inflight calibration of the near multispectral imager: ii. results from eros approach and orbit
TRANSCRIPT
Icarus 155, 229–243 (2002)
doi:10.1006/icar.2001.6746, available online at http://www.idealibrary.com on
Inflight Calibration of the NEAR Multispectral Imager
II. Results from Eros Approach and Orbit
Scott Murchie
The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland 20723E-mail: [email protected]
Mark Robinson
Department of Geological Sciences, Northwestern University, Evanston, Illinois 60208
Deborah Domingue
The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland 20723
Han Li
Department of Geological Sciences, Northwestern University, Evanston, Illinois 60208
Louise Prockter and S. Edward Hawkins, III
The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland 20723
William Owen
Jet Propulsion Laboratory, Pasadena, California 91109
Beth Clark
Center for Radiophysics and Space Research, Cornell University, Ithaca, New York 14853
and
Noam Izenberg
The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland 20723
Received November 27, 2000; revised May 17, 2001
During the Near-Earth Asteroid Rendezvous (NEAR) space-craft’s investigation of asteroid 433 Eros, inflight calibration mea-surements from the multispectral imager (MSI) have providedrefined knowledge of the camera’s radiometric performance, point-ing, and light-scattering characteristics. Measurements while atEros corroborate most earlier calibration results, although thereappears to be a small, gradual change in instrument dark currentand flat field due to effects of aging in the space environment. Themost pronounced change in instrument behavior, however, is a dra-matic increase in scattered light due to contaminants accumulatedon the optics during unscheduled fuel usage in December 1998.Procedures to accurately quantify and to remediate the scatteredlight are described in a companion paper (Li et al. 2002, Icarus
155, 00–00). Acquisition of Eros measurements has clarified therelative, filter-to-filter, radiometric performance of the MSI. Ab-solute radiometric calibration appears very well constrained fromflight measurements, with an accuracy of ∼5%. Pointing relativeto the spacecraft coordinate system can be determined from thetemperature of the spacecraft deck with an accuracy of ∼1 pixel.c© 2002 Elsevier Science (USA)
Key Words: asteroids; instrumentation; surfaces; Eros; NEAR.
1. INTRODUCTION
g
229
The Near-Earth Asteroid Rendezvous (NEAR) spacecraft be-an detailed investigations of its target asteroid, 433 Eros, on
0019-1035/02 $35.00c© 2002 Elsevier Science (USA)
All rights reserved.
E
spread function (PSF) due to scattered light. Following the
230 MURCHI
23 December 1998 following an abort on 20 December 1998 ofthe first of four orbit insertion burns intended to match NEAR’sheliocentric velocity with that of Eros. A successful firing ofthe main engine to match the spacecraft and asteroid velocitiestook place on 3 January 1999, placing NEAR at a distance ofover 400,000 kilometers from Eros. During the remainder of1999 additional small maneuvers closed the distance. NEAR’sapproach investigations began on 10 January 2000, culminatingin orbit insertion on 14 February 2000.
In the short term, this sequence of events led to upload tothe spacecraft and execution of an emergency flyby imaging se-quence on 23 December 1998. For several hours before and afterclosest approach at a distance of 3813 kilometers, seven-colorimaging sequences were taken that characterized gross asteroidshape, volume, and spectral properties. A broadband filter wasused only for satellite searches at the beginning and end of theencounter and for closest approach imaging. In the longer term,the orbital mission was replanned and modified from its originalconfiguration, retaining all of the same objectives (Veverka et al.1997a). Results of flyby imaging are described by Veverka et al.(1999), and first results from approach and orbital imaging aregiven by Veverka et al. (2000). More detailed orbital results arepresented in this issue.
The multispectral imager (MSI) is a keystone to accomplish-ing NEAR’s science objectives. The design and function of theinstrument are described in detail by Hawkins et al. (1997).Two major subassemblies make up MSI: a camera and a dataprocessing unit (DPU), which provides a digital interface to thespacecraft and supplies power and the master timing to the cam-era. A refractive optical telescope, a filter wheel, and a detectorwith its associated electronics are all part of the camera. Thecamera is a five-element (five-lens) refractive telescope with afocal length of 167.0 ± 0.2 mm. Seven narrow-band visible andnear-infrared spectral filters permit multispectral imaging, andone broadband filter is used for optical navigation and faint ob-ject imaging. A one-time deployable cover with a fused silicaaperture, which protected the optics from contamination duringlaunch, was opened on 1 May 1996. A fixed stray light baf-fle prevents out-of-field sources ≥36◦ from the optical axis ofthe camera from directly illuminating the outer optical element.A frame-transfer silicon charge-coupled device (CCD) convertsthe optical signal into an electrical one, which is then digitizedto 12 bits in the focal plane detector (FPD) and transferred tothe DPU. Exposure times are controlled electronically; thereis no mechanical shutter. The image is exposed for some time(0–999 ms), transferred to a memory zone in 0.9 ms, held for80 ms, and converted from analog to digital signal over 800 ms.The CCD is cooled passively to approximately −30◦C to mini-mize dark current. During flight, temperatures of −28◦ to −32◦Chave been maintained. MSI provides a field of view of 537 × 244rectangular pixels, which view solid angles of 162 × 96 µrad,yielding a total field of view encompassing 2.95◦ × 2.26◦. Thedetailed description of the instrument and results of onground
calibrations are given by Hawkins et al. (1997).ET AL.
Murchie et al. (1999) described the results of comprehen-sive inflight calibrations of MSI through the first Eros flyby,which were intended to update results from onground tests andto measure MSI attributes that cannot be well determined on-ground. Henceforth we refer to that manuscript as paper 1. Here,in paper 2, we describe further calibration results pertaining toimager radiometric and pointing characteristics obtained duringand since the Eros flyby through July 2000 (Table I). The mostpronounced change in instrument behavior is a dramatic increasein scattered light due to contaminants accumulated on the opticsduring an unscheduled fuel dump following the aborted burnof 20 December 1998. Procedures to accurately quantify and toremediate the scattered light are described in a companion paperby Li et al. (2002).
2. APPROACH
MSI testing and calibration were done using a three-tieredstrategy of testing at the component level, testing onground atthe instrument level, and retesting inflight. Onground instru-ment level tests were performed at the Optical Calibration Fa-cility (OCF) at APL (Hawkins et al. 1997). The OCF consistsof several large, linked, cryogenically cooled vacuum cham-bers, the largest of which (the instrument chamber) contains atwo-axis motion stage. Calibration sources can be placed at twolocations for viewing, at the focus of a collimator or outsidea fused silica port window. Either of two integrating spheres(a large Spectralon-lined sphere or a small gold-lined sphere)can be viewed through the port window or collimator. Resultsof onground component-level tests and MSI radiometric, wave-length, and geometric calibration are described by Hawkins et al.(1997).
