reach grounzeroes in the 12-bit DN words, and (c) low-level periodic nopicked up from spacecraft electronics. Odd and even coluhave slightly different biases (by∼6 DN), an inherent prop-erty of this Thomson CCD. This difference introduces a fix

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measurable effect on accuracy or signal-to-noise ratio ofimage data. The rms magnitude of the periodic noise isDN, depending on the image. It is apparent only in dark fieand is not evident in scenes with even low levels of illumintion, demonstrating that it is additive rather than multiplicatto measured signal. The periodic noise is similar in effectmagnitude to read noise (noise from pixel readout and cosion to digital signal). With MSI’s digitization at∼60 e-/DN(Hawkinset al. 1997), at moderate signal levels of even a fhundred DN out of a total 12-bit range of 4095, both perioand read noise are completely dominated by shot noise (thetistical uncertainty in measured signal resulting from the finnumber of photoelectrons counted).

Dark current has been monitored inflight by periodic acqsition, every several months, of 10- and 999-ms exposureimages of a dark field of space. The 1050-nm filter (which tramits the least signal) was used to mitigate effects of dim sUse of both exposure times allows tracking of both the base land the dependence on exposure time. Cosmic ray hits abovlevel of noise do occur in some frames; exclusion of thesefound to have an insignificant effect on mean DN. Mostflight calibration targets observed to date (typically one or mstars on a dark background) provide additional measuremendark current. Figure 2 summarizes dedicated dark-current msurements acquired to date. The accumulation of dark cuis<1 DN/s at MSI operating temperatures, and levels in mpixels have remained stable to within 1 DN since launch.the time of the encounter with Mathilde, four “bad” pixels happeared that accumulate dark counts at a higher rate (seDN/sec) than other pixels. In other respects (e.g., flat-field cacteristics) these pixels are undistinguished. Due to the snumber of affected pixels, no special corrective steps arerently being taken.

FIG. 2. Plots of dark-field DN levels at 10- and 999-ms exposure timas a function of mission-elapsed time (MET) since NEAR’s launch, for
and even image columns. Error bars, included for one case only, represenimage-wide cumulative effects of read and periodic noise.

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3.2. Response Uniformity

3.2.1. Flat-field. The pixel-to-pixel variation in CCD re-sponse to a uniform extended source, the “flat field,” was msured onground by averaging for each filter multiple imagof the interior of the large, Spectralon-lined integrating spheThe averaging reduces shot noise in the derived flat field toproximately 0.001 of the averaged signal. For each flat fieinstrument offset and stray light at the test apparatus weretracted using secondary images, e.g., with the integrating spoff, yielding background-free images of the integrating sphinterior. The rms variations in the flat field are typically± 2.2%and are highly reproducible between filters at the pixel scThese variations are unrelated to field angle, indicating thatfield variations result mostly from properties of the CCD anot from attributes of the optics.

There are no completely flat targets with which to rederflat fields inflight to determine their stability, but there are oportunities to observe “smooth” fields. One such opportunoccurred during the “low-Sun” test, as part of the preparatfor the encounter with Mathilde. The approach phase anglMathilde, 139◦, was only 5◦ off the orientation at which therewould be direct illumination of the outer optical element. Aquisition of optical navigation images was required for prectargeting of the Mathilde flyby. Star fields at 40◦–90◦ from theSun (in 10◦ increments) were imaged to assess the impactoptical navigation of scattered sunlight not stopped by the slight baffle. Figure 3 shows the appearance of the scatteredin a typical star field compared to that of one in which thereno scattered sunlight. The greater amount of scattered sunat one side of the image is consistent with one reflection offinside of the stray light baffle. Application of the flat-field corection to this smoothly varying field allowed testing of responstability over the dominant scale of flat-field nonuniformitytens of pixels or less (Fig. 4). The flat-field correction effetively removes pixel-to-pixel signal variations, with residualsthe levels expected for shot noise at measured DN levels (approximately one part in 200). This test showed that, durthe year between image acquisition and onground measureof the flat-field, the response uniformity of the instrument wunchanged at the level of detectability.

3.2.2. Subpixel nonuniformities.Although at large scales(more than tens of pixels) average responsivity is nearly uform, at subpixel scales response is highly nonuniform duethe design of the CCD (Fig. 5). Each pixel consists of 75% acarea, and adjacent to every second column of pixels therepair of anti-blooming channels that drain excess charge fromjacent pixels to minimize bleeding of saturation beyond a sinpixel. This feature prevents contamination of surrounding pixby a saturated pixel. However, it also presents a challengeto geometric calibration (discussed below) and to determinaof the responsivity to small or point sources such as stars,

t thecause the blur circle for a point source is comparable in size tothe pair of anti-blooming channels. The comparable sizes of the

72 MURCHIE ET AL.

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FIG. 3. These images acquired during the low-Sun angle test on 26 J1996 illustrate the effects of sunlight striking the interior of the baffle. At moerate phase angles shadowing by the spacecraft completely blocks sunlightthe imager. At phase angles≥112◦ measurable scattered sunlight begins to enthe imager. Its magnitude is largest on the antisunward side of the baffle, at

anti-blooming channels and blur circles of stars are of signcance because regular monitoring of responsivity during crurelies on star imaging.

The manufacturer’s specifications indicate that the fractionCCD area that is active (the “pixel fill factor”) is 0.5675. Threduction of the aperature with the lens cover closed duringfirst 120 days of NEAR’s cruise phase presented an opportuto rederive the pixel fill factor inflight, because the reduced ap

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FIG. 4. Application of the flat-field corrections derived onground to thimage of diffusely scattered sunlight acquired on 26 June 1996 effectivelymoves variations induced by pixel-to-pixel response nonuniformities. Themaining variations are consistent with the gradient in the scattered light andshot noise of the measured photons. The contrast is highly enhanced, withequaling a DN of∼1500 and white a DN of∼1600.

FIG. 5. At the subpixel scale, the MSI CCD has nonuniform responsity. White areas are sensitive portions of pixels, light gray areas in insensportions of pixels, and dark gray areas are insensitive pairs of antibloomchannels. The fractional area of each pixel that is sensitive is 0.5675.

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FIG. 6. Results from four pixels of the 6× 1-pixel slow scan of Canopuusing the 550-nm filter, on 29 April 1996. Variations in star signal level refrom subpixel nonuniformities in sensitivity due to a pixel fill factor of lethan unity. The derived ratio of brightness compared with that predictedextended sources is well within 1% of the pixel fill factor indicated by the Cmanufacturer (0.5675).

ture improves focus and provides very small spot sizes forfilter in best focus (550 nm). Just before opening of the lcover, a sequence of 30 550-nm, reduced-aperture imagCanopus was acquired in a 1-row by 6-column diagonal sconfigured to place the star image in a variety of subpixel lotions on the CCD. Results of a portion of this test (21 imagare shown in Fig. 6. The variations in signal result fromimage being scanned across nonresponsive sites and frouneven spacing of active pixel areas. Comparison of the mbrightness with that predicted from calibration measuremof extended sources (discussed below) yields a ratio of 0.which is a measure of the fraction of CCD area that is sensto light. This value agrees to better than 1% with the pixelfactor supplied by the manufacturer.

3.3. Readout Smear

MSI is shuttered electronically. An image is exposed fonominal integration (exposure) time, following which it is tranferred in 0.9 ms to a memory zone on the CCD from which anasignal is digitized line-by-line. Accumulation of signal contiues during the 0.9 ms of frame transfer. The finite duration offrame transfer therefore induces a streak or “readout smeathe wake of an illuminated object in the field of view, parallelthe direction of frame transfer. This effect is illustrated in Fig(middle), which is highly contrast-enhanced to accentuateeffect.

