1-D Flat Fields for COS G130M and G160M Tom Ake TIPS 17 June 2010

Status of COS FUV Flat Fields 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 mapped science region of detector with WD cross-dispersion positions with G130M, and 2 each with G160M and G140L Different cenwaves and FP-POS settings helped 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 1-D correction is somewhat better than 2-D

CALCOS Processing CALCOS was designed to apply a 2-D flat field prior to spectral extraction. A unity flat is currently implemented in the pipeline. 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

Generation of 1-D Flats Spectral iteration of X1D extracted spectra used 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 Each grating processed separately since spectra fall at different cross-dispersion locations

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 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

Spectral Iteration Example - Busy Spectrum

Final 1-D Flat Fields - G130M

Final 1-D Flat Fields - G160M

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 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 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 Flat fielding increases S/N close to Poisson noise for single exposures. With 4 FP- POS 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