Inflight calibrations provide the link between onground cal-ibrations and science measurements of Mathilde and Eros. Forradiometric calibration, this is done via multispectral imagingof planetary and stellar sources. The Moon was imaged throughall spectral filters three days after launch, in February 1996, andagain in January 1998. At both times highland regions, whichhave relatively well known spectral properties from measure-ments of returned lunar soil samples, were visible. Lunar cal-ibrations at these two times, plus comparison of MSI resultsfrom Mathilde (Veverka et al. 1997b) and Eros (Veverka et al.2002), provided a test for both the accuracy and the stabilityof MSI radiometric calibration. During intervening times and atEros, imaging of Canopus was used to track and characterizelight-scattering properties and any time variations in respon-sivity. Pointing calibrations were carried out by imaging starfields for which positions and angular separations are accuratelyknown. During the orbital mission, at least four pointing cali-bration frames were acquired daily.
The predominant inflight characterization of MSI performancesince the flyby has been quantification of the extended point
aborted burn of 20 December 1998, approximately 28 kg of
231
image rtributes
NEAR MULTISPECTRAL IMAGER CALIBRATION
TABLE IImage Data Used in This Paper
Figure Images (by METa)
2 0060963809, 0060964369
3, 4, 6, 7 0003093069, 0003093371, 0006172292, 0006172294, 0006427889, 0006427891, 0008211989, 0008211991,0011225789, 0011225791, 0046263701, 0046263703, 0052138903, 0052138906, 0056977304, 0056977306,0060948723, 0060948725, 0099386816, 0099386818, 0126673212, 0128881806, 0131642227, 0134353812,0136945335, 0139450936
5 0003093069, 0003093371, 0006172292, 0006172294, 0006427889, 0006427891, 0008211989, 0008211991,0011225789, 0011225791, 0046263701, 0046263703, 0052138903, 0052138906, 0056977304, 0056977306,0060948723, 0060948725, 0099386816, 0099386818
8 0003093071, 0003093371, 0006172292, 0006172294, 0006427889, 0006427891, 0008211989, 0008211991,0011225789, 0011225791, 0011234609, 0011234611, 0046263701, 0046263703, 0052138903, 0052138906,0056977304, 0056977306, 0060948723, 0060948725, 0099386816, 0099386818, 0141088270, 0141088272,0141088274, 0141088276, 0141173351, 0141173353, 0141173355, 0141173357, 0141351551, 0141351553,0141351555, 0141351557, 0141433811, 0141433813, 0141433815, 0141433817, 0141520211, 0141520213,0141520215, 0141520217, 0141599951, 0141599953, 0141599955, 0141599957, 0142380611, 0142380613,0142380615, 0142380617, 0142553411, 0142553413, 0142553415, 0142553417, 0142897211, 0142897213,0142897215, 0142897217, 0143154551, 0143154553, 0143154555, 0143154557, 0143494751, 0143494757,0144104892, 0144104894, 0144104896, 0144104898, 0145310832, 0145310834, 0145310836, 0145310838,0145508232, 0145508234, 0145508236, 0145508238, 0146634552, 0146634554, 0146634557, 0146634559,0148358713, 0148358715, 0148358717, 0148358719, 0148473433, 0148473435, 0148473437, 0148473439,0149601253, 0149601255, 0149601257, 0149601259, 0149769014, 0149769016, 0149769018, 0149769020,0150172814, 0150172816, 0150172818, 0150172820, 0150285314, 0150285316, 0150285318, 0150285320,0151980734, 0151980736, 0151980738, 0151980740, 0152087869, 0152087871, 0152087873, 0152087875,0152578635, 0152578637, 0152578639, 0152578641, 0153185775, 0153185777, 0153185779, 0153185781,0153300315, 0153300317, 0153300319, 0153300321, 0153790575, 0153790577, 0153790579, 0153790581,0153877575, 0153877577, 0153877579, 0153877581, 0154395975, 0154395977, 0154395979, 0154395981,0154574775, 0154574777, 0154574779, 0154574781, 0154998075, 0154998077, 0154998079, 0154998081
9 0011232329, 0011232749, 0011233169
10 0042826982
11 0125720165, 0125720167, 0125720169
12 0011232329, 0011232749, 0011233169, 0042826982, 0125720165, 0125720167, 0125720169
14 0089840640-0089840652, 0089841412-0089841424, 0089842184-0089842196, 0089843728-0089843740,0089844500-089844512, 0089845272-089845284, 0089846895-0089846907, 0089847667-0089847679,0089858630-089858642
15 0123597125-0123614299, 0124263125-0124281859, 0124543925-0124557967, 0125236683-0125256147,0125494925-0125496491
16 0006429784-0006430102, 0046265646-0046265964, 0052140842-0052141160
17, 18 Hundreds of broadband filter Images
19–21 0046264497, 0046265050, 0052139708, 0052140246, 0052140264, 0056978094, 0056978656, 0056978662,0099395893, 0099396075, 0099396271, 0099396579, 0099396754, 0099396964, 0101984301, 0101984455,0103798694, 0103798932, 0103799093, 0103799380, 0104822894, 0104823104, 0104823370, 0104823517,0104823741, 0104824042, 0104824175, 0104824392, 0107549232, 0107549477, 0107549729, 0107549988,0107550205, 0107550415, 0107550576, 0107550849
a Mission elapsed time, in seconds since launch.
hydrazine were consumed by unplanned firing of spacecraftattitude-control thrusters, creating a cloud of reaction productssurrounding the spacecraft. Subsequent images exhibit a dra-matic and stable increase in scattered light sufficient to seriouslydegrade image quality. 7225 long-exposure images of Canopuswere acquired to characterize the PSF, which is used as the in-put to an unprecedented and highly successful effort at planetary
estoration. In this paper we describe only the general at-of the scattered light and how its stability in time was
confirmed. The companion paper by Li et al. (2002) providesthe detailed description of how Canopus images were used tosynthesize the PSF for each filter and of the technical details andresults of the image restoration algorithm.