The spatial distribution and magnitude of the “smear”scene-dependent, but can be removed analytically and auto
ically from an image with a high degree of accuracy providthat no illuminated pixel in that column of the image is sat
NEAR MULTISPECTRAL IMAGER 73

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rated. The brightness distribution in the image itself is usedcalculate the smear (given known exposure and frame trantimes), which is then subtracted. This calculation is performafter dark current removal and flat-field correction, sequentifor each line with the removed smear calculated from the smcorrected DN values for the part of the image illuminating thline. (The procedure could alternately be performed beforefield correction, given knowledge of the flatfield nonuniformitThe magnitude of the smear to be removed from the pixecolumnx and liney in an image integrated for exposure timetis

Smearx,y,t =y−1∑

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t2t• DNx,y, f,T,t,c − Darkx,y,T,t − Smearx,y,t

Flatx,y, f,c,

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wheret2 is the time for frame transfer (0.9 ms) divided by tnumber of lines in the image in the direction of frame trans(244 lines).

The effectiveness of the smear-removal in onground datashown by Hawkinset al. (1997). It has been validated repeaedly inflight, first from lunar images acquired on 21 Februa1996 and more recently on images of 253 Mathilde acquon 27 June 1997 and Earth and the Moon acquired on 23January 1998. The center and right panels of Fig. 7 shohighly contrast-enhanced distant image of Mathilde beforeafter smear removal. During Earth imaging on 23–26 Janu1998 the smear-removal algorithm proved effective to the leof read noise for exposure times as small as 2 ms, extenthe usefulness of the CCD to exposure times shorter thanmanufacturer’s stated specification of≥10 ms.

3.4. Linearity

The underlying assumption of radiometric calibration of Mis that response of the CCD is linear with exposure timethe digitization limit of 4095. Onground testing summarizedHawkinset al. (1997) shows that this condition is satisfied oground. This assumption was tested again after launch by iming the Moon through the 550-nm filter at exposure times fr3 to 80 ms (above which saturation occurred). For each imagthis sequence, the total light collected from the Moon was msured by averaging over a 30-pixel box centered on the ceof the illuminated portion of the Moon. All data were correctfor dark levels, readout smear, and flat-field nonuniformitiResults are expressed in Fig. 8 in two manners, as the aveDN at each exposure time and as the DN accumulation ratemillisecond at each exposure time. The DN accumulation raa more sensitive indicator of small departures from linearityits value would change with exposure time if nonlinearity weto occur. This test places an upper limit of 1% on the variatiin linearity with signal level; most of the uncertainty results fro

edu-low exposure times at which read noise significantly affects thesignal.

74 MURCHIE ET AL.

97. (Left)nt. Resi

FIG. 7. The effects of readout smear removal are apparent in this 700-nm-filter, 6-ms-exposure-time image of Mathilde acquired on 27 June 19Raw image. (Center) Raw image with the contrast enhanced to bring out the smear. (Right) Smear-removed image with the same contrast enhancemeduals
are at or below the level of the read noise.
tl

a-

v

e

oorl

edlity,

lewh theessbe-igh

nalat

all-

pre-,

D,

andoff.

3.5. Cover Attenuation

The attenuation of light by the lens cover is of significanmostly because the first of the two scheduled lunar calibrasequences was acquired with the cover on. The secondcalibration sequence, in January 1998, was acquired withcover off.

Cover attenuation was measured onground by observinglarge integrating sphere through each spectral filter withcover on and off. The attenuation measured onground hmean value in all filters of 0.2357± 0.0010 with no clear wavelength dependence. No similar extended calibration sourceavailable for observation inflight before and after cover remoso an extended source was simulated by imaging Canopudiagonal spacecraft slew at a rate of 8–16 pixels/s was usspread the starlight over a representative variety of subpixe

FIG. 8. Linearity of the MSI CCD determined from 550-nm imagesthe Moon taken at different exposure times. Results are expressed here baverage measured signal at each exposure time (heavy line) and as thesignal accumulation per ms at each exposure time (fine line). Response isto within 1%.

ceionunarthe

thethes a

wasal,s. Ad tol el-

fth as

ate ofinear

ements of the CCD and to prevent “trapping” of the streakstar image in a less responsive area of the CCD. For practicaimages were acquired for all spectral filters during a single sat a constant exposure time (999 ms), and in each case (witcover on or off) two imaging slews were performed to assreproducibility of results. The slew rate used was a balancetween a low rate to enhance signal level in each pixel and a hrate to spread the signal more.

Figure 9 shows the reproducibility of the measured sigof Canopus under different conditions. With stable pointingCanopus at different locations on the CCD, the CCD’s smscale nonuniformities limit measurement precision to∼14%.Streaking the star image into an extended source improvescision. At the higher (1600µrad/s) slew rate with the cover offthe length of the streaked image was>10 pixels, providing arelatively representative sample of subpixel regions of the CC

FIG. 9. Reproducibility of Canopus measurements with the cover on

N

s

aineab

s

ntnhemu

ros

e

d

dughereo-ref-use

s ofoth-ibleths.inale in-

Nure

os,tral

ef-lti-andsionngu-

n asromear,forde-vermal-werenar


FIG. 10. Onground and inflight measurements of attenuation by the Mcover.

and the integrated signal was reproducible typically to∼1%. Atthe lower (800µrad/s) slew rate with the cover on, a lower signlevel and more uneven sampling of the CCD resulted in lereproducibility of results, near 5%.

Comparison of the integrated signals with the cover onoff provides measures of the attenuation (Fig. 10). Uncertain the derived attenuation is dominated by imprecision of msurements with the cover on. On average, the derived attenuis comparable to the onground value, but there appears towavelength dependence in cover attenuation that was notdent in onground measurements. The calibration implicationthe wavelength-dependence of cover transmission are discuin more detail below.

3.6. Radiometric Responsivity

Radiometric responsivity results from the efficiency in coverting photons of light into photoelectrons measured byinstrument, and is the key variable for converting DN levels iphysical units of radiance,I /F , etc. Because of the finite widtof the MSI filters (Fig. 11), calibration coefficients are affectnot only by instrumental parameters but also by the spectruthe irradiance source and the reflectance spectrum of the illnated target. Since the calibration coefficients derived for Mare based on a lunar reflectance spectrum, some level of erthe radiometric calibration will be introduced for objects whospectra differ from that of the Moon over the width of a givbandpass filter.

3.6.1. Onground measurement.Responsivity was measureonground by imaging NIST-traceable integrating spheres
known radiance and determining coefficients for convertindark-, flat-field-, and smear-corrected DN levels into units

SI

alser

ndtya-tione a

evi-of

ssed

-heto

dof

mi-SIr inen

of

radiance (Hawkinset al.1997). Both the large, Spectralon-linesphere and the small, gold-lined sphere were imaged throthe port window to the calibration chamber. The large sphwas also imaged through the collimator. For initial use, cefficients derived using the large sphere were chosen in perence to coeffiecients derived from the small sphere beca(a) the large sphere lacks the vignetting present in imagethe small sphere and (b) its Spectralon is spectrally smoand flat over the finite width of the MSI filters. The gold lining of the small sphere has a steeply sloping spectrum at viswavelengths but a nearly flat spectrum at infrared wavelengThese “calibration coefficients” necessarily assume a nomexposure time; 100 ms is used as a baseline value. Given thstrument’s linearity, for any given exposure time measured Dlevels need be normalized only to that expected for an exposof 100 ms.

3.6.2. Testing using lunar images.The Moon is an appropri-ate inflight radiometric calibration target because, like 433 Erit is spectrally red and solar-illuminated and because its speccharacteristics are relatively well-known. The calibration coficients determined onground were tested inflight using muspectral images of the Moon acquired on 20 February 1996on 23 January 1998 (Fig. 12). The second lunar imaging sesprovided superior data to the first session, because (a) the alar diameter of the Moon was∼5 times larger than during thefirst session and (b) the lens cover was off, the same conditioat the asteroid targets 253 Mathilde and 433 Eros. For data fboth sessions, corrections were made for dark current, smand flat-field. For the first session a correction was madecover attenuation using the wavelength-independent valuerived onground. Disk-integrated DN levels were measured oan aperture enclosing all measurable scattered light and norized to signals expected at 100 ms. These measurementscompared to predictions based on Apollo 16 soil and a lu

gof

FIG. 11. Relative responsivities within the MSI filters, with values scaledto unity at the most sensitive wavelengths in each filter.

