5.4 Flat Fields
5.4.1 Ground Flats (P-flats)
In early 2001, flat field images for the ACS were produced in the laboratory using the Refractive Aberrated Simulator/Hubble Opto- Mechanical Simulator (RAS/HOMS). The RAS/HOMS is an HST simulator capable of delivering OTA-like, external, monochromatic point-source and broad-band, full field illumination above its refractive cutoff wavelength of ~3500 Å. Because the RAS/HOMS optics are opaque below 3500 Å, flats for the UV filters F330W and F344N (see ACS ISR 03-02) were created using inflight observations of the bright earth. (See Section 5.4.4 for more information on the ACS Earth flat program.)
The resulting flat fields include both the low frequency (L-flat) and the high frequency pixel-to-pixel (P-flat) structure. A total signal of about 100,000 electrons per pixel is required for each flat field to avoid degrading the intrinsic pixel-to-pixel rms response of <1% for the ACS CCD detectors. The flats are normalized by dividing by the average number of counts in the central 1% of the frame. In the case of full WFC frames, the chip 2 images are divided by the chip 1 central value, in order to preserve the overall sensitivity difference between the two CCD chips across the ~50 pixel gap that separates the two independent pieces of the WFC detector. For small filters that cover just part of one quadrant of the WFC (i.e., F892N, polarizers), flats were masked to unity below 90% of the central value of the data, so that no flat field correction is done on the scattered light outside the physical edge of the filter.
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 do not have the same total counts after the flat fielding step. Thus, point source photometry extracted from a flat fielded image must be multiplied by the effective pixel area map (see Section 6.1.3). This correction is accounted for in pipeline processing by MultiDrizzle (see Chapter 4), which uses the geometric distortion solution to correct all pixels to equal areas. In the drizzled images, photometry is correct for both point and extended sources.
More information on the HRC and WFC ground flats may be obtained from ACS ISR 01-11 for the standard filters, polarizers, and coronagraph, from ACS ISR 02-01 for the ramp filters, and from ACS ISR 02-04 for the prism and grism. Ground flats for the SBC detector are described in ACS ISR 99-02.
5.4.2 Inflight L-flat Correction
The large scale uniformity of the WFC and HRC detector response, as provided by the ACS laboratory flats, has been improved inflight by using multiple dithered pointings of stars in 47 Tucanae. By placing the same stars over different portions of the detector and measuring relative changes in brightness, low frequency spatial variations in the response of each detector have been measured. Photometric errors of up to +/-9% corner-to-corner have been found in the original WFC and HRC laboratory flat fields (see ACS ISR 02-08). The derived L-flats are based on a 4th-order polynomial fit and are shown in Figure 5.11 and Figure 5.12 for WFC and HRC, where white indicates that the photometry produced using the laboratory flats is too faint with respect to the true stellar magnitude, and black indicates that the photometry is too bright. There is a continuous gradient in the L-flat correction along the diagonal of the detector which corresponds to the axis of maximum geometric distortion.
L-flats were determined from inflight observations using filters F435W, F555W, F606W, F775W, F814W, and F850LP for both the WFC and HRC. The HRC study included two additional filters: F475W and F625W. The L-flat correction for the remaining filters was derived by using linear interpolation as a function of wavelength. The pivot wavelength of each filter was used for the interpolation, where the resulting L-flat is equal to the weighted average of the L-flat for the two filters nearest in wavelength. For a detailed discussion of ACS L-flat corrections, refer to ACS ISR 02-08. For a discussion of the mathematical algorithm used to derive the L-flats, refer to ACS ISR 03-10.
To verify the interpolated L-flat corrections, additional observations of the same stellar field have been acquired and are undergoing analysis. The L-flat calibration program revisits the same target several times per year; therefore, observations are obtained for a range of orientations due to the roll of the telescope. Differential photometry of stars falling on different portions of the detector as the telescope rolls will provide a verification of the L-flats for the interpolated filters and will also provide an independent test of the absolute sensitivity dependence with time for the full suite of ACS filters. Initial testing indicates that the photometric response for a given star is now the same to ~1% for any position in the field of view for filters which were observed during the inflight L-flat campaign. For the interpolated L-flats, the expected accuracy is ~2-3%. Once the analysis for the additional stellar observations is complete, new flat fields will be delivered to the pipeline and the errors are expected to be reduced to <1%.
Figure 5.11: WFC low frequency (L-flat) flat-field corrections required for the laboratory data. While the pixel-to-pixel (P-flat) structure of the laboratory flats is robust, a low frequency correction is required to achieve uniform detector response. This correction ranges from +/- 5% for the F555W filter to +/- 9% for the F850LP filter.
Figure 5.12: HRC low frequency (L-flat) flat-field corrections required for the laboratory flats. This correction ranges from +/- 3% for the F555W filter to +/- 6% for the F850LP filter.
5.4.3 Pipeline Flats
The inferred low-frequency (L-flat) corrections were applied to the laboratory flats, and the resulting flat fields have been in use in the calibration pipeline. Observers are encouraged to check the ACS web site for the most recent flat fields available for recalibration at http://www.stsci.edu/hst/acs/analysis/reference_files/flatimage_list.html
Figure 5.13 shows the corrected WFC laboratory flats for several broadband filters. Note that on the sky a gap of ~50 pixels exists between the top and bottom halves that is not shown here. The central donut-like structure is due to variations in chip thickness (see ACS ISR 03-06) and is dependent on wavelength. Pixels in the central region, for example, are less sensitive than surrounding pixels in the blue F435W filter and more sensitive in the red F850LP filter.
