the status of cos flat fields

The Status of COS Flat Fields

Tom AkeTIPS

21 August 2008

Status of COS Flat Fields

• Material covered today– COS Description and Signal-to-Noise Requirements– NUV Ground Calibration and SMOV4 Plans– FUV Detector Characteristics – FUV Ground Calibration and SMOV4 Plans

• Most of the results are from COS IDT analyses– Thermal Vac 2003 at Ball Aerospace– Thermal Vac 2006 at GSFC

COS Detector Overview

• COS comprised of two spectrograph channels with two different types of micro channel plate (MCP) detectors

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COS S/N Requirements

• Designed to obtain S/N = 30 per resel routinely, 100 per resel with special effort

• S/N requirements achieved by various techniques– Flat fielding to divide out small scale fixed pattern noise– FP-POS grating steps to average out small scale fixed pattern noise– Pulse height amplitude (PHA) screening (FUV time-tag only)

• As a performance metric, the S/N ratio is taken to be the reciprocal of the RMS scatter around a smooth fit to the data

Example of Detector Artifacts

• Features correctable by flat fielding– Hex pattern– Moiré pattern– Divots/blemishes– Grid wire shadows (on

FUV only)

• Uncorrectable features – Dead spots– Hot spots

• Spectrum location not known until SMOV4 alignment

Flat Field Calibration

• Internal calibration system consists of two deuterium lamps illuminating a flat field calibration aperture (FCA)

– Light takes nearly the same optical path as an external target– Only the science areas of the detectors are illuminated, not the wavelength

calibration region – FCA (X=1750 µm, Y= 750 µm) is larger than the PSA (700 µm diameter) – Aperture mechanism moves in both dispersion and cross-dispersion directions

• External flat field calibration exposures were taken through the PSA during thermal vacuum tests in 2003 and 2006

– Preserved internal lamp– Allowed characterization of illumination angle dependence between PSA and

FCA

COS Optical Layout

NUV Ground Calibration



• G185M grating used since D2 lamp throughput has peak continuum flux there

• Internal and external exposures taken at various FCA Y positions to paint science area

– Internal data - 210 counts/pixel in 65 Ksec

– External lamps - 8700 counts/pixel in 25 Ksec

• Superflat constructed from all data– Polynomial fits performed along dispersion for L-

flat

– Total counts high enough to yield pixel-to-pixel variations (P-flat)

– S/N = 95 from photon statitsics



NUV Flat Field Assessment

• Distribution of P-flat variations give maximum S/N without a flat field

– Histogram of variations in each NUV stripe fit with Gaussian profile

– Widths indicate S/N (= 1/) ~ 50 per resel can be obtained without a flat

• NUV high quality test spectrum obtained using external D2 lamp through O2 absorption cell

– Extracted spectrum with slit 25 pix high in cross-dispersion direction

– Compare run of S/N with count level– S/N ~ 100 realized with FP-POS and flat fielding



Penton

Penton

NUV SMOV4 Calibration

• Since MAMA pixels are mapped to physical anode wires, expect ground superflat will be valid

– Flat field will be used by CalCOS

• Baseline plan is to replicate TV2003 internal lamp exposures (60 Ksec with G185M grating)

– Only nominal Y location of FCA

– Mapping in Y performed by using different gratings

• On-orbit data will achieve S/N ~ 15 per pixel (~ 45 per resel)

• New P-flat will be compare to ground flat to verify it has not changed

– If there is a difference, another set of exposures taken in SMOV4

– If necessary, more taken during Cycle 17 calibration

FUV Detector Characteristics

• The FUV XDL detector is inherently different from the NUV MAMA– Photon locations are defined by the difference in time it takes for

the MCP charge cloud pulse to reach the ends of the delay lines

– Positions needs to be thermally corrected using stim pulses and geometrically corrected to equalize pixel area

– COS team prefers the term “detector element” to “pixel” since the pixels are not physical

– In time-tag mode, PHA data provide measure of charge cloud produced by the photon, which is a function of the gain distribution at each pixel