76 MURCHIE ET AL.

caled innhan

FIG. 12. Color composite MSI images of Mathilde and the Moon, created from 450-, 550-, and 700-nm filter images. The images at left are sbrightness and color so that the brightest spot on each body is white. In the images at right, color saturation and low brightness levels have been eced toexaggerate color differences and low-magnitude scattered light. Note the bluish hue of the scattered light surrounding each body, and the reddened margins of the
illuminated areas.
a

se

es

revigi

ure-ofack-

intimesselythe

is re-ndi-CCDnce

photometric model (Helfenstein and Veverka 1987), and cbration coefficients were rederived. The rederived coefficiefrom inflight data and the effective wavelengths and bandpafor each filter (from Fig. 11) are given in Table IV. The filtbandbasses shown in Fig. 11 are tabulated in Appendix C.

The “calibrated” brightness of the Moon in each filter differbetween the two observing sessions, as shown in Fig. 13 aratio of brightnesses of the Moon in the two sessions, scaleunity in the 550-nm filter. Three possible sources for the discancy are (a) experimental error, (b) time variation in responsiof the CCD, and (c) wavelength-dependent attenuation of lby the lens cover, which was on during the first lunar calibrat
session but not during the second. ros.
li-ntssesr

dthe

d top-ityht

on

Experimental error was eliminated by repeating the measments while integrating over slightly different subregionsthe images and by using various techniques to remove bground signal, yielding results that agreed to within≤1%. Time-variation in CCD responsivity is eliminated by consistencythe brightnesses of Canopus measured at widely separatedduring cruise (Fig. 14). Instead, the discrepancy appears clorelated to the wavelength-dependent attentuation of light bylens cover, as measured inflight using Canopus (Fig. 13). Thsult, combined with the Canopus imaging described below, icates no measureable differences in the responsivity of theover the 2 years between lunar calibrations, providing confidein extrapolation of the lunar calibrations to data acquired at E


TABLE IVMSI Filter Positions and Responsivity Based on Onground and Inflight Tests

Filter λ Filter width Effective center Effective width Cal. coefficient from Moon-2Filter No. (nm, nominal) (nm, nominal) (nm, from OCF data) (nm, from OCF data) at−29.6◦C (DN/W m−2µm−1 sr1)

2 450 50 462 23 141.51 550 30 554 24 486.20 700 200 700 133 3923.43 760 20 755 19 506.25 900 40 900 33 467.94 950 40 951 38 309.46 1000 50 996 44 165.87 1050 80 1033 51 63.3

Inla

ooaig

u

ts

tsD

co-rat-

nar-rgehere,, the20%fac-theffi-fectse ac-), or

t theured

r 18cy of

3.6.3. Monitoring using Canopus images.Lunar observa-tions provide a test of MSI responsivity at two points in timjust after launch and after nearly 2 years of exposure of MSthe space environment. To track changes during the intervetime as well as subsequently, Canopus is monitored reguusing streaked observations as described above. All calibrasteps described above are performed on the Canopus datathe integrated star signal is measured as is done for the MThe integrated star signal is divided by the pixel fill factor0.567 to correct for the difference between a point sourcean extended source, and the result is compared with the spredicted based on the brightness of Canopus (V=−0.73 mag),a blackbody spectrum with an effective temperature of 7350and application of lunar-derived calibration coefficients. Resfor the first three Canopus observations, spanning nearlymonths of cruise operations, are shown in Fig. 14. Overall,

FIG. 13. Comparison of the difference in measured lunar signals withcover off and on, compared with the wavelength dependence of cover transivity derived inflight from images of Canopus. The small formal error barsthe 1000- and 1050-nm cover transmissivities from Canopus measuremenquestionable, because larger deviations in these two values resulted fromdifferent techniques for removing uncorrected dark current from the low
level Canopus measurements.
e,toingrly

tion, andon.fndnal

K,lts18

ab-

hemis-fors areubtlyN

solute calibration appears accurate for Canopus to∼5%, and nosystematic change with time is evident.

3.6.4. Comparison with onground measurements.Figure 15compares the lunar-derived calibration coefficients with theefficients derived onground using the large and small integing spheres, by ratioing the onground coefficients to the luderived coefficients. The coefficients derived using the lasphere are more precise than those derived using the gold spas expected from the discussion in Section 3.6.1. Howevercoefficients from the large sphere appear consistently 10–higher than those derived inflight. One or more of severaltors may be involved in this discrepancy: systematic error inlunar photometric modeling required to derive calibration coecients, systematic error in reduction of groundbased data, efof false images on groundbased measurements (which werquired at short exposure times of 3–30 ms; see Section 5.2calibration drift of the integrating spheres.

3.6.5. Validation using Mathilde. The first test of MSI cal-ibration at an asteroidal target came on 27 June 1997 aMathilde encounter. The spectrum of the asteroid meas

FIG. 14. Comparison of measured brightnesses of Canopus ovemonths inflight. No clear time variation is evident, and the absolute accurathe measurement is∼5%.

78 MURCHIE ET AL.

lu-smb

m

gdeigton

-erd

om

rind

omoon

as-tionforingsin

thisthetoo

redof aISon-

theIS

e theab-

terin

d byye

ster,calepeonsossfrom

ge-m)uiredirst,rmper-.onlyac-an

the

FIG. 15. Comparison of calibration coefficients derived inflight usingnar measurements with coefficients derived onground using the large andcalibration spheres, viewed either out the port window of the calibration chamor through a collimator. The large calibration sphere provides better agreeas expected because of its smooth spectral shape.

with a groundbased telescope (Binzelet al.1997) is flat, nearlyfeatureless, and typical of C-type asteroids at the wavelencovered by MSI. The disk-integrated MSI spectrum of Mathilmeasured as for the Moon by integrating all measured lthrough each filter, is similarly smooth but systematically“blue” (Fig. 16). The bluer spectrum measured by MSI is ulikely to be a photometric effect. The Binzelet al. measure-ments were acquired at a phase angle of 6◦, whereas MSI measurements were obtained at a 42◦ phase angle, and the largphase angle of the MSI measurements would be expectemake Mathilde appear redder (Gradieet al. 1980, Gradie andVeverka 1986).

FIG. 16. MSI spectrum of 253 Mathilde compared with the spectrum frthe Small Main-Belt Asteroid Survey (SMASS) (Binzelet al. 1997). The errorbars on the MSI data represent mean absolute calibration uncertainty defrom Canopus measurements and represent a good estimate of the boucalibration error over a large wavelength range.

aller

ent,

ths,

hto-

to

veds on

Systematic calibration error with wavelength could result frdifferences between the real spectrum of the portion of the Mmeasured on 23 January 1998 and the laboratory spectrumsumed for it. We believe this to be likely, as the measured porof the Moon included part of the South-Pole Aitken basinwhich there is no published, well-calibrated spectrum coverthe full wavelength range of MSI. The South-Pole Aitken bais known to be redder than typical highlands regions (Headet al.1993); forcing the disk-integrated lunar spectrum containingregion to appear less red, like typical highlands, would haveobserved effect of making Mathilde appear systematically“blue.”

3.6.6. Validation using Earth observations.During theEarth swingby on 23 January 1998, MSI and the near-infraspectrometer (NIS) conducted simultaneous observationslarge, nearly uniform region of East Antarctica (Fig. 17). Nwas calibrated independently of MSI onground, and itsground calibration is being applied to flight data (Izenberget al.1998). In addition, its fifth spectral channel is centered onsame wavelength of MSI filter 5, and the location of the Nfield-of-view within the MSI field-of-view is known from in-flight measurements, as described in Section 4.4. ThereforNIS observations provide a valuable independent check ofsolute calibration using an extended, field-filling source. Afapplication of the corrections for image artifacts describedSection 5.2.3, the derived MSI radiance of the spot observeNIS is 103.5± 5.2 W m−2 sr−1 µm−1. The radiance derived bNIS is 98.5± 1.7 W m−2 sr−1µm−1. The agreement between thtwo values supports the estimated∼5% uncertainty in absolutecalibration.

4. GEOMETRIC CALIBRATION

4.1. Image Scale and Distortion

Eleven broadband filter images of stars in the Praesepe cluimaged 2 May 1996 (Fig. 18), were used to derive the folength and distortion parameter for the MSI optics. The Praesimages were acquired with the star cluster in 9 different locatiwithin the field-of-view to accumulate as great a sampling acrthe CCD as possible. Magnitudes of measured stars ranged5.7 to 8.7.

