The flats in use from WFC3’s commissioning in 2009 (t*_pfl.fits) were derived from ground test data (
WFC3 ISR 2008-46
). These flats were found to contain a large internal window reflection affecting ~45% of the detector field of view at a level of 1-2%. A simplified geometric model was used to remove this feature from the ground flats (
WFC3 ISR 2011-16
). Corrections for additional low frequency components, due to differences between the ground and in-flight optical paths, were computed from on-orbit photometry of bright stars in Omega Centauri dithered across the detector (
WFC3 ISR 2013-10
). In-flight solutions (v*_pfl.fits) were delivered on December 14, 2011 for the 42 full-frame filters (excluding the QUAD filters which currently still use the ground flats). Flats for 2 ×
2 and 3 ×
3 binned modes (w*pfl.fits) were delivered on August 29, 2012 (WFC3 TIR 2012-04).
To support the new chip-dependent photometric calibration, a new set of flat fields (z*pfl.fits) were delivered on February 23, 2016 (
WFC3 ISR 2017-07
). These include revised L-flat corrections based on CTE-corrected Omega Centauri photometry, and a new temperature-dependent correction for the UV filters. The photometric repeatability of white dwarf standards stepped across the field of view is now better than 1% r.m.s. and 3% peak-to-peak over the full wavelength range of the detector (
WFC3 ISR 2016-04
WFC3 ISR 2016-05
With a total signal of at least 75,000 electrons, the UVIS flat fields achieve an r.m.s. error better than 0.4% per pixel. The ground flats were normalized by the median value in a 100 ×
100 pixel region in UVIS1 amplifier A, coordinates [1032:1133,328:429]. This region was selected to avoid the small dark rings ("droplets") that are spread across the UVIS field of view. The droplets are likely mineral residue on the outer window of the flight detector caused by a condensation event that occurred before TV3 (
WFC3 ISR 2008-10
). About one-third of these droplets moved in a coherent way during launch (
WFC3 ISR 2009-27
A subsample of ground test UVIS flat fields are shown in
. A complete set may be found in the ‘UVIS CASTLE Photometric Filter Flat-Field Atlas’ (
WFC3 ISR 2008-46
). On-orbit corrections to the flat fields are described in the next two sections.
Initial on-orbit data confirmed that the ground flats did not fully correct low- and mid-frequency structure, and SMOV calibration program 11452 (UVIS Flat-Field Uniformity) showed differences of ~1.5 to 4.5% in a subset of 6 filters based on observations of two galactic globular clusters (see (
WFC3 ISR 2009-19
). Supplemental observations were obtained in Cycles 17 and 18 (programs 11911 & 12339) in a key set of 10 broadband filters used most frequently by observers: F225W, F275W, F336W, F390W, F438W, F555W, F606W, F775W, F814W, F850LP.
The same mathematical algorithm for deriving ACS in-flight corrections (
ACS ISR 2003-10
) was used to characterize the accuracy of the UVIS ground flats. Aperture photometry was performed on the cluster data with a radius of 0.2” (5 pixels) to minimize uncertainties due to crowding. Then, a spatially variable aperture correction to 0.4” (10 pixels) was applied to each image to account for variations in the PSF with detector position and telescope focus, including temporal effects such as breathing and long term focus drifts (
WFC3 ISR 2013-11
). Because of spatial and temporal variability in the UVIS encircled energy, the aperture corrections for radii smaller than 0.4” (10 pixels) are most accurate when computed directly from bright stars in each image.
The first set of L-flat corrections showed evidence of an extended wedge shaped artifact in the ground flats (see
where the wedge is most prominent in the F606W flat). This same feature was noticed in on-orbit flats obtained by observing the dark side of the Earth during periods of full moon illumination (programs 11914 and 12709). This extended feature, dubbed the UVIS 'flare', is a result of the tilted UVIS focal plane, where light is reflected multiple times between the detector and the two chamber windows. When a point source is positioned in the lower right quadrant of the UVIS detector, out of focus reflections between the CCD and the two windows appear along a diagonal from the source towards the upper left (see
). When the detector is illuminated by a uniform surface brightness source, the summation of defocused ellipses creates the wedge shaped ‘flare’ apparent in both the ground flats and the on-orbit Earth flats. The L-flat residuals showed a negative imprint of the flare, implying that it is not a true feature of the L-flat, but instead an internal reflection in the ground flat which is imprinted on the Omega Centauri images during the flat fielding process.
A geometric model (
WFC3 ISR 2011-16)
was used to predict the shape and extent of the flare and the relative strength of the four internal reflections with respect to the primary source (see
). The absolute strength of the flare as a function of wavelength was estimated from the preliminary L-flat solutions, and this was used to scale and divide the model from the ground flats. L-flats were then recomputed from cluster photometry recalibrated with flare-free ground flats. Sensitivity residuals for the 10 broadband filters are shown in
, represented as a chessboard grid of basis functions of order 2n
, where the 4th
order solutions result in a 16 ×
16 and 32 ×
32 pixelated version of the UVIS detector (with each grid pixel representing an independent solution). With the exception of the two bluest UV flats, which were obtained with the detector running warmer than usual during ground testing, the sensitivity residuals show a general wavelength dependence, where the required correction deviates from unity more at longer wavelengths compared to shorter ones.
Rather than carry a separate set of L-flat reference files, revised LP-flats were created by multiplying the ground P-flats (flare removed) with smoothed versions of the pixelated L-flat.
shows the ratio of the improved 2011 pipeline flats with respect to their 2009 solutions for the 10 broadband filters with on-orbit calibration data. The 2011 solutions differ by 0.6 to 1.8% rms, with a maximum change of ~2.8 to 5.5%, depending on filter (
WFC3 ISR 2013-10
The quantum efficiencies of the two UVIS chips differ at wavelengths shorter than about 400 nm (see
Section 5.4.1 of the WFC3 Instrument Handbook
). Motivated by this, a chip-dependent approach to the photometric calibration was adopted in February 2016 to better track any changes with time as the two detectors age. Prior to this date, sensitivity offsets between chips were derived from stars in Omega Centauri dithered between UVIS1 and UVIS2, as described in
, and these were corrected by calwf3
as part of the flat fielding step (FLATCORR). Photometric keywords such as PHOTFLAM, the inverse sensitivity, were then populated in the image header via the PHOTCORR photometry switch, where a single value of PHOTFLAM applied to both UVIS1 and UVIS2.
In the chip-dependent approach, the L-flats were computed from CTE-corrected star cluster data using stars dithered over a single chip only. The LP-flats were then normalized by the median value of each chip. To ensure that count rate photometry in calibrated data remains continuous across the two chips, calwf3
(version 3.3+) makes use of a new calibration switch (FLUXCORR), which multiplies the UVIS2 science extension by the sensitivity ratio (PHTRATIO) of the two chips (see
WFC3 ISR 2014-16
for details). This ratio is empirically derived from photometry of white dwarf standards measured for UVIS1 and UVIS2 separately as part of the UVIS photometric calibration program (
WFC3 ISR 2016-03
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 (
WFC3 ISR 2016-05
shows the ratio of the 2016 to 2011 flats for F275W, where the crosshatch correction is more pronounced for UVIS1.
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
Section 5.4.2, 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. For the most accurate UV photometry, users may wish to ‘back out’ the UVIS2 scaling performed by calwf3
and apply separate zeropoints to each detector (
WFC3 ISR 2017-07
). For targets with multiple populations observed in the UV, corrections to the photometric zeropoints as a function of spectral type may be derived using pysynphot (see