The Status of COS Flat Fields
Tom Ake
TIPS
21 August 2008
Status of COS Flat Fields
• Material covered today
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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
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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
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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
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– Internal data - 210 counts/pixel in 65 Ksec
– External lamps - 8700 counts/pixel in 25 Ksec
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Superflat constructed from all data
– Polynomial fits performed along dispersion for Lflat
– Total counts high enough to yield pixel-to-pixel
variations (P-flat)
– S/N = 95 from photon statitsics
NUV Flat Field Assessment
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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
Penton
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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
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Penton
NUV SMOV4 Calibration
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Since MAMA pixels are mapped to physical anode
wires, expect ground superflat will be valid
– Flat field will be used by CalCOS
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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
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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
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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
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For COS, pulse height is digitized to values
between 0-31
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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
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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)
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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
Penton
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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
FUV Ground Calibration (TV2006)
• 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
FUV Ground Calibration (TV2006)
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Penton
FUV Flat Field Assessment (TV2006)
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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
Penton
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Penton
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
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