The required data for measurement of the focal length andometric distortion are accurate star centers in millimeter (mcoordinates on the detector plane. Several steps were reqto obtain these coordinates starting from the raw images. Flow-level periodic noise was removed using Fourier-transfotechniques. Next, dark current and bias were removed. Afectly rectangular array of 27× 16-µm pixels was assumedBecause the nonresponsive anti-blooming channels occurevery second pixel column, creating nonuniform column sping, the effective coordinates of a particular pixel requiredadjustment of±0.125 pixels from the ideal 16µm spacing.The center of the stellar image was then found by modeling

9

frg


FIG. 17. Location of simultaneous MSI and NIS observations of Antarctica on 23 January 1998, shown inset in an image acquired later on the same day.

d

A

t

wasfindorme

anandis.

0etsonthentyrlytionsinalhis

sampled DNs as a simple Gaussian fall off. The data recorfor each star were its mm coordinates at the focal plane, toDNs, and star catalog information on stars’ right ascension (Rdeclination (Dec), and magnitude. Measurements of 233 starsitions were made in the 11 images.

The star positions were fit by an iterative least squares maing of predicted location from spacecraft attitude informatioand observed location in the images, starting with an appro

FIG. 18. Image of the Praesepe star cluster acquired on 2 May 1996. T
ame is one of those used to validate the MSI focal length and to estimeometric distortion.
edtal),

po-

ch-nxi-

his

mate RA, Dec, and twist of the camera. The solution thenstepped through focal lengths and distortion parameters tothe best fit of focal length, radial distortion, and pointing feach image. Our initial distortion parameters follow the scheof Davieset al. (1994),

x = 0.016× (sample-268) (3)

y = 0.027× (line-122) (4)

R2 = x2+ y2 (5)

xc = x(1+ k1R2

)(6)

yc = y(1+ k1R2

), (7)

wherexc andyc are the corrected star locations in mm that cbe fit to the angular positions in the star catalog. The sampleline references are approximate coordinates for the optic ax

The best focal length is 166.85 mm, compared to 167.±0.2 mm derived from onground images of geometric targ(Hawkinset al. 1997). The mean residual for the best solutiis 0.00486 mm, or 0.23 of an average pixel. Uncertainty infocal length is about 0.1 mm when accounting for uncertaiin the distortion parameter. The radial distortion factor is pooconstrained, and with the residuals of 0.2 pixels, the best solufor k1 is −0.00007 mm2. This factor implies about 0.4 pixeldistortion in the sample direction at the edge, which is a margdetection given the residuals in the fit positions of the stars. T

atecompares to 0.13 pixels of distortion predicted from the opticaldesign. Tests for other distortions or systematic effects on the

80 MURCHIE ET AL.

h

ca

I

a

er

i

1

yo

6exnu

i

lrt

S

la-of

callye-, in

e-ofged.se aromhatsmalltarSIof

esisff-f

solution show no reduction in residuals by inclusion of any otmodel effects. These results translate into accurate angulamensions for a mean pixel of 95.9× 161.8µrad and a field ofview of 2.95◦ × 2.26◦.

4.2. Imager Alignment

The Praesepe images also provided the orientation of theter of the field-of-view (the MSI boresight) in the spacecraftordinate system, an essential measurement for locating imon an asteroidal target using spacecraft attitude information.principal coordinate system of the spacecraft includes az axisnormal to the solar panels, an orthogonalx′ axis located alongthe nominal common boresight of MSI, NLR, XGRS, and Nand ay′ axis orthogonal tox′ andz (Chenget al. 1997). Theorientation of MSI assumed initially for calculation of the foclength and distortion is parallel to thex′ axis. Attitude infor-mation comes from NEAR’s star camera. In a variety of msurements, attitude measurements on a given day have pprecise generally to∼20µrad or∼0.2 MSI pixels, i.e., the limitof detectability.

With the origin of an image (pixel 1,1) at upper left as dplayed on most monitors, and with positive coordinates exteing to the right across columns and down across rows (Fig.in the Praesepe images “true” vector to the star is offset fromcalculated vector (as inferred from the star’s pixel coordinatethe image, with the imager looking exactly down the spacecx′ axis) by+15.04± 0.35 pixels (1442± 33 µrad) across thenarrow pixel dimension (long image dimension, in the−zdirec-tion) and+2.99± 0.22 pixels (484± 36µrad) across the longpixel dimension (short image dimension, in the−y′ direction).Stated differently, instead of thex′ axis being located exactlat the center of the image (column 268.5, row 122.0), it isset 15.04± 0.35 pixels (1442± 33µrad) to the left and 2.99±0.22 pixels (484± 36µrad) up. It is located at column 253.4row 119.01 (Fig. 19). Another way of stating this is that the vtor to an object is oriented down and to the right from the piposition of that object in the image. This offset of 15 colum3 rows compares with 4 columns, 7 rows determined ongroin air at 1 g and∼25◦C. Inflight, MSI is pointing slightly moretoward the+z axis, consistent with disappearance of a smamount of flexure of the aft deck by the instrument massesder terrestrial conditions of 1 g. (Note: In some software paages the first line of an image is displayed at the bottom. Twould invert the sign of the row positions cited in the precedparagraph.)

This measurement was repeated on images where mustars are visible, that were acquired at a variety of times ducruise (e.g., in images acquired at Mathilde and during validaof software uploads) to track the stability of imager pointingthe spacecraft experienced different thermal environments otrajectory to Eros. Through mid-1998, approximately 400µradof variation in MSI pointing was recognized.

Two hypotheses for explaining this time variation in Mpointing were developed and tested. First, accuracy of the

err di-

cen-o-ges

The

S,

l

a-oven

s-nd-9),thes inraft

ff-

,c-els,nd,

allun-ck-hisng

tipleingionasn its

Iatti-

FIG. 19. Alignment of MSI in the spacecraft coordinate system and retive to the boresight of NIS, at NIS mirror position 75. For NIS, the locationsboth the wide and narrow slit fields of view are shown.

tude provided by the star camera may be affected systematiby which stars are detected in its field-of-view. This hypothsis was ruled out by a test performed on 19 August 1998which images were acquired over the course of two 40◦ space-craft rolls. One roll, around the optic axis of MSI (the spaccraft x′ axis), maintained the same stars in the MSI fieldview, while the stars seen by the star camera were chanThe second roll, around the spacecraftz axis, changed the starin both instruments’ field-of-view. In neither case was thersystematic change in apparent imager pointing as derived fstars in the MSI field-of-view. The second hypothesis is ttemperature changes in the spacecraft structure causes aamount of flexure which shifts the relative pointing of the scamera and MSI. A high degree of correlation between Mpointing derived from cruise imaging and the temperaturethe aft (instrument) spacecraft deck supports this hypoth(Fig. 20). Application of the following equations yields the oset in MSI pointing from the spacecraftx′ axis to an accurary o∼30 µrad:

offset inx′–z plane= 1291µrad+ 49µrad× (T,◦C) (8)

offset inx′–y′ plane= 670µrad+ 18µrad× (T,◦ C). (9)

N

xt

di

esurebe-

f an

NISSId

aranif-ISo-and

in-ays

entthe

on-

oonir”


FIG. 20. Dependence of pointing of MSI on temperature of the aft deckthe spacecraft. Pointing offsets from thex′ axis in thex′–z plane (across imagecolumns) are shown as “x” symbols fitted with thin lines; offsets from thex′axis in thex′–y′ plane (across image rows) are shown as filled squares and fiwith thick lines.

