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WFC3 Data Handbook > Chapter 5: WFC3-UVIS Sources of Error > 5.4 Flat Fields

5.4
The UVIS flat fields provide a map of the total instrument response and represent the combined throughput of various components along the optical path (see WFC3 ISR 2013-10). These LP-flats include both 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. Revised flat fields for the complete set of 42 full-frame filters were computed from on-orbit data and delivered on December 14, 2011 for use in the WFC3 calibration pipeline.
The flat fields in use prior to this date were derived from ground test data, and these were found to contain a set of four internal window reflections affecting ~45% of the detector field of view. A simplified geometric model was used to remove this artifact from the ground flats, and the resulting flats map the pixel-to-pixel response of the detector. Additional low-frequency residual corrections, caused by differences in the ground and in-flight optical paths, were computed from on-orbit data using relative photometry of bright stars in Omega Centauri measured at multiple positions across the detector. The new LP-flat reference files typically improve the accuracy of point source photometry by 0.6 to 1.8% (rms), with maximum differences of ~3 to 6% depending on filter.
5.4.1 Ground Flats
During spring 2008, flat-field images for the UVIS channel were produced at the Goddard Space Flight Center during the third and final thermal vacuum campaign (TV3) using the CASTLE Optical Stimulus. CASTLE is an HST simulator capable of delivering OTA-like, external, monochromatic point source and broadband full field illumination. Flats were acquired only in the standard CCD readout configuration of four amplifiers (ABCD), gain=1.5 and binning=1x1. Flat fields with the OS Xenon and Halogen lamps were taken with the detector at its nominal operating temperature of -82C. A subset of ultraviolet (UV) flat fields was also acquired at a warmer temperature (-49C) using the deuterium lamp to achieve higher count rates.
A total signal of about 75,000 electrons per pixel was required for each flat field to avoid degrading the intrinsic pixel-to-pixel rms response of <0.4%. The UVIS flat fields were normalized to unity in a 100x100 pixel box 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 (see WFC3 ISR 2008-10). About 1/3 of these droplets moved in a coherent way during launch (see WFC3 ISR 2009-27).
For the full-frame UVIS filters, the chip 2 flat fields were divided by the chip 1 normalization value in order to preserve the overall sensitivity difference between chips. Unlike the ACS/WFC detector, the two UVIS chips were not cut from the same silicon wafer and as such are two independent detectors. Because sensitivity differences are accounted for during flat fielding, users need only apply a single zeropoint value when performing photometry for both chips using calibrated flat fielded (flt.fits) or drizzled (drz.fits) images. For the QUAD filters, each quadrant was normalized to unity in a 100 x 100 pixel box of that quadrant.
Because of geometric distortion effects, the area of the sky seen by a given pixel is not constant; therefore, observations of a constant surface brightness object will have count rates per pixel that vary over the detector, even if every pixel has the same sensitivity. In order to produce images that appear uniform for uniform illumination, the observed flat fields include the effect of the variable pixel area across the field. A consequence of dividing by these flat fields is that two stars of equal brightness falling on different portions of the detector would not have the same total counts in a calibrated *_flt.fits image. To correct for this effect, point source photometry extracted from flat-fielded images may be multiplied by the effective pixel area map (see Section 9.2.3). Alternatively, this correction is accounted for in the pipeline by AstroDrizzle (see Section 4.2), where the geometric distortion solution is used to correct all pixels to equal area on the sky in the *_drz.fits data products.
A subsample of ground test UVIS flat fields are shown in Figure 5.3. A complete set is 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 section
Figure 5.3: A subset of flat fields obtained during ground testing.
Dark regions correspond to lower response pixels in the LP-flats. Notable features include a crosshatch pattern at bluer wavelengths due to structure in the detection-layer, a large diffuse spot in chip 2 for wavelengths greater than ~600nm due to variations in the detector thickness, vertical striping in long pass filters which see through the detector to glue adhesive on the other side of the CCD, and fringing in narrow-band filters redder than ~600nm.
5.4.2 On-orbit Corrections
Due to differences in the HST optical path, flat fields acquired in flight were expected to differ from those obtained during ground testing. Neither the laboratory flat-field illumination nor the on-orbit internal lamp flats provide an accurate simulation of the OTA. For this reason, low-frequency residuals in the spatial response (L-flats) were characterized by observing dense star clusters with multiple telescope orientations and large dithered steps across the detector. By placing the same stars over different portions of the detector and measuring relative changes in brightness, local variations in response may be computed. Initial on-orbit data confirmed that the ground test flats did not fully remove low- and mid-frequency structures.
