April 29, 2009
During the spring of 2008, flat field images for the UVIS channel were produced in the laboratory by simulating the sky illumination using the CASTLE optical stimulus and providing a mean signal-to-noise ratio of ~250, yielding corrections to a level ~0.4% per pixel (ISR 2008-46). The flat field reference files were delivered to CRDS (Calibration Reference Data System) in April 2009 with filenames ‘t*pfl.fits’ populated in the image header ‘PFLTFILE’ keyword.
Dithered star cluster data acquired during Servicing Mission Observatory Verification (program 11452) revealed that the ground flats did not fully correct for low-frequency variations in the spatial sensitivity, with photometric residuals ranging from 1.5% to 4.5% rms, due differences in the ground-based and inflight optical paths (ISR 2009-19).
December 14, 2011
Additional star cluster observations were obtained in Cycles 17 and 18 (programs 11911 and 12339, respectively), and the photometric residuals with position revealed that a major source of error in the flats was caused by an internal reflection affecting ~40% of the field of view at a level of ~1-2%. Because of the tilted UVIS focal plane, light from the CASTLE stimulus was reflected multiple times between the detector and the two chamber windows creating an extended ‘wedge-shaped’ feature (ISR 2011-16) in the ground flats.
A geometric model of the light reflections was used to remove this feature, and any remaining low-frequency (L-flat) sensitivity residuals were derived using photometry of Omega Centauri. These calibration observations were acquired at a range of roll angles, with large dithered steps across the detector, and were used to quantify magnitude differences between measurements of the same stars at different positions on the detector (ISR 2013-10).
The total correction with respect to the ground flats ranged from 3.6% to 5.6% peak-to-peak, increasing with wavelength from F336W to F814W. Flats for the remaining 35 imaging filters were computed by interpolating the L-flat correction based on the filter pivot wavelength and then multiplying by the filter-dependent ground flat. The updated flat reference files were delivered in December 2011 with filenames ‘v*pfl.fits’. UVIS flats for 2 × 2 and 3 × 3 binned modes ‘w*pfl.fits’ were delivered on August 29, 2012.
Spatial Stability Tests
December 30, 2015
To quantify the accuracy of the inflight flat corrections, observations of bright HST standard stars were stepped across the detector field of view in eight filters to measure the photometric repeatability. For filters with pivot wavelength greater than 300nm, photometry was shown to be consistent to better than 0.7% rms and 2.7% peak-to-peak. The flux residuals showed a weak correlation with the number of y-transfers, indicating that some of the spatial variation was due to CTE losses during readout.
For UV filters (pivot wavelength less than 300nm), the photometric residuals were larger at 1.8% rms and 6.7% peak-to-peak. Flats for these four filters were acquired in ambient conditions during ground testing, and these temperature-dependent residuals were predicted in ISR 2008-46. The stepped UV photometry also showed offsets between the two UVIS chips of up to 5% for the bluest filters. These offsets were attributed to color differences between the blue white dwarf standards and the average color of Omega Centauri, which was used to compute inflight corrections.
Chip-Dependent Flats & Ultraviolet Corrections
February 23, 2016
In early 2016, a new methodology was adopted for UVIS photometric calibration, treating the two chips as separate detectors when computing the flats and zeropoints. This approach was adopted to better track any changes with time as the two detectors age. Prior to this date, the flats were normalized to unity in a 100x100 pixel box in amplifier A, and UVIS2 was divided by the same normalization value to preserve the overall sensitivity difference between chips. With the new approach, each chip was instead normalized by its median value, removing any inherent sensitivity offsets from the flats ISR 2016-04, as determined from the dithered star cluster data. For the majority of filters, changes in the L-flat correction from the 2011 reduction were less than 1% and primarily a result of correcting the star cluster data for CTE losses prior to computing the L-flat. As in 2011, corrections for additional UVIS filters were computed by interpolating the L-flat solutions based on the filter pivot wavelength and then multiplying by the filter-dependent ground flat. Updated ‘PFLTFILE’ flat reference files were delivered in February 2016 with filenames ‘z*pfl.fits’ for both standard and binned modes.
To ensure that count rate photometry in calibrated data remained continuous across the two chips, calwf3 (version 3.3+) was updated with a new calibration switch (FLUXCORR), which multiplies the UVIS2 science extension by the sensitivity ratio (PHTRATIO) of the two chips. This ratio was empirically derived from photometry of white dwarf standards measured in the corners of UVIS1 and UVIS2 as part of the photometric calibration program (ISR 2016-03)
Flats for the four bluest UVIS filters (F218W, F225W, F275W, F280N) included an additional correction to the spatial response with temperature (ISR 2016-05). Due to equipment limitations, these flats were obtained at a warmer detector temperature (−49◦C) during ground testing and have now been corrected with inflight data (obtained at −82◦C). White dwarf standard stars stepped across the two chips showed flux residuals correlated with a crosshatch pattern in the UV flat fields at spatial scales of ~50 – 100 pixels. These data were used to derive a correction model which improved the spatial repeatability from ~7% to ~3% peak-to-peak.
The UVIS chip-dependent calibration was intended to produce uniform count rates across the two chips for sources similar in color to the white dwarf standards. As discussed in the ‘Spatial Stability Test’, count rate offsets of several percent were found in the UV filters for blue versus red stars, but these appear to impact only the photometric zeropoints and not the flat fields themselves. Monochromatic flats obtained during ground testing with an optical stimulus at wavelengths 310, 530, 750, and 1250 nm showed no evidence of spatial residuals in the flat ratio with wavelength for a given chip.