HRC and WFC Flat Fields: Ramp Filters

Computer Science

Scientific paper

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Hubble Space Telescope, Hst, Space Telescope Science Institute, Advanced Camera For Surveys, Acs, Stsci

Scientific paper

The ACS has three ramp filters in each of five filter wheel-2 slots. The five middle segments are available to the HRC, while all 15 ramps cover portions of the WFC field of view. Therefore, each pipeline WFC ramp pixel-to-pixel P-flat is composed of three segments, where each segment is from the broadband LP-flat that is nearest to the central wavelengths of each ramp segment. Since the P-flats vary slowly with wavelength, the surrogate P-flats for the ramp filters generally reduce the intrinsic CCD rms noise from 1% to ~0.2%. Along the ramps, the transmitted wavelengths are a function of position; and the large-scale relative response depends on the shape of the continuum flux distribution. Instead of an L-flat measured by a laboratory lamp, ramp filters will have an absolute sensitivity as a function of wavelength defined for pipeline flux calibration. Across the ramps, i.e. perpendicular to the direction of changing wavelength, a traditional L-flat could be applied; however, any variation in the total throughput integrated over the bandpass along this axis causes this L-flat to differ for the cases of monochromatic and continuum illumination.

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