4.3. Image Timing

Under the condition of significant changes (at the 1-pilevel) in spacecraft pointing on the time scale of an image,exact time of image acquisition relative to the time of mesurement of spacecraft attitude must be known. Both kindsmeasurements receive a time stamp of MET from a 1-Hz pugenerated by a spacecraft clock. For an MSI image, the ena commanded exposure is fixed at 919 ms after the assoc
time stamp (Fig. 21). The duration of exposure time is varied by
e (MET)

pattern centered on the center of NIS’s field of view as mea-ired
adjusting the time of the beginning of accumulation in signal.
FIG. 21. Timing of acquisition of images having two different exposure times. The end of an exposure is fixed relative to the mission elapsed tim

sured onground. MSI images and NIS spectra were acqu

stamp. Frame transfer from the image zone to the memory zone of the CCoccur at fixed rates and times, independent of the commanded exposure tim


of

tted

elhea-of

lseof

ated

Thus, for an exposure time of≤919 ms, the beginning of thexposure is at or after the time stamp, whereas for an expotime of 920–999 ms the beginning of the exposure comesfore the time stamp. The equation describing the midpoint oexposure is therefore:

MET+ 919 ms− (exposure time/2). (10)

4.4. MSI-NIS Alignment

Coordination of spectral observations between MSI andrequires accurate knowledge of NIS’s footprint within an Mframe. NIS’s nominal 0.38◦ × 0.76◦ footprint can be scanneusing a mirror, in 0.4◦ increments in thex′–z plane (in the±zdirections), over 350 positions ranging from 30◦ from the MSIboresight (toward+z) to 110◦ from the MSI boresight (toward−z) (Warrenet al.1997). NIS mirror position 75 is located nethe center of the MSI field of view, and positions 71–79 spthe MSI field of view. The positional relationship between dferent mirror positions is known from onground testing of N(Warrenet al.1997), but the location of an index scan mirror psition (e.g., position 75) depends on the coalignment of MSINIS.

The coalignment of the two instruments was measuredflight on 26 January 1998 using the Moon as a target, 3 dafter closest approach, when its disk subtended only 0.1◦. Thissmall size was easily imaged by MSI and provided sufficisignal to be detected by NIS, yet is small enough to defineedges of the NIS field-of-view as well as was done duringground testing (Warrenet al.1997). With the NIS mirror fixed inposition 75, the spacecraft was slewed slowly to move the Malong two orthogonal 1.5◦ transsects that formed a “cross-ha

D and readout of image signal from the memory zone for analog-to-digital conversiones.

82 MURCHIE ET AL.

n

i

dfga

t

l

-re

e

inen–-nmimbto-sonen-

ster-lt ofich

orenm

oonrtiesedan

redionlde,itudeands thehn onthelightt. At

simultaneously. Position of the centroid of the Moon’s radia(as measured by MSI) was interpolated to the times at whichMoon became detectable in the NIS field of view at 2-σ abovethe background noise, defining the edges of NIS’s field of vin the coordinate space of MSI pixels (Fig. 19).

5. SCATTERED LIGHT

Two inflight tests provide information on light-scatterinproperties of MSI. The low-Sun test of 26 June 1996 provithe measurements necessary to assess the effect of out-osources at distances 40◦–90◦ from instrument boresight. Durinthe Earth swingby on 23 January 1998, several hundred imof Earth, the Moon, and the space surrounding the Moon wcollected to assess the effects of infield light sources and oufield sources to 12◦ off the boresight.

5.1. Infield Sources

Onground measurements of scattered light utilized smaltended sources several pixels in size, measured against aened interior to the OCF (Hawkinset al. 1997). These measurements provided useful data to∼0.1◦ (20 pixels across theinarrow dimension) from the source, at which distance scattlight in the test apparatus is a significant concern. Inflight calibtions provided extremely useful supplementary measuremof sources of scattered light in both the near and the far fiagainst the perfectly black backdrop of deep space.

catterede plot

pr

FIG. 22. Magnitude of scattered light from the limb of the moon, normalized to the radiance just inside the “reddened limb” in Fig. 12. (a) Falloff in slight in the+z direction from the lunar limb. At distances of≤20 pixels (≤2 mrad) from the limb, shorter wavelengths are more strongly scattered. (b) Samin linear space, showing spectral effects of scattered light near the limb of the Moon. The limb’s reddened appearance and the blue halo are caused byeferentialscatter of 450- and 550-nm light.

cethe

ew

ges-field

gesere-of-

ex-dark-

redra-entsld,

Possible effects of infield scattered light were observedcolor reconstructions of images of Mathilde. In a red–greblue composite image constructed from 700-, 550-, and 450images, Mathilde appears slightly reddened along its l(Fig. 12). A similar reddening could be expected from phometric effects on carbonaceous chondrite-like material (Johnand Fanale 1973), thought to make up Mathilde. However,hancement of the image shows a blue “halo” around the aoid, strongly suggesting that the reddened limb is a resuwavelength-dependent scattering in the imager itself, in whlight transmitted through the 450- and 550-nm filters is mhighly scattered than is light transmitted through the 700-filter.

Infield scattering was characterized using images of the M(acquired 23 January 1998), the color and photometric propeof which are relatively well understood. The Moon was imagat three positions in the field of view, and at each positionunsaturated and a∼3x saturated image (to enhance scattelight) were acquired through each filter. A color reconstructfrom unsaturated images (Fig. 12), like that used at Mathishows the same bluish halo and a reddened limb of a magnnot expected from lunar photometric effects (HelfensteinVeverka 1987). Figure 22 shows transsects of signal acroslunar limb (in the+z direction, cf. Fig. 19) extracted from botexposure-time sets. Signal is normalized to that at a locatiothe Moon’s illuminated disk, just inside the interior edge ofreddened limb. Figure 22a, which emphasizes the scattereditself, shows the wavelength dependence of scattered ligh

N

th

0atit

o

aiollt

tedtai

h

itm

rr)i

aceo

s

r

htshiz8

e toof 60the

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ered

ellipsealest,on-signt

il-red

aryemat-ngelln of


distances of≤20 pixels from an abrupt brightness contrast,four shorter wavelengths (450, 550, 700, and 760 nm) are mscattering than the four longer wavelengths (900, 950, 11050 nm). The 450- and 550-nm filters exhibit the strongestsecond strongest light scattering at these distances, respecwhereas at 20–70 pixels from an abrupt brightness contras1050-nm filter becomes most scattering.

In the 450- to 550- to 700-nm composite image of the Moin Fig. 12, color saturation and low brightnesses have beenhanced in the right panel. The origin of the reddened limbsthe Moon, as on Mathilde, is demonstrated by the above ansis. In shorter-wavelength filters light from the near-limb regis scattered to the region beyond the limb, giving rise simuneously to a reddened limb and a bluish halo. The 450-nm fishows the largest effect, and it is primarily responsible forlimb reddening and the blue halo.

Two approaches can be used to mitigate the effects of scatlight on spectral and photometric measurements. For whole-measurements, the correct approach to measurement of todiance is “aperture photometry.” That is, the total light witha box enclosing all of the scattered light is measured, andeach image in a multispectral set, this sum is corrected fortarget’s range to the Sun and range from the imager. Thisproach was used to determine the average spectrum of Matshown in Fig. 15 and was used by Clarket al. (1999). The re-quired dimensions of the aperture box depend on the magnof scattered light in the worst filter at large distances (1050 nand the SNR of the imager at high DN levels (∼500; cf. Hawkinset al.1997). To enclose all scattered light measurable at an Sof ≥1 in an unsaturated exposure, we require a box 50 (nadimension) pixels larger in radius in thez direction (see Fig. 19than the object being measured and 30 pixels (long dimenslarger in radius in they′ direction than the object being mesured. The second, analytical approach, “removal” of the stered light directly by filtering of the Fourier transform imagwas employed very successfully with Galileo images to remwavelength-dependent scattered light (Gaddiset al. 1995). Asimilar approach is now being implemented for MSI.

5.2. Out of Field Sources

5.2.1. Scattered light. During the “low-Sun test,” imageacquired with the Sun at positions 40◦–90◦ off the MSI bore-sight provided valuable information on the effects of scattesunlight on asteroidal observations at high phase angles,those≥90◦. The scattered light originates as solar irradiancethe interior of the stray light baffle, which is reflected into toptics. In Fig. 23, the magnitude of the scattered light fromSun at some off-axis angle is compared with the radianceMathilde and Eros when observed at the corresponding pangles. For Mathilde, a gray color, 4% albedo, and generalasteroidal photometric function (Helfenstein and Veverka 19were assumed. For Eros, the disk-integrated spectral prope
of Murchie and Pieters (1996) were assumed. In both casesincidence angle of 60◦ (near the terminator) was also assume

eore00,ndvely,the

nen-only-n

ta-terhe

rediskl ra-nfortheap-ilde

ude),

NRow

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ofaseed9)

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FIG. 23. Magnitude of scattered sunlight at Mathilde and Eros, relativasteroid radiance expected at different phase angles at incidence angles◦,predicted from the “low-Sun test” of 26 June 1996. Unfilled symbols are for550-nm filter; filled symbols are for the broadband 700-nm filter.