3. RADIOMETRIC CALIBRATION
For radiometric calibration, the main objective of inflight cal-ibration is to solve for variables in the calibration equation for
E
∼20 DN of leaked light ignored in Eq. (1) but under-removes
232 MURCHI
converting image data numbers (DNs) to physical units of radi-ance, W m−2 µm−1 sr−1. Two forms of the calibration equationwere applied to flight images. Each term in these equations is amatrix or vector of coefficients, and the equation itself assumesstable, predictable instrument behavior as described in detail inpaper 1. All coefficients are given in paper 1.
In the first variant,
Radiancex,y, f,T,t,c ={[DNx,y, f,T,t,c − Darkx,y,t ] − Smearx,y,t − Leakx,y} × 100
Flatx,y, f,c × Coef f × Resp f,T × Atten f,c × Expt.
(1)
Here DNx,y, f,T,t,c is raw DN measured by the pixel in columnx , row y through filter f at exposure time t and CCD temper-ature T with the cover status c open or closed. Darkx,y,t is thedark current modeled for this pixel at exposure time t , derivedfrom short and long dark-field exposures. Smearx,y,t is the scene-dependent frame transfer smear for the pixel at exposure time t .Leakx,y is the scene-dependent but exposure-time-independentleakage of light falling onto the memory zone of the CCD, whichcreates an additive background consisting of a highly attenu-ated (∼10−3) image of the scene imaged onto the memory zone(Fig. 1; see paper 1 for a detailed description). Flatx,y, f,c is the flatfield for filter f with the cover status c open or closed. Coef f
is the coefficient for converting dark-removed, flat-field-, andsmear-corrected DN from filter f to radiance, for a baseline ex-posure time of 100 ms. Resp f,T is the responsivity for this filterat temperature T relative to the baseline, inflight operating tem-perature (−29.6◦C). Atten f,c, if appropriate, is the attenuationof incoming signal by the lens cover in filter f when the coverstatus c is closed. Expt is exposure time in milliseconds between1 and 999 ms. (The constant 100 is the exposure time in millisec-onds for which the calibration coefficient Coef f is applicable.)
In this version of the equation, dark current and frame trans-fer smear are modeled analytically but leaked light is explicitlyignored. Typically, this version of the calibration equation wasused for monochrome mapping where absolute radiometric ac-curacy is not a major concern, or where the image of the asteroiddoes not fall onto the memory zone of the CCD. Where light doesfall onto the memory zone (Fig. 1), a small fraction of the lightleaks though an imperfectly opaque metallic mask, generatinga weak “ghost” of the image on the memory zone. This “leakedlight image” represents a scene located one field- of- view (FOV)away from the intended frame position and is typically attenu-ated by a factor of ∼10−2 to 10−3. For morphologic analysis, theeffects of leaked light are small and acceptable, but the magni-tude of leaked light is unacceptably large for photometry or formeasurement of Eros’s weak color variations.
In the second variant,
Radiancex,y, f,T,t,c
{DNx,y, f,T,t,c − DNx,y, f,T,0,c} × 100
=Flatx,y, f,c × Coef f × Resp f,T × Atten f,c × Expt, (2)
ET AL.
FIG. 1. Origin of the false image and other image artifacts as inferred fromonground and inflight tests. The orientation of the CCD is shown as it “sees” animaged scene. The CCD is divided into two zones, an image zone centered onthe instrument’s optical axis and a memory zone of the same size, coated with ametallic film and lying off-axis. Following an exposure of an object, the imageframe of data is transferred to the memory zone in 0.9 ms. During that time,the object image accumulates a weak “trail” of frame transfer smear behind theprimary image. Over the next 80 ms, the image data remain “stationary” in thememory zone. Readout of the data and conversion from analog to digital signaloccur slowly and at a fixed rate over the next ∼800 ms. During that time theimage data slowly are migrated across the memory zone in the same directionin which frame transfer occurred. Leaked light is superimposed on the intendedimage where highly attenuated light passing through the metallic film on thememory zone creates a “ghost” image of the scene one field of view away,imaged onto the memory zone.
where DNx,y, f,T,t,c is raw DN of an intended image scene andDNx,y, f,T,0,c is an image acquired a few seconds later at an ex-posure time of 0 ms. The 0-ms image contains no real sceneinformation but has the same transfer smear and leaked light asthe primary image. It differs only in (a) the exact position of thescene at the subpixel level and (b) a slightly lesser accumulationof dark current at the shorter exposure time, such that a verysmall amount of dark current remains unremoved. For a typicalEros image exposed to a DN of ∼2000, this approach removes
dark current by <1 DN—a large improvement in calibration
NEAR MULTISPECTRAL IMAGER CALIBRATION 233
FIG. 2. Removal of the false image from a 760-nm frame showing Antarctica, using a 0-ms exposure. (Left) Nominal 2-ms exposure, containing an imageof 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. There is no
image of the scene; instead only the false image and readout smear are present. (Re
the higher contrast than in the left panel, without the flattening effects of the falsaccuracy. Application of this version of the equation requires tworaw images to produce one calibrated, reduced image (Fig. 2),and this is much more resource-intensive. This version of the cal-ibration equation was therefore used for monochrome sequenceshaving as an objective photometric accuracy, or for color se-quences, in either case when the asteroid overfills the FOV.
3.1. Dark Current
The signal in an image of black space is the sum of three majorcomponents: (a) dark current from thermal electrons, (b) a biasof ∼80 DN intentionally added to the output to prevent occur-rence of negative values which would reach ground as zeroes inthe 12-bit unsigned DN words, and (c) low-level periodic noisepicked up by instrument electronics. Odd and even columns haveslightly different biases (by ∼6 DN), an inherent property of thisCCD. Once dark current is subtracted from the data, this differ-ence has no measurable effect on accuracy or signal-to-noiseratio of the image data.
Dark current was monitored inflight by periodic acquisition,through the Eros flyby, of 10- and 999-ms exposure time images
where Ax,y represents bias and Bx,y represents the accumu-lation of thermal electrons over a finite exposure time t (in
FIG. 3. Representative dark-current measurements at 10 and 999 ms in oddis shown using dashed lines. The new model implemented in final data processin
ight) Difference image showing Antarctica with the false image removed. Noteimage. All three images are contrast-enhanced.
of a dark field of space. The 1050-nm filter (which transmitsthe least signal) was used to mitigate the effects of dim stars.Use of both exposure times allows tracking of both the baselevel and the exposure-time-dependent portions of dark current.Between the Eros flyby and July 2000, the deep space fieldsused for regular pointing calibrations (all long exposures) werealso used for dark-current monitoring because these frames arealmost entirely black space. Subsequently, regular dark-currentmonitoring using paired 10- and 999-ms images was resumed.Figure 3 shows a subset of the frame-averaged dark measure-ments acquired through July 2000. What is noticeable is a gen-eral increase in the bias with time since launch (mission elapsedtime, in seconds, or MET).
