The flat fields for the WFC combine information from two sources. Ground-based flats were obtained for all filters at a signal-to-noise of ~300 per pixel. To refine the low-frequency domain of the ground flats, inflight observations of a rich stellar field with large scale dithers have been analyzed (see ACS ISR 2002-08
and 2003-10
). The required L-flat correction is a corner-to-corner gradient of 10 to 18%, dependent on wavelength. The resulting flat field supports photometry to ~1% over the full WFC field of view.
Figure 4.2 shows the corrected WFC ground flats for several broadband filters. Note: the 50 pixel gap seen in external images, between the top and bottom chips, is not shown here. Since the two CCDs were cut from the same Si wafer, and have undergone similar treatments, there is a significant continuity in the response across the gap.The central donut-like structure is wavelength dependent, where pixels in the central region are less sensitive than surrounding pixels in the blue F435W filter, for example, and more sensitive in the red F850LP filter. For further discussion of WFC flat fields, see ACS ISRs 2001-11
, 2002-04, 2003-10, 2003-11, 2005-02
,
and 2005-09
.
As for the WFC, the HRC ground flats were refined using in-flight observations of a rich stellar field with large scale dithers to determine the low-frequency domain of the flat fields. The correction required for the visible filters is a corner-to-corner gradient of 6% to 12%, dependent on wavelength. For the NUV filters, flats were taken in-flight using observations of the bright earth (see ACS ISR 2003-02
) and include both the pixel-to-pixel and low-frequency structure of the detector response.
Currently, HRC flat fields have a signal-to-noise of ~300 per pixel and support photometry to ~1% over the full HRC field of view. Figure 4.3 shows the corrected HRC ground flats, derived for 6 broadband optical filters. The donut-like structure seen in the WFC response is not found in the HRC flats. For further discussion of HRC flat fields, see ACS ISRs 2001-11
and 2002-04
.
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