## Overview

The UVIS flat fields provide a 2-D spatial map of the detector response for each filter. These include a high spatial frequency component that accounts for variations in the pixel-to-pixel response (P-flat) and a low spatial frequency component (L-flat) that corrects for large-scale modulations across the detector, typically used to correct for differences in the inflight response.

High signal-to-noise P-flats were derived from ground test data, however the optical stimulus and the tilted UVIS focal plane resulted in large internal reflection in the data. This was corrected in the P-flats using a geometric model, and an L-flat correction was derived from dithered photometry of cluster stars in an outer field of Omega-Centauri.

A summary of the UVIS flat field calibration is provided here, with further details in Chapter 5 of the WFC3 Data Handbook. Ground flats for a subset of filters are shown in Figure 1.

## Current Solutions

### February 23, 2016

#### Chip-Dependent Flats

The most recent set of PFLTFILE reference files were delivered in February 2016 with filenames ‘z*pfl.fits ’. These reflect a revised methodology for the UVIS photometric calibration (flats and zeropoints), which treats the two CCD chips as separate detectors. This approach was adopted to better track any changes with time as the two detectors age (ISR 2016-03).

Previously, the flats were normalized to unity in a 100x100 pixel box in UVIS1, and UVIS2 was divided by the same normalization value to preserve the average sensitivity ratio between chips. With the new approach, each chip was normalized separately removing any sensitivity differences computed from dithered star cluster data (ISR 2016-04). For the majority of filters, changes in the L-flat were < 1% compared to the 2011 solutions and were primarily due to correcting the star cluster photometry for charge transfer efficiency (CTE) losses.

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) that multiplies the UVIS2 science array by the chip sensitivity ratio (PHTRATIO). This ratio was computed from the white dwarf standards measured in the corners of UVIS1 and UVIS2 as part of the photometric calibration program, rather than from the dithered star cluster data. (This approach assumes the flat fields have similar accuracy in the detector corners as in the central region of the detector.)

#### UV Flats

Flats for the four bluest UVIS filters (F218W, F225W, F275W, F280N) include an additional correction to the spatial response with temperature. Due to ground equipment limitations, these flats were obtained at a warmer detector temperature (−49°C) during ground testing and have now been corrected with in-flight data (obtained at −82°C). White dwarf standard stars stepped across the two chips showed flux residuals which correlate with a crosshatch pattern in the UV flat fields at spatial scales of ~50 – 100 pixels. These data were used to derive a linear correction model which improves the spatial repeatability from ~7% to ~3% peak-to-peak (ISR 2016-05).

The UVIS chip-dependent calibration is 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 Tests section, count rate offsets of several percent are found in the UV for red versus blue 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.

## Spatial Stability Tests

### December 30, 2015

To check the accuracy of the 2011 flats, CALSPEC white dwarf standards were stepped across the detector to measure the photometric repeatability. For filters with pivot wavelength greater than 300nm, photometry in a 10-pixel aperture was repeatable to better than 0.7% rms and 2.7% peak-to-peak. The flux residuals showed a weak correlation with position (e.g. the number of y-transfers), suggesting that some of the spatial variation was due to CTE losses during readout at a level of 0.5-1.0%. For UV filters, the photometric residuals from stepped photometry were larger, at 1.8% rms and 6.7% peak-to-peak, as a result of these ground flats being acquired at a warmer temperature (ISR 2015-18). For details, see the UVIS Flats section below.

The 2011 in-flight flats corrected for sensitivity offsets between the two chips using dithered stellar photometry, but the average cluster population is significantly redder than the stepped white dwarfs used to derive the zeropoints. When dividing the cluster stars into separate populations by their intrinsic color, differential photometry of sources dithered between the chips showed offsets in the measured count rate that correlated with the source’s intrinsic color. For the UV filters, this offset was as large as 5% in F225W for blue versus red populations, though red leaks limit the accuracy of this prediction at the 1-2% level.

## Inflight Corrections

### December 14, 2011

Flat fields acquired inflight were expected to differ from those obtained during ground testing, since neither the laboratory flat-field illumination nor the on-orbit internal lamp flats provide an accurate simulation of the Optical Telescope Assembly (OTA). Corrections to the spatial response were characterized by observing dense star clusters with multiple telescope orientations and large dithered steps across the detector (ISR 2009-06). By placing the same stars over different portions of the detector and measuring relative changes in brightness, local variations in the response (L-flats) were computed.

Aperture photometry from the dithered cluster data showed flux residuals correlated with a large, wedge-shaped artifact in the ground flats. This feature, dubbed the "flare", is a result of the tilted UVIS focal plane, where light from the CASTLE optical stimulus was reflected multiple times between the detector and the two chamber windows, affecting ~40% of the detector at a level of ~1-2% (ISR 2011-16). A geometric model was used to remove the flare, and the cluster data were recalibrated with the corrected ground flats to compute an inflight L-flat correction (ISR 2013-10). The updated flat field reference files differed by 0.6 to 1.8% rms from the 2009 solutions, with the maximum change ranging from ~2.8 to 5.5%, depending on filter.

PFLTFILE reference files were redelivered to CRDS in December 2011 with filenames v*pfl.fits. Flats for 2 × 2 and 3 × 3 binned modes w*pfl.fits were delivered in August 2012.

## Ground Flats

### April 29, 2009

Flat field images for the UVIS channel were produced in the laboratory by simulating the sky illumination using the CASTLE optical stimulus. These provide a mean signal-to-noise ratio of ~250 and errors of ~0.4% per pixel (ISR 2008-46). In April 2009, flat field reference files were delivered to the Calibration Reference Data System (CRDS) with filenames t*pfl.fits, populated by calwf3 in the image header keyword "PFLTFILE".

LAST UPDATED: 11/30/2022