The dark-current model used throughout the mission con-sisted of a linear equation of the form
Darkx,y,t = Ax,y + Bx,yt, (3)
and even columns (solid symbols). The dark-current model implemented inflightg is shown in unfilled symbols.
E
234 MURCHImilliseconds). The model is independent of CCD temperature.One update was made to the initial model to provide a minimalaccounting for changes in dark current since launch, yieldingsets of coefficients A and B applicable to pre-Eros cruise and toEros operations. The resulting predicted dark currents at 10 msand 999 ms exposures are also shown in Fig. 3.
In general the model accurately predicted dark current towithin ∼1 DN. Although this level of accuracy is adequate forcalibrating typical images of Eros, greater accuracy is requiredto formulate the stellar PSF used as an input to the image restora-tion algorithm discussed in Section 5. This is because the PSFis constructed from multiple coadded star images, and in thedistal part of the PSF unremoved dark current approaches themagnitude of the stellar signal. Of particular concern would bean unremoved dark-current gradient. The large number of darkobservations and their very long baseline now provides the op-portunity to derive and test a refined model that can be appliedfor final end-of-mission data processing. This model was de-veloped in two steps. First, the relationships of frame-averageddark current to MET, exposure time, and CCD temperature werederived to demonstrate the general form of the model. Second,the row- dependence of the relationships was derived, so thatthe final model predicts dark current as a function of columnx , row y, MET, exposure time t (in milliseconds), and CCDtemperature T (in ◦C).
Figure 4 shows, for selected pairs of 10- and 999-ms expo-sures acquired during cruise, the estimated frame-averaged val-ues of the dark-current bias and dark-current accumulation rate(A and B in Eq. (3)) as a function of MET. Over short time pe-
riods, the two exhibit an inverse correlation. Over a long timeperiod tof the coefficients a1, a2, a3, b1, and b2 are given in Table II.through
he dark current bias A shows some relationship to MET. On a frame-averaged basis, dark measurements acquiredFIG. 4. Dark-current biases and accumulation rates calculated from
ET AL.
The dark-current accumulation B rate shows no dependence onMET, but it is expected to increase with CCD temperature due tothe origin of the dark current as thermal electrons kicked loosefrom the Si in the CCD. Figure 5 shows that in fact the lattercorrelation is strong.
Using the temperature dependence of dark-current accumu-lation rate shown in Fig. 5, the bias for any dark image canbe calculated following Eq. (3). Figure 6 shows the resultingbiases as a function of MET, extending through July 2000. Thehigh correlation of bias with MET is presumably due to agingof the CCD due to radiation damage. However, the inverse rela-tionship over short time intervals between dark bias A and darkaccumulation rate B (the latter a strong function of temperature)suggests that bias A may, in fact, have an additional weak rela-tionship with temperature. Figure 7 shows the residuals betweenframe-averaged calculated biases A and those expected from alinear function of MET (Fig. 6). The residuals do exhibit a weakcorrelation with temperature, suggesting lesser bias at higherCCD temperatures.
Combining the relationships between CCD temperature T anddark bias A and dark accumulation rate B, and the relationshipof dark bias A with MET and with temperature, Eq. (3) can berewritten as
Darkx,MET,T,t = a1x + (a2x MET) + (a3x T )
+ (t(b1x + (b2x T ))), (4)
where the subscript x denotes odd or even columns. Definitions
pairs of 10 and 999 ms dark field exposures, as a function of MET.
NEAR MULTISPECTRAL IMAGER CALIBRATION 235
TABLE IIVariables in Dark Current Model for End-of-Mission Data Processing
Even Odd
Parameter Explanation Offset Coefficient Offset Coefficient
a1 Bias at MET = 0, reference CCD temp. 80.336 4.939 × 10−3 84.543 5.467 × 10−3
a2 Change in bias with MET 1.918 × 10−8 1.037 × 10−11 1.736 × 10−8 1.054 × 10−11
a3 Change in bias with CCD temperature −5.272 × 10−2 1.159 × 10−4 −4.406 × 10−2 1.345 × 10−4
b1 Dark accumulation rate per ms, reference 8.071 × 10−3 2.549 × 10−6 8.491 × 10−3 8.571 × 10−7
CCD temp.b2 Change in d.c. accumulation rate 2.355 × 10−4 8.767 × 10−8 2.249 × 10−4 2.942 × 10−8
per ms per 1◦ change in CCD temperature
FIG.
FIG. 5. Dark-current accumulation rates as a function of CCD temperature.
6. Dark-current bias as a function of MET.
FIG. 7. Relationship between CCD temperature and the residual to theMET-based model of dark-current bias shown in Fig. 6.
23
6 MURCHIE ET AL.FIG. 8. Relationship between row position in an image and the dark-current model parameters from Eq. (4).
37
NEAR MULTISPECTRAL IMAGER CALIBRATION 2C
tended source of scattered light provides an opportunity to ob-
FIG. 8—
July 2000 are fit by the relationship for even columns
DarkEVEN,MET,T,t = 78.97 + (2.27 × 10−8 MET) − (0.100 T )
+ (t(5.70 × 10−3 + (1.56 × 10−4 T )))
(residual = ±0.27 DN). (5)
and for odd columns
DarkODD,MET,T,t = 84.41 + (2.14 × 10−8 MET) − (0.050 T )
+ (t(5.70 × 10−3 + (1.56 × 10−4 T )))
(residual = ±0.20 DN). (6)
Figure 3 shows this frame-averaged model in the unfilled sym-bols. It represents a several times more accurate determinationof dark current than is provided by the previous model.
To determine any dependence of the parameters a1, a2, a3, b1,and b2 on the row index y, all dark current measurements takenthrough January 2001 were used. Each row y in the image datawas treated separately, and row-specific values of the parameterswere derived. These are illustrated separately for odd and evencolumns in Fig. 8. These plots show significant row dependencein the initial level of bias at launch and at reference temperature(parameter a1, Figs. 8a and 8b), in the increase of bias with MET(parameter a2, Figs. 8c and 8d), and in the decrease of bias withincreasing CCD temperature (parameter a3, Figs. 8e and 8f ).In even columns only, there is a weak row dependence in thedark-current accumulation rate (parameter b1, Figs. 8g and 8h)and in its sensitivity to CCD temperature (parameter b2, Figs. 8iand 8j). The dark-current model parameters can be expressedas a linear function of row in the image, so that the expressionfor frame-averaged dark current in Eq. (4) can be rewritten inrow-dependent form as
Darkx,y,MET,T,t = (a1off + a1coef y) + ((a2off + a2coef y)MET)
+ ((a3off + a3coef y) T ) + (Expt ((b1off
+ b1coef y) + ((b2off + b2coef y) T ))), (7)
ontinued
where, e.g., a1 is determined using the row value y which in-crements from 1 to 244, an offset a1off, and a coefficient a1coef.There are separate offsets and coefficients for odd and evencolumns; all values are given in Table II.