FUV Detector Gain

• Detector gain can be important for flat fields– Photon position errors can arise from pulse shape

differences at different PHAs– Detector structures can be different– Noise reduction by eliminating low and high

PHAs, which are not likely real photons

• For COS, pulse height is digitized to values between 0-31

• Detector gain is not the same as sensitivity– Since detector is photon counting rather than

charge integrating, intensity of charge cloud unimportant as long as event can be discerned above the noise



FUV Ground Calibration (TV2003)

• The two segments of the FUV are treated separately– More spectral features in D2 occur in FUV region– To avoid D2 structure, G130M was used for FUVA, G160M for FUVB.

This doubled the amount of exposure time needed.

• 95 internal D2 lamp exposures were taken at all central wavelengths and FP-POS positions– 19 Ksec per segment– Yielded median counts of 276 (FUVA) and 296 (FUV B) per pixel– S/N ~ 18 per pixel (~130 per resel )

• External lamp data not useful due to low lamp throughput, so no supplemental data used to create a ground flat

FUV Flat Field Assessment (TV2003)

• As with NUV, L-flats and P-flats constructed for each segment

– S/N ~ 5/6 per detector element

– Maximum S/N without a flat field estimated to be ~20 per resel

• FUV high quality spectrum test with CO absorption cell and Kr lamp

– S/N >30 obtained with FP-POS technique

– TV2003 flat did not improve data beyond what could be achieved with FP-POS

– Plateau in S/N reached at ~ 2000 counts

Penton

Penton


• In TV2006, the calibration system delivered more light from the external D2 source than the TV2003 set up– External source produced continuum only > 1600 Å, so long wavelength

portion of segment A analyzed – Series of tests conducted to investigate S/N characteristics from a 1-D flat

field

• Twenty high S/N exposures acquired to simulate a point source in the PSA– G160M grating used at 5 central wavelengths, 4 FP-POS settings each– Divided data into two sets of 10 exposures and used one set to flatten the

other




Penton

FUV Flat Field Assessment (TV2006)

• 1-D flat fields created and one group used to flatten the other

– Data co-added in detector space and normalized by linear fit

– Data aligned in wavelength space for S/N evaluation with and without flat fielding

• S/N close to photon statistics achieved with flat fielding and FP-POS merging

– Photon limited result factors in quality of the flat (estimated to be 3%)

– No plateau in the S/N distribution, so maximum S/N achievable is higher

– External flat should be obtained on-orbit since illumination appears to be important

Penton

Penton

FUV SMOV4 Calibration

• No on-orbit D2 lamp exposures planned for SMOV4

• FASTEX standard observed in PSA with all gratings– Each grating has slightly different Y location (G140L top, G130M

middle, G160M bottom of science area

– Use Y POS-TARG steps in PSA to ±1.2” with 0.6” spacing to increase area covered

– Spectra shifted to four locations in X by combination of central wavelength and FP-POS selections

FUV SMOV4 Calibration

• Chose high declination DA white dwarf , WD0320-539 (V=14.9)– Provides count rate near time-tag limit (~ 20,000 counts/sec) so we can

maximize counts and still obtain PHA data– Exposure times chosen to achieve S/N = 35-45 per resel– Will take 11 orbits

• Data will be combined to create 2-D flat field using an iterative technique – Methodology first used with GHRS and described in STIS ISR 98-16

(Gilliland)– Iterate between wavelength and pixel space in merging and correcting

data sets– Solve simultaneously for the stellar spectrum and underlying fixed pattern

noise

COS Flat Field Summary

• NUV flat field will be in good shape after SMOV4– Enough counts obtained in TV to create P-flat at pixel level– Expect on-orbit flat field to be consistent with or correctable to ground

flat– CalCOS set to perform flat field correction as default

• FUV flat field requires more work– Investigating P-flat at resel level or 2-D S-flat – Uncertainty about flat field changes with detector aging – CalCOS currently set NOT to perform flat field correction, but can be

turned on with a switch– Best technique for improving S/N is through an FP-POS strategy

the status of cos flat fields

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