The significance of scattered sunlight in high-phase angleages depends both on the phase angle and on the albedoerties of the measured asteroid. At different distances fromSun, differences in scattered irradiance on the optics areanced by the differences in irradiance incident on the asteso the ratio of scattered sunlight to asteroid radiance at a gphase angle depends on asteroid reflectance properties anon solar distance. Thus at any phase angle, scattered suwas a greater concern at Mathilde than it is expected to bEros. At a 120◦ phase angle, the highest planned for normEros imaging science, scattered sunlight is only a∼0.1% effect,but at lower-albedo Mathilde at the same geometry the scattsunlight is an∼1% effect.

5.2.2. False images.The imaging strategy at Mathildwas to acquire repeated image mosaics of the uncertainty ehaving a 2-σ probability of containing the asteroid, for a totof 10 mosaics from different viewing geometries. Near closapproach, the uncertainty ellipsoid was∼10 image frames wideand Mathilde was located near the center of that ellispoid. Csequently many images of black space were acquired by dewith the asteroid up to 10◦ outside the field of view. A subseof these images exhibited a highly attenuated (∼10−3), grainyimage of Mathilde (a “false image”) near the limit of detectabity. Several hypotheses involving internal reflections appeacapable initially of explaining the false images.

Just after radiometric imaging of the Moon on 23 Janu1998, the Moon was used as a source to characterize systically the magnitude and distribution of stray light originatifrom out-of-field sources, including the “false image” as was any other manifestations. This test consisted of acquisitio
, and.three orthogonal strips of overlapping images, which placed theMoon at distances up to 12◦ from the center of the MSI field of

84 MURCHIE ET AL.

ag

fo

s

ede

o its

theed

the

int,ewed

ig-rafthar-

focuse thesurentuatems)

FIG. 24. Primary and “false” images of the Moon obtained on 23 Janu1998 during the test for stray light from out-of-field sources. The “false imaappears from sources located 2.26◦ out of the field of view in the+y′ directionin the spacecraft coordinate system.

FIG. 25. Strength of the “false image” relative to the primary image,
false images appearing near the center and edges of the frame.
rye”

r

view in the+z,+y′, and−y′ directions (cf. Fig. 19). The imagecontaining the Moon out of field of view in the+y direction con-tained false images like those of Mathilde (Fig. 24), displac2.26◦ in the−y′ direction from the Moon’s actual position. Thfalse image is both highly attenuated (Fig. 25) (∼10−2 to 10−3)and grainy, and there is a clear wavelength dependence tintensity, which increases at longer wavelengths.

An extensive set of Earth images with the south pole atcenter of the field of view, acquired over 36 h as NEAR recedfrom Earth, revealed a similar false image (Fig. 26). Whereimage of Earth extends outside of the field-of-view in the+y′

direction, there is superimposed on the true Earth image a fafull-frame false image of the scene adjacent to the field-of-viin the+y direction (cf. Fig. 27). Its attenuation was measur

FIG. 26. (Top) 1050-nm image of Earth showing the false image that orinates from the part of the planet just outside the field of view in the spacec+y′ direction. The false image is marked by the arrow. Note the defocused cacter of the false image near the edge of the frame, compared to the in-false image appearing near the center of the frame in Fig. 22. (Bottom) Herfalse image has been removed with a simple technique that utilizes two expotimes of the same scene. Both images have been highly enhanced to accelow brightness levels; the intensity of the false image in this exposure (48
is∼60 DN.


FIG. 27. Origin of the false image and other image artifacts as inferred from onground and inflight tests. The orientation of the CCD is shown as it “sees” animaged scene.

86 MURCHIE ET AL.

tuo

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by comparing signal of the real and “false” images of the sacloudmass across the edge of the field-of-view. Its magniand wavelength dependence are similar to those for the Mexcept for the 1050-nm filter (and to a lesser extent the 1000filter) in which the level of the “false” image is significantelevated at the edge of the frame (Fig. 25).

The structure and layout of the CCD itself are critical to undstanding the origin of the false image. The CCD is divided itwo zones, an image zone centered on the instrument’s opaxis and a memory zone of the same size, coated with a mefilm and lying off-axis. As shown in Fig. 21 and illustrated gometrically in Fig. 27, following an exposure of an object, timage frame of data is transferred to the memory zone in 0.9During that time, the object image accumulates a weak “trailreadout smear behind the primary image. Over the next 80the image data remain “stationary” in the memory zone, supedly protected against accumulation of additional signal bymetallic film. Readout of the data and conversion from anato digital signal occur slowly and at a fixed rate over the n∼800 ms. During that time the image data slowly are migraacross the memory zone in the same direction in which fratransfer occured.

Figure 27 shows the origin of the false images deduced fthe information described above, and supported by ongromeasurement of a spare CCD like that used in MSI. The angoffset of the false image from the primary image (2.26◦ in the−y′

direction) strongly suggests that it is not a true ghost imagerather a primary image of an out-of-field scene that is partitransmitted through an imperfectly opaque metallic film othe memory zone. Such leakage is reproducibly demonstronground using the spare CCD and also results in a “grainy fimage” (the graininess possibly being due to micrometer-simperfections in the metallic film). Additional inflight measurments also support this hypothesis. First, the DN level offalse image is independent of commanded exposure time, brelated to radiance of the out-of-field source. This situatioexpected from the constant, exposure time-independent 8
during which the transferred frame remains stationary for an
an imageis no

ov

At exposure times of tens to hundreds of milliseconds the false

FIG. 28. Removal of the false image from a 760-nm frame showing Antarctica, using a 0-ms exposure. (Left) Nominal 2-ms exposure, containingof the scene plus a superimposed weak, smeared false image of the scene adjacent in the+y′ direction. (Center) 0-ms exposure of the same scene. Thereimage of the scene; instead only the false image and readout smear are present. (Right) Difference image showing Antractica with the false image remed. Notethe higher contrast than in the left panel, without with the flattening effects of the false image. All three images are contrast-enhanced.

medeon,

-nm

er-toticalallic-e

ms.ofms,os-thelogxt

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in-focus image to penetrate the memory zone. Due to the fiaccumulation time for the false image, the proper measure foattenuation as shown in Fig. 25 is comparison with the matude of the primary image expected during an 80-ms exposSecond, the false image shows a trail similar to that causereadout smear (Fig. 24). This trail too is expected, from conued penetration of the out-of-field image into the memory zas the image data slowly shift in the+y′ direction as they areread out.

The increasing intensity of the false image near the edgethe frame in the 1050-nm filter (and to a less extent the 1000filter) may be due to propagation of light leaked near the edof the metallic film covering the memory through the undering Si detector material, which is more transmissive at lonwavelengths. This hypothesis is supported by the apparenfocusing of 1050- and 1000-nm false images near the edgthe frame, relative to the false image in the other filters.

The false image phenomenon increases in intensity at shoposure times, when the memory zone is filled by an illuminascene, and at higher scene radiances. All of these conditioncurred simultaneously during the highest resolution imageAntarctica during the Earth recession sequence (Fig. 28).false image lacks recognizable structure because it is hismeared by the accumulation throughout readout. The inteof the false image increases toward the bottom of the frameto the longer time that image data in those rows spend inmemory zone prior to readout.

In summary, the major source of out-of-field stray light isweak false image of the scene offset 2.26◦ in the+y′ directionfrom any part of the MSI field of view. This out-of-field scenis imaged onto the memory zone of the CCD, and is weatransmitted through an imperfectly opaque metallic coverThis “false image” appears dominated by an in-focus comnent, but for scenes that fill the memory zone, accumulathroughout image readout blurs recognizable structure. In1050-nm filter (and to a lesser extent the 1000-nm filter) thean additional out-of-focus component at the edges of the sc

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image magnitude is∼10−3 that of the primary image, but ivery short exposure time images (e.g., through the broad700-nm filter) and at the edges of images though the 10001050-nm filters the false image is larger in magnitude.