Once this improved dark-current model was implemented forend-of-mission data processing, the first variant of the calibra-tion equation (1) remained essentially unchanged. However, oneadditional term is added to the second variant of the calibrationequation (in which a zero-exposure image is subtracted), so thatEq. (2) becomes
Radiancex,y, f,T,t,c
= {[DNx,y, f,T,t,c − Darkx,y,MET,T,t ] − [DNx,y, f,T,0,c − Darkx,y,MET,T,0]} × 100
Flatx,y, f,c × Coef f × Resp f,T × Atten f,c × Expt
,
(8)
so that separate dark currents are subtracted from the zero andnonzero exposures.
3.2. Response Uniformity
The pixel-to-pixel variation in CCD response to a uniformextended source, the “flat field,” was measured onground by av-eraging for each filter multiple images of the interior of the large,Spectralon-lined integrating sphere. The averaging reduces shotnoise in the derived flat field to approximately 0.001 of the aver-aged signal. For each flat field, instrument offset and stray lightat the test apparatus were subtracted using secondary images,e.g., with the integrating sphere off, yielding background-freeimages of the integrating sphere interior. The RMS spatial vari-ations in the flat field are typically ±2.2%, due to pixel-scalevariations in responsivity that are highly reproducible betweenfilters. There is no evidence that the flat field varies with fieldangle or quadrant of the CCD. The measured flat-field variationsmust therefore result mostly from small-scale properties of theCCD and not from attributes of the optics.
There are no completely flat targets with which to moni-tor stability of the flat field inflight, but any well-exposed ex-
serve “smooth” fields. One such opportunity occurred during
238 MURCHIE
the “low-Sun” test (27 June 1996), when fields at 40◦–90◦ fromthe Sun (in 10◦ increments) were imaged to assess the impacton optical navigation of scattered sunlight. Two other opportu-nities occurred during the Mathilde flyby (27 June 1997), whenhighly overexposed images were acquired during a search forsmall satellites, and during approach to Eros, when highly over-exposed images were taken for optical navigation to determineEros’s position relative to the star background. Application ofthe flat-field correction to these smoothly varying fields allowstesting of flat-field stability over the dominant scale of flat-fieldnonuniformity of several pixels.
In each case, flat-field stability was evaluated using purelystatistical means. There are no visually obvious degradations.Based on first principles, changes in the flat-field can be de-termined by examining the relationship between noise and DNin partially calibrated images (i.e., images in units of DN butcorrected for dark current, frame transfer smear, and flat-fieldnonuniformity). The total noise can be defined as the deviationin a partially calibrated image of the DN of a single pixel fromits local average and will have the form
√(read2 + periodic2) + shot2 + other2 + � flat, (9)
where read is read noise, shot is the uncertainty in mea-sured photoelectrons from counting uncertainties (shot noise),periodic is periodic noise, � flat is the change in flat field sinceits measurement onground, and other is any other sources ofnoise. The RMS of read and periodic noises was measured byaveraging the uncertainties in a large number of 10-ms darkframes and is ∼1.3 DN or ∼78 e−, assuming 60 e−/DN fromHawkins et al. (1997). Shot noise is simply the square root oftotal photoelectrons, which is calculated by multiplying raw DNcorrected for dark current, frame transfer smear, and flat field by60 e−/DN.
For each smooth field, “noise” was measured as the RMSdifference between each pixel and the local average within a5 × 5 pixel box. This was then fitted to Eq. (9) as a functionof locally averaged DN, neglecting other. The values of � flatwere then plotted as a function of MET to determine if thereis evidence for any systematic degradation of the flat field withtime. The fitted noises are shown in Figs. 9, 10, and 11, andthe upper limits to � flat are plotted as a function of MET inFig. 12.
During early cruise, the nominal change in the flat field(∼0.001) is insignificant, at the level of uncertainty in the flatfield. However, the upper limit to changes in the flat field doesincrease significantly with MET to ∼0.003, about one-eighth ofthe total RMS pixel-to-pixel variation pre-launch.
3.3. Signal-to-Noise Ratio
The signal-to-noise ratio (SNR) of MSI is a useful quan-
tity in limited circumstances but esoteric in general, becausethe major source of uncertainty in scattered-light remediatedET AL.
FIG. 9. Fitted noise in scattered light measured during the low-Sun test of27 June 1996, fitted as a function of DN using Eq. (9). The three fits are theactual (middle), a lower limit assuming no change in flat field, and a worst caseassuming that the flat field had been randomized (±2.2%).
FIG. 10. Fitted noise in scattered light measured during the Mathilde satel-lite search of 27 June 1997, fitted as a function of DN using Eq. (9). The threefits are the actual (middle), a lower limit assuming no change in flat field, and aworst case assuming that the flat field had been randomized (±2.2%).
FIG. 11. Fitted noise in scattered light measured during the final Erosoptical navigation frames exposed for the star background (11 February 2000),fitted as a function of DN using Eq. (9). The three fits are the actual (middle), a
lower limit assuming no change in flat field, and a worst case assuming that theflat field had been randomized (±2.2%).
I
NEAR MULTISPECTRALFIG. 12. Upper limits to the change in flat field as a function of MET.
images (Li et al. 2002) is the accuracy of the scattered-lightcorrection.
From the total noise derived for the three episodes of themission represented in Figs. 9–11, the SNR can be derived as afunction of dark-, smear-, and flat-field-corrected DN. This rela-tionship, shown in Fig. 13, assumes that the very small changesin the flat field are uncorrected. Typical MSI images of Erosare exposed to a DN of ∼1800 for monochrome morphologicmapping; this DN was chosen to limit linear motion smear to<0.5 pixels for the typical scene motion. The resulting peakSNR is ∼190. Multicolor sequences are exposed to a DN of∼2500, yielding an only slightly higher peak SNR of ∼210.