The HRC corrected laboratory flats for the same broadband filters are shown in Figure 5.14. The donut-like structure seen in the WFC response is not found in the HRC flats. The small dark rings are shadows of dust on the CCD window (see Section 5.5.1). The large dust mote seen in the WFC F606W flat is due to dust on the F606W filter. That portion of the filter is not "seen" by the HRC detector.
The accuracy of pipeline flats can be verified using a variety of complementary methods. In Section 5.4.2, we discussed how follow-up observations of the same stellar field can be used to verify the derived L-flat corrections. Alternately, observations of the bright earth can provide a uniform flat field source for the complete OTA optical complement and incorporate both the low frequency L-flat and the high frequency pixel-to-pixel P-flat response. Earth flats are described in Section 5.4.4. For most filters, the flats agree to within ~1%, except for the interpolated L-flat filters which usually agree to within ~2%.
The third method for verifying the ACS pipeline flats is with sky flats. These can be made by filtering and summing many observations of a sparse field. Sky flats have been created for a few of the most frequently used broadband WFC filters and are discussed in detail in Section 5.4.5. The sky flats are generally similar to the corrected ground flats at the 1% level, in accordance with the results of the previous two methods. While the WFC can show residuals in the central donut-region which are as large as 2%, these are most likely due to differences in the color of the spectrum of the sky from that of the bright globular cluster stars used for the L-flat determination.
To summarize, the pipeline flats were created by correcting the laboratory flats by low-frequency corrections derived from dithered stellar observations. For the most used modes, the flats are accurate to better than 1% across the detector. For a few filters, improved L-flats will be derived from additional stellar observations to improve the flat field errors to <1% for all modes. Additional verification of the pipeline flats will be provided by both earth flats and sky flats, thus confirming the derived L-flats and setting limits to their dependence on the color of the source spectrum.
Figure 5.13: WFC Flat Fields
Figure 5.14: HRC Flat Fields
5.4.4 Earth Flat Verification
Because the RAS/HOMS optics are opaque below 3500 Å (see Section 5.4.1), flat fields for the UV filters were created using inflight observations of the bright Earth. The Earth is a poor flat field source at optical wavelengths because structure in the cloud cover can cause streaking in the flat. However, HRC modes that utilize the F220W, F250W, F330W, and F344N are immune to streaks because of the large optical depth down to the tropospheric cloud layers. The bright Earth then provides a uniform flat field source for the complete OTA+HRC optical complement. The required calibration flat, which incorporates both the low frequency L-flat and the high frequency pixel-to-pixel P-flat response, can be easily produced from these observations. Unfortunately, the red leaks in F220W and F250W are so large that the out of band light dominates, and the lab flats made with the deuterium lamp illumination (see ACS ISR 01-11 for details) are superior to the observed earth flats for the modes that include these two filters. Because no L-flat correction has been applied, errors in the flats of +/- 2 to 4%corner-to-corner are expected for these two filters.
The HRC earth flats at wavelengths longward of 4000 Å often show streaking and non-uniformities from clouds or the terminator. However, a number of the images are free from these defects and provide a promising alternative to sky flats as well as an independent verification of the stellar L-flats. A comparison of the current pipeline flats to the new HRC earth flats shows differences of typically less than ~1% for the broadband filters and ~2% for the interpolated L-flat filters. An ISR describing these results is in preparation and will be posted on the ACS site:
http://www.stsci.edu/hst/acs/documents/isrs.
At the shortest exposure time of 0.5 sec, most WFC modes saturate, except for a few narrow band filters (i.e., F502N, F658N, F660N, and F892N) which are only saturated approximately half the time. Earth flats in these narrow band filters would nicely compliment the stellar L-flat program and sky flat results, which use only broadband filters. More observations in the WFC are required to produce good enough earth flats for comparison with the pipeline flats.
5.4.5 Sky Flats
The ACS sky flat program takes advantage of the many survey programs that have executed on ACS, such as the GOODS program, to build high signal-to-noise flats. These non-proprietary data consist mostly of sparsely populated images with relatively uniform sky that can be stacked to further quantify the pixel-to-pixel variation of the instrumental response.
The GOODS program consists of either 15 or 16 unique pointings over 5 epochs for both the Chandra Deep Field South (CDFS) and the Hubble Deep Field North (HDFN) using the most commonly used F606W, F775W and F850LP filters in the WFC. In total between 50-70 images were stacked in each filter.
The sky flats were created by median combining the pipeline reduced FLT files after removing cosmic rays and masking all of the sources. Because the GOODS data contained 2-4 dithers at each pointing, depending on the filter, masking was necessary to eliminate the high values at each pixel.In addition before combining, each image was corrected for the pedestal bias signature and inspected for scattered light.
The combined sky flat in each filter was box medianed down to a 32x32 image, in order to directly compare it to the corrected ground flat. Since the sky flats were created starting from the pipeline calibrated FLT files, it will show any flat field signatures that were not accounted for by the pipeline.The resulting sky flats show variations across the field of view of <2% for each of the 3 filters. The CDFS and HDFN fields were analyzed separately and compared. The two fields showed excellent agreement (<1%) for all 3 filters. The inferred sky flats will be made available on the ACS web site and a future ISR will describe the sky flats in more detail. Please check the ACS web site for updates on this issue.