The results of SMOV calibration program 11452 (UVIS Flat Field Uniformity) showed differences of ~1.5 to 4.5% in a subset of 6 filters using observations of two galactic globular clusters (see WFC3 ISR 2009-19). Additional cluster observations were obtained in Cycles 17 and 18 (programs 11911 & 12339, *UVIS L-flats*), 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 developed for computing the ACS L-flats (ACS ISR 2003-10) was used to characterize the accuracy of the UVIS flat fields. Aperture photometry was performed on the cluster data with a radius of 0.2 arcsec (5 pixels) to minimize uncertainties due to crowding. A spatially variable aperture correction to 0.4 arcsec (10 pixels) was then applied to the photometry for each image to account for variations in the PSF with detector position and telescope focus, including both breathing and long term focus trends (see WFC3 ISR 2013-11). Because of strong variability in the UVIS encircled energy, users are advised to apply local aperture corrections when computing aperture photometry with radii smaller than 0.4 arcsec (10 pixels).
A preliminary set of L-flat sensitivity residuals showed evidence of an extended wedge-shaped artifact in the ground flats. This same feature was noticed in on-orbit Earth 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 Section 5.5.1). 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 as well as the relative strength of the four reflections with respect to the primary source (see Figure 5.4). The absolute strength of the flare as a function of wavelength was computed from the preliminary L-flat solutions, and this was used to divide the flare model from the ground flats. L-flats were then recomputed from cluster photometry that was flat fielded by the flare-free ground flats, and the sensitivity residuals derived for the 10 broadband filters are shown in Figure 5.5. The solutions are represented as a chessboard grid of basis functions of order 2n where the 4th and 5th order solutions result in a 16x16 and 32x32 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 under ambient conditions during ground testing, the residuals show a general wavelength dependence, where the required correction deviates from unity more at longer wavelengths compared to shorter ones.
Figure 5.4: Geometric model of the UVIS flare, a set of 4 internal reflections for a source, which uniformly illuminates the detector.
Figure 5.5: Low-frequency corrections to the ground LP-flats (flare removed).
Blue indicates that the flare-corrected ground flats overcorrect the data, making the photometry too faint. The solutions are presented as a 32x32 grid for most broadband filters, such that one grid pixel corresponds to 128x128 detector pixels. The exceptions are F225W, F275W, and F850LP for which 16x16 grid solutions were used, where one grid pixel corresponds to 256x256 detector pixels.
5.4.3 Pipeline Flats
Rather than carrying a full set of separate L-flat reference files, which would need to be combined with the ground flats (P-flats) during the flat fielding step in calwf3 the new reference files are revised LP-flats. These were created by multiplying smoothed versions of the pixelated L-flat solutions by the existing ground flats (corrected for the flare). The ratio of the revised pipeline flats with respect to the original ground flats is presented in Figure 5.6 for the 10 broadband filters sampled by the flat field calibration program. The new reference files are different from the ground flats by 0.6 to 1.8% rms, with a maximum change of ~2.8 to 5.5%, depending on filter (see WFC3 ISR 2013-10).
For the remaining 32 UVIS filters, the residual flat-field correction (including contributions from the flare and the stellar L-flat) was computed via interpolation based on the filter pivot wavelength, where the resulting correction is equal to the weighted average of the residual flat for the two filters nearest in wavelength. The general wavelength dependence of both the flare correction and the L-flat solutions suggests that interpolation is a valid method for generating these reference files.
The photometric response for any given star using the revised flat fields is now accurate to 1% over the detector for most UVIS filters. Flat fields for the 4 bluest UV filters were obtained warm (-49C versus -82C) during ground testing, and these show 2-3% residuals in a crosshatch pattern across the detector. Several calibration programs are underway to model and correct this residual flat-field structure. For sources with SEDs significantly different than the average Omega Centauri population, color terms of up to a few percent may be present.
Figure 5.6: Ratio of the revised pipeline flats to the original ground flats.
Blue indicates that the ground flats overcorrect the data, making sources too faint. Note that the residuals include contributions from both the flare and the stellar L-flat.

WFC3 Data Handbook > Chapter 5: WFC3-UVIS Sources of Error > 5.4 Flat Fields

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