5.2.3. False image removal.The false image accumulatover a fixed time interval, so it is an additive component toages acquired at different exposure times. Because it is addthe false image can be removed once it is isolated using imof the same scene acquired at two different exposure timesa nominal and a very short exposure. The equation for a linsolved pixel-by-pixel, where the independent variable is exsure time and the dependent variable is dark-corrected DNoffset, ory-intercept, represents the sum of the false imagereadout smear. This is then removed from the nominal expoto produce a false image- and readout-smear-free image. Tsult can then be flat-fielded and calibrated to physical units,radiance. This procedure was validated using Earth imagequired at two exposure times through each of the seven spefilters, and in each case the false image was removed to wthe level of its detectability (Fig. 26). Additive periodic noipicked up from the spacecraft was also removed, improvingaccuracy of low-DN measurements.

In practice during flight, the false image can be measuand removed more directly by removal of a zero exposureimage from an image exposed for a nominal period. Usezero exposure is preferred to a very short exposure, becauzero exposure contains only a false image and readout swithout the image of a scene. It is a direct measure of the odescribed in the preceding paragraph. Figure 28 shows aexposure, 760-nm image of Antartica with a strong false ima 0-ms exposure of the same scene, and the “cleaned” diffeimage. In the difference image, the false image is removed∼97% effectiveness, i.e., to the level of the noise in the nomexposure, despite a shift in the scene of several pixels betthe nominal and zero exposures.

The false image phenomenon is also an issue with theground flat-field images, which are of the interior of the fiefilling large integrating sphere. In the initial versions of thefields (Hawkinset al. 1997), those acquired through differefilters are nearly the same except for the 1000- and 1050flat fields which are elevated at the edges at the same locawhere the out-of-focus false images were found in Earth imaWe applied the same procedure as described above to deteremove false images from the flat fields, using images of thtegrating sphere acquired at different exposure times. Theimage was found to be detectable only in the 1000- and 1050flat fields, and these were updated from the previous versshown by Hawkinset al. (1997).

6. SOFTWARE IMPLEMENTATION

The calibration for MSI described in this paper has been
plemented in a user friendly graphical interface called “MSICAL.”The program may be run in single image mode or list mode.

andand

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Thenduree re-.g.,ac-

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the list mode an ascii file of image names is input to the pgram, each image in the list is then calibrated. The user can sthe output calibrated images to be in units ofI /F or radianceand can select different stages of partial calbration in unitDN if desired. A companion image viewer (“NVIEW”) has alsobeen developed that allows the analyst to display the datan X interface. Functions in the viewer include four differeaspect correction algorithms, contrast stretching, pseudocand screen dump to FITS or TIFF format. This software is avable with the image data from the PDS. Periodic updates tocalibration will be provided as available.

7. SUMMARY

MSI is a keystone to accomplishment of NEAR’s sciencejectives at the S-type asteroid 433 Eros, and it provided valuimage data during cruise on the C-type asteroid 253 MathiMSI has been subjected to a comprehensive series of inflightto validate its radiometric characteristics measured ongroto characterize instrument stability, pointing, and coalignmwith other instruments, and to determine light-scattering chateristics. The results of these tests, described herein, suppoalgorithms designed to convert into scientifically useful produthe images of 253 Mathilde released to the PDS. Key inflicalibration results include:

(1) During cruise, dark durrent exhibits stability to withinDN.

(2) Images of smooth fields show that the flat-field correctappears accurate to within the limits of inflight detectability.

(3) Calibration images of the Moon and Canopus revealsolute radiometric accuracy of∼5%, with no evident systematichange with time.

(4) The focal length was determined inflight to be 166.85 mcompared to 167.0± 0.2 mm measured onground.

(5) In preparation for coordinated MSI–NIS science invesgations at Eros, MSI pointing and coalignment with the neinfrared spectrometer (NIS) have been characterized under flconditions.

(6) Sources and intensity of scattered light from inside aoutide the field-of-view have been quantified, and proceduhave been developed to remove the out-of-field conponent.

APPENDIX A: MSI FILE NAMING CONVENTIONS

The file naming convention for MSI images isMnnnnnnnnnnXmpY.FIT,whereM is the name of the instrument (MSI);nnnnnnnnnnis the MET (missionelapsed time) stamp on the image;X is “F” for full image or “S” for summaryimage;m is the filter wheel position, 0 to 7;p is virtual channel “0”, “2”, or “3”;andY is “P” for production, “R” for real-time, or “B” for brassboard test. Eacof these terms is explained below.

nnnnnnnnnn. The MET corresponds to the last spacecraft 1-Hz pubefore the end of image integration, i.e., 918 ms before the end of image

Ingration. See Section 4.3, for a detailed explanation of relative timing of imageintegration and the spacecraft pulse.

88 MURCHIE ET AL.

terms is given in Tabl

TABLE BIExplantion of Terms in MSI Radiometric Calibration Equation

DNx,y, f,T,t,c Raw DN measured by the pixel in columnx, row y, through filter f at exposure timet andtemperatureT with the cover statusc open or closed

Darkx,y,t,T Dark level modeled for this pixel at exposure timet and temperatureTSmearx,y,t Scene-dependent readout smear for the pixel at exposure timetFlatx,y, f,c Flat field for filter f with the cover statusc open or closedCoeff Coefficient for converting corrected DN from filterf to radiance.Respf,T Responsivity for this filter at temperatureT relative to that at−29.6◦CAttenf,c Attenuation in filter f with the cover statusc closedExpt Exposure time in milliseconds

e

ftai

lrm

e

f

xparka.

ofbed

-ofoonaluessedThe7 ind onanddlyy bethe

re ater

ht

X. A full image (“F”) is the 537-column, 244-line image returned by MSA summary image (“S”) contains 22 lines and 26 columns of 4-bit superpixeach representing an area 20 pixels wide and 11 pixels high in the original imIt is used primarily as a vehicle for download of header information.

m. Filter wavelengths for positions “0” through “7” are given in Table IV

p. Virtual channel (VC) represents the route of image transmission toground. VC2 images are sent from MSI via a 2 Mbit/s link directly to thesolid-state recorder. VC0 images are sent to the recorder via the spacecraEither VC0 or VC2 images are played back minutes to days after acquisidepending on the schedule of image acquisitions and downlinks. VC3 imare transmitted to the ground in real time immediately after image acquisit

Y. Real-time (“R”) processed images are onground products created asas possible after downlink, before attitude information dowlinked separateassociated with the images. Production (“P”) images have attitude infotion extracted from spacecraft telemetry and associated with the images. Bboard (“B”) images are acquired as parts of onground tests using the spacsimulator.

APPENDIX B: MSI RADIOMETRIC CALIBRATION

The radiometric calibration equation has the form

Radiancex,y, f,T,t,c = {[DNx,y, f,T,t,c − Darkx,y,t,T ] − Smearx,y,t } • 100

Flatx,y, f,c • Coeff • Respf,T • Attenf,c • Expt,

(B1)

where radiance is in units of W m−2µm−1 sr−1. The summary explanation o
e BI.
-05

-0505

operating temperature of−29.6 C.

TABLE BIIMSI Calibration Coefficients Based on Onground and Inflight Tests

Coeff (fromEff. center Eff. width Moon for Attenc from Respf,T Respf,T Respf,T

Filter no. (nm, OCF data) (nm, OCF data) effectiveλ,−29.6◦C) Moon coeff.a coeff.b coeff.c

2 462 23 141.5 0.2182 0.9022 −0.0045827 −4.3198e-051 554 24 486.2 0.2357 0.94105 −0.0029599 −3.2714e-050 700 133 3923.4 (0.2774) 1.0057 0.00019236 —3 755 19 506.2 0.2444 1.0499 0.0016854 —5 900 33 467.9 0.2432 1.1049 0.0051262 5.3421e4 951 38 309.4 0.2322 1.1311 0.0041073 −1.0833e-056 996 44 165.8 0.2305 1.1965 0.0070161 1.2722e7 1033 51 63.3 0.2330 1.3238 0.012328 4.6893e-

I.ls,

age.