3.4. Radiometric Responsivity
Relative radiometric calibration. Responsivity was mea-sured onground by imaging NIST-traceable integrating spheresof known radiance and determining coefficients for convertingdark-, flat-field-, and smear-corrected DN levels into units ofradiance (Hawkins et al. 1997). For initial use, “version 0” cali-bration coefficients (Coef f in Eqs. (1), (2), and (8)) were derived
from ground-based measurements using the large Spectralon- 700-nm broadband filter, so an update to the broadband filter’s lined sphere (Table III).TABLE IIIMSI Radiometric Calibration Coefficients Derived from Different Sources
Coeff. from version 0 Coeff. from version 1 Coeff. from version 2 Coeff. from version 3 (Mathilde +Filter Eff. λ (nm) (large Integ. sphere) (MSI lunar spectra) (MSI Mathilde spectra) Eros + Canopus spectra)
2 462 160.0 141.5 149.9 163.41 554 572.0 486.2 486.2 530.00 700 — 3923.4 3923.4 4041.13 755 596.0 506.2 464.6 506.45 900 559.0 467.9 429.4 468.04 951 344.0 309.4 291.2 317.46 996 189.0 165.8 154.1 168.0
calibration had to await further measurements.
7 1033 76.5 63.3
MAGER CALIBRATION 239
FIG. 13. MSI signal-to-noise ratio as a function of dark-, smear-, and flat-field-corrected DN at three representative phases of the NEAR mission.
The coefficients have been tested and refined inflight usingimaging of the Moon, Mathilde, and Eros. Paper 1 presented“version 1” calibration coefficients derived from a comparisonof spectra of lunar soil samples with inflight images of the lunarfarside. It was noted, however, that these coefficients made thespectrum of Mathilde appear too blue-sloped. It was suggestedthat the coefficients were in error because the region of theMoon covered included the very red materials inside South Pole–Aitken basin, which are dissimilar spectrally to the Apollo 16soils for which the spectrum was assumed. By forcing the MSIspectrum of Mathilde to match the telescopic spectrum of Binzelet al. (1997) resampled into the MSI bandpasses, version 2 ofthe inflight calibration coefficients (Table II) was derived, andthese were applied to orbital measurements of Eros.
Figure 14 compares the spectrum of Eros measured byMSI during the 23 December 1998 flyby with ground-basedobservations. The MSI spectrum was obtained at a high phaseangle; the match with the highest phase-angle ground-basedspectrum is significantly improved using version 2 of the cali-bration coefficients. The flyby multicolor sequences were takenonly in the seven narrow-band filters and did not include the
58.7 64.0
E
FIG. 16. Comparison of expected radiance of Canopus with measured val-
240 MURCHI
FIG. 14. Comparison of average MSI spectrum of Eros acquired during the23 December 1998 flyby (phase angle range of 82◦–110◦) with ground-basedspectra at low and medium phase angles. The spectrum compares much morefavorably with ground-based observations if Mathilde is used as a calibrationstandard instead of the moon.
During the January–February 2000 approach to Eros, mul-ticolor sequences were taken using all eight filters at phaseangles of 52◦–57◦, a close match to the geometry of the ground-based observations of Vilas and McFadden (1992). To recalibratethe clear filter relative to the remaining seven filters, Vilas andMcFadden’s spectrum was resampled through MSI bandpasses,and the expected reflectance ratio of the broadband filter was cal-culated. The broadband calibration coefficient was then scaledto yield a consistent result (Fig. 15, Table III).
Absolute radiometric calibration. Although the above met-hods provide a precise determination of relative, filter-to-filter
FIG. 15. Comparison of average MSI spectrum of Eros acquired duringthe January–February 2000 approach to Eros (phase angle range of 52◦–57◦),after application of version 2 algorithms and version 3 algorithms (the latter of
which updates calibration of the broadband 700-nm filter).ET AL.
radiometric calibration of MSI, derivation of absolute calibrationof the instrument from flight measurements requires an indepen-dent source of well-known absolute intensity. As described inpaper 1, absolute calibration can be evaluated by comparingcalibrated values of Canopus’s total radiance with values pre-dicted from a model irradiance spectrum. To measure Canopus,a diagonal spacecraft slew at 16 pixels/s was used to spread thestarlight over a representative variety of subpixel elements ofthe CCD, simulating an extended source. For practicality, im-ages were acquired for all spectral filters during a single slewat a constant exposure time (999 ms), with two imaging slewsto assess reproducibility of results. The integrated star signalwas measured inside a box encompassing all scattered light vis-ible above the noise. The total was divided by the pixel fillfactor of 0.567 to correct for the difference between a pointsource and an extended source, and the result was comparedwith the signal predicted based on the brightness of Canopus(V = −0.73 mag) assuming a blackbody spectrum with an ef-fective temperature of 7350 K. In addition, calibrated MSI radi-ance measurements of Earth (23 January 1998) were comparedwith simultaneous near-infrared spectrometer (NIS) measure-ments of the same sources. Both methods suggested an uncer-tainty in absolute calibration to radiance of ∼5% (Murchie et al.1999).
Following the update to relative calibrations of the MSI fil-ters described above, absolute calibration was reevaluated usingCanopus images. As shown in Fig. 16, the filters at 550–1050 nmyield the consistent result that version 2 calibrated radiances are9 ± 3% too high. (The low value for the 450-nm filter is ex-cluded from this determination, because it is thought to resultfrom inaccurate convolution of Canopus’s spectrum through thisrelatively broadband filter.) This result is consistent with com-parison of a variety of simultaneous MSI and NIS observationsof Eros from low orbit, which show that version 2 calibratedMSI radiances are 6 ± 6% higher than radiances measured by
ues calibrated using version 2 coefficients.
NEAR MULTISPECTRAL
NIS. We therefore conclude that the most accurate absoluteradiometric calibration of MSI requires that version 2 calibra-tion coefficients be divided by 1.09. Table III lists the resultingversion 3 coefficients, which incorporate both this adjustmentand the adjustment of the clear-filter calibration described ear-lier. Version 3 coefficients were applied to end-of-mission dataprocessing.
The uncertainty in absolute calibration is estimated as ±5%,based on the RMS difference between measured and predictedradiances of Canopus after application of the version 3 calibra-tion (Fig. 16). In the companion paper on Eros’s color and albedoproperties as determined by MSI, Murchie et al. (2002) show thatthe asteroid’s 950-nm reflectance, corrected to i = 30◦, e = 0◦,is 0.116. This agrees with a value of 0.118 derived indepen-dently from NIS data (Clark et al. 2002), to within the estimateduncertainity in MSI’s absolute calibration.
