1-D Flat Fields for COS G130M and G160M
Tom Ake
TIPS
17 June 2010
Status of COS FUV Flat Fields
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SMOV Program and Results
CALCOS Processing
Generation of 1-D Flats Through Spectral Iteration
1-D Flat Field Evaluation and Achievable S/N
Caveats and Plans
SMOV Program and Results
• SMOV 11491 mapped science region of
detector with WD0320-539
• 5 cross-dispersion positions with G130M,
and 2 each with G160M and G140L
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• Different cenwaves and FP-POS settings
helped separate spectral and detector features
• 2-D flats were made for each grating and
segment
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• Flats removed prominent dips due to grid
wire shadowing, but induced some structure
due to low S/N
• 1-D correction is somewhat better than 2-D
CALCOS Processing
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CALCOS was designed to apply a 2-D flat field prior to spectral extraction. A
unity flat is currently implemented in the pipeline.
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Grid wire shadows are the largest FPN features (20% deep, every 840 pixels)
• When CALCOS coadded different
FP-POS exposures into an X1DSUM
spectrum, features were reduced in
depth, but appeared in more places
• For 4 FP-POS steps, 30% of pixels
were affected by grid wires
• Until we have a flat field, changed
CALCOS SDQFLAG keyword to ignore
grid wires when creating X1DSUM
spectra
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Generation of 1-D Flats
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Spectral iteration of X1D extracted spectra used to create 1-D flats
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Technique had been developed for GHRS
Requires data taken at different grating settings (cenwave and/or FP-POS position)
Iterate between wavelength and pixel space in merging and correcting data sets
Solves simultaneously for the stellar spectrum and underlying fixed pattern noise
Each grating processed separately since spectra fall at different cross-dispersion
locations
PID
11491
11494
11897
11491
11494
11897
Program
SMOV Flat Field
SMOV High S/N
C17 Sensitivity
SMOV Flat Field
SMOV High S/N
C17 Sensitivity
Target
WD0320-539
WD0947+857
WD0947+857
WD0320-539
WD1057+719
WD1057+719
Grating
G130M
G130M
G130M
G160M
G160M
G160M
Cenwave
1291,1309
1309
1291,1309,1327
1600
1600
1577,1589,1600,
1611,1623
FP-POS
1,3
1, 2, 3, 4
3
1, 2, 3, 4
1, 2, 3, 4
3
Spectral Iteration Example - WD
• 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 bothand
dispersion
and cross-dispersion
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• 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
Spectral Iteration Example - Busy Spectrum
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Final 1-D Flat Fields - G130M
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Final 1-D Flat Fields - G160M
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Flat Field Evaluation
Consistency check performed by dividing final 1-D flat into each contributing flat
• Grid wire shadows are nicely corrected
• Detector dead spots leave residuals since spectra were taken at different Y position. These
regions were never expected to be correctable and are flagged by CALCOS
• Long wavelength end of segment A
(X>11000) shows either misalignment
of flats or low S/N effects
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Signal-to-Noise Achieved
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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
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• Maximum S/N for single
grating setting (~14 per pixel)
reached at ~700 counts/pixel
• Current CALCOS
X1DSUM ignoring grid wires
improves global S/N by
smoothing FPN
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Flat fielding increases S/N
close to Poisson noise for
single exposures. With 4 FPPOS steps, S/N=45 per pixel
possible.
• Caveat - using same data to evaluate as what went into the flats, although
different targets, various cenwaves, and multiple FP-POS steps were averaged
Conclusions and Plans
• 1-D flats show promise. Need to check against more data
• Current flats cannot be used to correct old data since the flux
calibration was created without flat fielding
• Need to investigate why long wavelength side of segment A is so
noisy
• G140L still to be studied. Criteria for iteration convergence may
need revision since spectrum covers only part of the detector
segments