.

the

t bus.ion,ges

on.

soony is

a-rass-craft

DNx,y, f,T,t,c is the image data, whereas exposure time in milliseconds Et

and the filterf used are both given in the image header. The flat-field and dfiles, Flatx,y, f,c and Darkx,y,t,T , respectively, are archived with the image datThe equation for removal of the smear is given in the text of this paper.

The three remaining terms, Coeff , Attenf,c, and Respf,T , are given inTable BII. The coefficients for converting corrected DN in each filter into unitsradiance (Coeff ) were derived from lunar observations and tested as descriin this paper.

The attenuation by the cover (Attenf,c) was believed from onground measurements (Hawkinset al.1997) to be wavelength-neutral and to have a magnitude0.2357. However, as described in this paper, inflight measurements of the Mand Canopus clearly indicate wavelength dependence. The measured DN vfor the Moon in the 550-nm filter are in good agreement with predictions baon a photometric model and assuming this value for the cover attenuation.values for cover attenuation in Table BII therefore assume a value of 0.235the 550-nm filter, with the values for the other filters scaled to this one basethe difference in signal levels between lunar observations with the cover onoff. The value for the broadband 700-nm filter, although nominal, is markegreater than that of the others. Part of the reason for the discrepancy mainaccurate knowledge of the filter’s passband, which was used to calculateexpected signal from the Moon.

The temperature dependence of responsivity in each filter (Respf,T ) was de-termined from fitting onground measurements of the large integrating sphedifferent temperatures (Hawkinset al.1997). This is expressed as a second-ordpolynomial,

Respf,T = a+ bT + cT2, (B2)

whereT is temperature in Celsius and the function is unity at the nominal inflig◦

alednse

237744107564331042


TABLE CIMSI Filter Bandpasses, Visible Wavelengths

Filter 2 Filter 1 Filter 0 Filter 3

Wavelength Scaled Wavelength Scaled Wavelength Scaled Wavelength Sc(nm) response (nm) response (nm) response (nm) respo

405.000 0 .003 525.000 0.001 590.000 0.000 730.000 0.00410.000 0.004 528.000 0.004 595.000 0.005 733.000 0.00415.000 0.003 531.000 0.017 600.000 0.032 736.000 0.00420.000 0.004 534.000 0.140 605.000 0.106 739.000 0.01425.000 0.018 537.000 0.546 610.000 0.200 742.000 0.06430.000 0.041 540.000 0.583 615.000 0.287 745.000 0.29435.000 0.103 543.000 0.567 620.000 0.366 748.000 0.85440.000 0.174 546.000 0.597 625.000 0.448 751.000 1.00445.000 0.277 549.000 0.662 630.000 0.524 754.000 0.94450.000 0.445 552.000 0.751 635.000 0.578 757.000 0.96455.000 0.608 555.000 0.821 640.000 0.614 760.000 0.94460.000 0.716 558.000 0.862 645.000 0.651 763.000 0.77465.000 0.827 561.000 0.937 650.000 0.703 766.000 0.37470.000 1.000 564.000 1.000 655.000 0.774 769.000 0.11475.000 0.348 567.000 0.469 660.000 0.838 772.000 0.03480.000 0.056 570.000 0.098 665.000 0.875 775.000 0.01485.000 0.017 573.000 0.023 670.000 0.900 778.000 0.00490.000 0.008 576.000 0.007 675.000 0.914 781.000 0.00495.000 0.005 579.000 0.002 680.000 0.934500.000 0.004 685.000 0.970500.000 0.004 690.000 0.999

695.000 1.000700.000 0.972705.000 0.935710.000 0.912715.000 0.912720.000 0.923725.000 0.923730.000 0.894735.000 0.838740.000 0.770745.000 0.721750.000 0.707755.000 0.724760.000 0.734765.000 0.673770.000 0.522775.000 0.368780.000 0.280785.000 0.265790.000 0.341795.000 0.301800.000 0.069805.000 0.015810.000 0.004
815.000 0.001
s

ngthnalrated,ader

APPENDIX C: MSI FILTER BANDPASSES

The bandpasses of the MSI filters, mounted in the instrument, were meain the OCF as described by Hawkinset al. (1997). A monochometer beam
imaged near the center of the CCD was stepped through the wavelength rof each filter as stated in the manufacturer’s specifications. The bandpas
ured

each filter was recovered by measuring the integrated signal at each wavele(after subtraction of background levels) and normalizing to the maximum sigmeasured. The incandescent source in the monochrometer was uncalibhowever, introducing uncertainty into the shape of the bandpass in the bro

anges offilters, especially the broadband 700-nm filter. The OCF-derived bandpasses foreach of the eight filters are given in Tables CI and CII.

90 MURCHIE ET AL.

aledonse

13255849197900139263914610577363378627619425

TABLE CIIMSI Filter Bandpasses, Infrared Wavelengths

Filter 5 Filter 4 Filter 6 Filter 7

Wavelength Scaled Wavelength Scaled Wavelength Scaled Wavelength Sc(nm) response (nm) response (nm) response (nm) resp

870.000 0.007 920.000 0.011 950.000 0.013 990.000 0.0873.000 0.019 923.000 0.025 953.000 0.016 993.000 0.0876.000 0.059 926.000 0.076 956.000 0.022 996.000 0.0879.000 0.211 929.000 0.248 959.000 0.024 999.000 0.1882.000 0.581 932.000 0.620 962.000 0.043 1002.000 0.4885.000 0.775 935.000 0.877 965.000 0.074 1005.000 0.8888.000 0.757 938.000 0.902 968.000 0.134 1008.000 1.0891.000 0.776 941.000 0.914 971.000 0.245 1011.000 0.8894.000 0.834 944.000 0.948 974.000 0.435 1014.000 0.6897.000 0.893 947.000 0.982 977.000 0.679 1017.000 0.6900.000 0.935 950.000 1.000 980.000 0.886 1020.000 0.6903.000 0.948 953.000 0.986 983.000 0.985 1023.000 0.7906.000 0.941 956.000 0.939 986.000 1.000 1026.000 0.8909.000 0.960 959.000 0.891 989.000 0.988 1029.000 0.8912.000 1.000 962.000 0.899 992.000 0.974 1032.000 0.8915.000 0.808 965.000 0.977 995.000 0.959 1035.000 0.8918.000 0.360 968.000 0.845 998.000 0.941 1038.000 0.8921.000 0.118 971.000 0.382 1001.000 0.915 1041.000 0.7924.000 0.043 974.000 0.119 1004.000 0.886 1044.000 0.7927.000 0.019 977.000 0.042 1007.000 0.856 1047.000 0.6930.000 0.010 980.000 0.018 1010.000 0.822 1050.000 0.5

983.000 0.009 1013.000 0.771 1053.000 0.55986.000 0.005 1016.000 0.680 1056.000 0.51

1019.000 0.536 1059.000 0.4831022.000 0.365 1062.000 0.4531025.000 0.226 1065.000 0.4241028.000 0.125 1068.000 0.3921031.000 0.069 1071.000 0.3491034.000 0.039 1074.000 0.2891037.000 0.022 1077.000 0.2111040.000 0.013 1080.000 0.1351043.000 0.007 1083.000 0.0751046.000 0.004 1086.000 0.0391049.000 0.002 1089.000 0.021

1092.000 0.0121095.000 0.0071098.000 0.004

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. Lee

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NEAR photometry of Asteroid 253 Mathilde.Icarus140, 53–65.

ACKNOWLEDGMENTS

The detailed inflight testing of MSI was made possible by the dedicaand hard work of many members of the NEAR Mission Operations, spaceand software teams and the mission manager in supporting these efforespecially thank M. Holdridge, B. Ballard, P. Carr, R. Dickey, R. FarquB. Geldzahler, C. Hersman, C. Kowal, J. Landshof, R. Molloy, T. MuliR. Nelson, A. Posner, A. Santo, R. Weir, and K. Whittenburg.

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inflight calibration of the near multispectral imager

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