4. POINTING CALIBRATION
Broadband images of stars in the Praesepe cluster, imaged2 May 1996, provided the first inflight measurement of ori-entation of the center of the FOV (the MSI boresight) in thespacecraft coordinate system. The principal coordinate systemof the spacecraft includes a z axis normal to the solar panels, anorthogonal x ′ axis located along the nominal common boresightof MSI, NLR, XGRS, and NIS, and a y′ axis orthogonal to x ′ andz (Cheng et al. 1997). The orientation of MSI assumed initiallyfor calculation of the focal length and distortion is parallel to thex ′ axis. Attitude information comes from NEAR’s star camera.See paper 1 for a detailed background discussion of this subject.
In the Praesepe images the “true” vector to the star is offsetfrom the calculated vector (as inferred from the star’s pixel coor-dinates in the image, with the imager looking exactly down thespacecraft x ′ axis) by +15.04 ± 0.35 pixels (1442 ± 33 µrad)across the narrow pixel dimension (long image dimension, inthe +z direction in the spacecraft reference frame) and +2.99 ±0.22 pixels (484 ± 36 µrad) across the long pixel dimension(short image dimension, in the −y′ direction in the spacecraftreference frame). Stated differently, instead of the x ′ axis beinglocated exactly at the center of the image (column 268.5, row122.0), it is offset 15.04 ± 0.35 pixels (1442 ± 33 µrad) to theleft (−z) and 2.99 ± 0.22 pixels (484 ± 36 µrad) up (+y′). It islocated at column 253.46, row 119.01. Another way of statingthis is that the vector to an object is oriented down and to theright from the pixel position of that object in the image. Thisoffset of 15 columns, 3 rows compares with 4 columns, 7 rowsdetermined onground, in air at 1 g and ∼25◦C. Inflight, MSIwas pointing slightly more toward the +z axis, consistent withdisappearance of a small amount of flexure of the aft deck bythe instrument masses under terrestrial conditions of 1 g.
This measurement was repeated on starfields acquired atnumerous times, and several hundred microradians of variation
in MSI pointing was recognized. Pointing has been found to bestrongly correlated with temperature of the aft, or instrument,IMAGER CALIBRATION 241
deck. In paper 1, we reported the relationship between decktemperature and row or column offset derived during cruise.At those times deck temperatures were relatively low, and typi-cally the MSI and the X-ray/gamma-ray spectrometer (XGRS)were the only instruments on the aft deck that were powered.
During the daily pointing calibrations conducted throughoutEros approach and orbit, a wider variety of conditions possi-bly affecting pointing were encountered: more instruments werepowered, with either or both the NIS and NEAR laser rangefinder(NLR) on; the deck itself ran warmer; and Eros occupied vary-ing portions of the hemisphere of deep space viewed thermallyby the aft deck during cruise. In addition, the cruise calibrationswere conducted with the spacecraft pointing inertially, whereasduring orbit pointing was commanded to be 90◦ or 180◦ awayfrom Eros. In the latter configuration, the onboard guidance andcontrol system caused temporal oscillations in attitude. Thesehave a typical period of about a minute and variable amplitudeof about 100 µrad that remains coherent over tens of minutes.The pointing calibrations (in groups of four images 2 s apart)sample different parts of these oscillations and perceive them asattitude “noise.”
Figure 17 compares pointing relationships derived separatelyfor different combinations of powered instruments during Erosapproach and orbit. There is significant overlap but all share thecommon features of temperature independence of the pointingoffset in the row direction and increasingly negative pointingoffset from the x ′ axis in the column direction with increasingdeck temperature. Figure 18 shows the results from all point-ing calibrations over the mission, through July 2000.
With about 150 µrad (1 pixel) accuracy, pointing over thecourse of the entire mission may be defined by simple linearequations. In an image reference frame, with the origin at theupper left, the column (sample) number increases in the +zdirection in the spacecraft reference frame. The line (row) num-ber in an image increases in the −y′ direction in the spacecraftreference frame. The offset position in the column or spacecraft
FIG. 17. Offsets and 1-σ uncertainties (dashed lines) in pointing derived
for MSI during Eros operations, for different combinations of instruments onthe aft deck being powered in addition to XGRS.
E
242 MURCHIFIG. 18. Best fit pointing relationship of MSI to aft deck temperature forthe entire NEAR mission (black line) with 1-σ uncertainties (black dashed lines).This relationship encompasses the 1-σ uncertainties in the relationships derivedseparately for thermal conditions during cruse (dashed lines at lower tempera-tures) and Eros orbital operations (dashed lines at higher temperatures).
+z direction is approximated by
column offset (µrad) = −1260 − 45.81 T (±113),
r = 0.83. (10)
The offset position in the line or spacecraft −y′ direction isapproximated by
line offset (µrad) = −744 ± 99. (11)
The stated uncertainties in these fits form an envelope that in-cludes both the relationships determined for orbital operationsand the relationship reported in paper 1 for cruise operations.
5. SCATTERED LIGHT
Evidence of enhanced scattered light was evident in theEros flyby images returned on 23 December 1998, but it was notimmediately evident whether the problem was transient or per-manent. By February 1999 the magnitude of halos around brightstars in optical navigation images made it clear that the problemwas at least long-lived. Therefore a series of measurements ofthe MSI PSF using Canopus was carried out to determine thenature and time stability of the scattered light. Six of eight fil-ters were used successfully on 15 April 1999; the remaining twoplus one repeat filter were used on 13 May 1999. On 3 June 1999four filters were used. On 15 June 1999 and again on 16 July1999 all eight filters were used successfully. These data werecompared with a limited set of previous measurements of Cano-pus acquired during 1996 and 1997. Here we report only ontime stability and the general nature of scattered lift using thesedata. Subsequently, thousands of Canopus images have been ac-quired through all filters to better characterize scattered light. Adetailed set of analyses and development and testing of an image
restoration algorithm are reported by Li et al. (2002).ET AL.
FIG. 19. Comparison of the integrated radiance from Canopus during 1999with earlier measurements. The mean ratio is 1.0 ± 0.1.
The exact placement of a star image on the MSI CCD causesvariations in integrated signal of around ∼13% RMS and vari-ations in peak brightness of a factor of 4, due to the occurrenceof subpixel inactive areas on the CCD. To assure centering ofthe star on one or more pixels in each filter, the sets of imageswere acquired during slow diagonal scans across 2 × 2 pixels inthe course of 16 images. To make valid comparisons of Cano-pus images from different times, the images selected for furtheranalysis are those with the highest signals in the central pixel ofthe star image.
