COS FUV Flat Fields and Signal-to-Noise Characteristics Tom Ake (STScI/CSC) and the COS STScI and IDT Teams 2010 STScI Calibration Workshop 21 July 2010

Status of COS FUV Flat Fields SMOV Program and Results CALCOS Creation of X1DSUM Spectra Generation of 1-D Flats Through Spectral Iteration 1-D Flat Evaluation and S/N Achieved Conclusions and Plans

SMOV Program and Results Prelaunch analyses indicated an FUV flat field could be made more easily using an external target Program SMOV 11491 mapped science region of detector with WD0320-539 Exposures taken at 5 cross-dispersion positions with G130M, and 2 each with G160M and G140L Different cenwaves and FP-POS settings used to separate spectral and detector features 2-D flats were made for each grating and segment Flats removed prominent dips due to grid wire shadowing, but induced some structure due to low S/N Extracting a 1-D flat and dividing it into the extracted spectrum is somewhat better than flat fielding the image prior to spectral extraction Decided not to install the 2-D flat field files in CALCOS at this time, but to change merging of FP-POS exposures into X1DSUM spectra

CALCOS Creation of X1DSUM Spectra Grid wire shadows are the largest FPN features (20% deep, every 840 pixels) Originally, when CALCOS coadded FP-POS exposures into an X1DSUM spectrum, wire shadows 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, CALCOS SDQFLAG keyword changed to ignore grid wires when creating X1DSUM spectra Cycle 18 GOs are required to use at least two FP-POS or cenwave settings to minimize chance that spectral features fall on bad spots and to smooth out FPN

Generation of 1-D Flats Through Spectral Iteration Since 1-D correction appeared to be better than 2-D, investigated spectral iteration of X1D extracted spectra to create 1-D flats 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 (FPN) Started analysis of G130M and G160M exposures to see if FPN residuals could be made into general 1-D flat fields Each grating still has to be processed separately since spectra fall at different cross-dispersion locations

Spectral Iteration Results Example

Final 1-D Flat Fields - G130M

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

Signal-to-Noise Achieved Maximum S/N for single grating setting (~20 per resel) reached at ~1500 counts/resel (red line) Current CALCOS FP-POS summing while ignoring grid wires improves global S/N by smoothing FPN (violet line) S/N from flat fielding approaches Poisson noise for single exposures (open blue circles). With 4 FP-POS steps, S/N=100 per resel possible (blue line) 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 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 New flats cannot be used to correct data with the current flux calibration since sensitivity curves were created without flat fielding. A self-consistent flux calibration is being worked 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 More high S/N observations are being obtained to characterize the flats at other cenwave settings