To evaluate whether any signal is being lost, or is just be-ing scattered, the integrated radiance through each filter wasevaluated. Figure 19 shows that the mean radiance ratio of de-graded images to earlier images is 1.0 ± 0.1, indicating thatwithin measurement uncertainties no signal is being absorbedby the contaminants on the optics.
Figures 20 and 21 summarize the wavelength dependence ofthe scattering. The fraction of the total light falling in the central
FIG. 20. Comparison of the fraction of light falling in the central pixel ofa point source image as a function of time and wavelength.
I
NEAR MULTISPECTRALFIG. 21. Same as Fig. 20, except normalized to the pre-anomaly baselinebehavior to show the degradation caused by the contaminants.
pixel for a point source image decreased by 50–90%, depend-ing on the filter. In the longest wavelength filter, up to 20%of the light falls in the central pixel; in the shortest wavelength(450 nm) filter, less than 2% falls in the central pixel. The behav-ior shows no significant change with time over 3 months, from4 to 7 months after the contamination was deposited, indicat-ing that the contaminant(s) is relatively stable against ablation.Fitting the fraction of light falling in the central pixel as a func-tion of wavelength shows that the light scattering is also highlywavelength dependent, becoming progressively worse at shortwavelengths, showing that the contaminants probably occur asextremely fine particulates.
6. SUMMARY
Continued inflight characterization of the performance of MSIhas improved our understanding of imager calibration, pointing,and performance.
(1) Dark current increased since launch by ∼3 DN. It canbe modeled highly accurately as a function of mission elapsedtime, CCD temperature, image exposure time, and location inan image, and it can be removed to an accuracy of 0.2–0.3 DNon a frame-averaged basis.
(2) The flat field is quite stable. There is a small but measurableamount of change in it that has no visual impact on calibratedimages but does degrade the signal-to-noise ratio slightly.
(3) Relative filter-to-filter radiometric calibration is now wellcharacterized and provides good agreement with ground-basedspectra of Eros.
(4) Absolute radiometric calibration has been revised, yield-ing an accuracy of ∼5%.
(5) The instrument boresight in the spacecraft reference frame
shifts with temperature of the instrument deck. Based on know-MAGER CALIBRATION 243
ledge of deck temperature, pointing can be predicted with anaccuracy of ∼1 pixel.
(6) The major change in instrument behavior since launch hasbeen a dramatic increase in scattered light due to contaminantsdeposited on the optics at the time of the NEAR “anomaly” of20 December 1998. The increase in light scattering is stablewith time and highly wavelength dependent, suggesting that itoriginates with extremely small, nonablating particulates.
REFERENCES
Binzel, R., T. Burbine, and S. Bus 1997. Groundbased reconnaisance of Asteroid253 Mathilde: Visible wavelength spectrum and meteorite comparison. Icarus119, 447–449.
Cheng, A., A. Santo, K. Heeres, J. Landshof, R. Farquhar, R. Gold, and S. Lee1997. Near-Earth Asteroid Rendezvous: Mission overview. J. Geophys. Res.102, 23,695–23,708.
Clark, B., P. Helfenstein, J. Bell, C. Peterson, J. Veverka, N. Izenberg, D.Domingue, D. Wellnitz, and L. McFadden 2002. NEAR infrared spectrometerphotometry of Asteroid 433 Eros. Icarus 155, 189–204.
Hawkins, S. E, III, E. H. Darlington, S. Murchie, K. Peacock, T. Harris,C. Hersman, M. Elko, D. Prendergast, B. Ballard, R. Gold, J. Veverka, andM. Robinson 1997. Multispectral imager on the Near Earth AsteroidRendezvous mission. Space Sci. Rev. 82, 31–100.
Li, H., M. S. Robinson, and S. L. Murchie 2002. Preliminary remediation ofscattered light in NEAR MSI images. Icarus 155, 244–252.
Murchie, S., and C. Pieters 1996. Spectral properties and rotational spectralheterogeneity of 433 Eros. J. Geophys. Res. 101, 2201–2214.
Murchie, S., M. Robinson, S. E. Hawkins III, A. Harch, P. Helfenstein, P.Thomas, K. Peacock, W. Owen, G. Heyler, P. Murphy, E. H. Darlington,A. Keeney, R. Gold, B. Clark, N. Izenberg, J. F. Bell III, W. Merline, andJ. Veverka 1999. Inflight calibration of the NEAR multispectral imager. Icarus140, 66–91.
Murchie, S., M. Robinson, B. Clark, H. Li, P. Thomas, J. Joseph, B. Bussey, D.Domingue, J. Veverka, N. Izenberg, and C. Chapman 2002. Color variationson Eros from NEAR multispectral imaging. Icarus 155, 145–168.
Veverka, J., J. F. Bell III, P. Thomas, A. Harch, S. Murchie, S. E. HawkinsIII, J. Warren, E. H. Darlington, K. Peacock, C. Chapman, L. McFadden,M. Malin, and M. Robinson 1997a. An overview of the NEAR multispec-tral imager–near-infrared spectrometer investigation. J. Geophys. Res. 102,23,709–23,727.
Veverka, J., P. Thomas, A. Harch, B. Clark, J. F. Bell III, B. Carcich,J. Joeseph, C. Chapman, W. Merline, M. Robinson, M. Malin, L. McFad-den, S. Murchie, S. E. Hawkins III, R. Farquhar, N. Izenberg, and A. Cheng1997b. NEAR’s flyby of 253 Mathilde: Images of a C asteroid. Science 278,2109–2114.
Veverka, J., and 25 colleagues. 1999. Imaging of Asteroid 433 Eros duringNEAR’s flyby reconnaissance. Science 285, 562–564.
Veverka, J., M. Robinson, P. Thomas, S. Murchie, J. F. Bell III, N. Izenberg,C. Chapman, A. Harch, M. Bell, B. Carcich, A. Cheng, B. Clark, D. Domingue,D. Dunham, R. Farquhar, M. Gaffey, E. Hawkins, J. Joseph, R. Kirk,H. Li, P. Lucey, M. Malin, L. McFadden, W. Merline, J. Miller, W. Owen,C. Peterson, L. Prockter, J. Warren, D. Wellnitz, B. Williams, andD. Yeomans 2001. NEAR at Eros: Initial imaging and spectral results. Science289, 2088–2097.
Vilas, F., and L. McFadden 1992. CCD Reflectance spectra of selected asteroids.Icarus 100, 85–94.