Other
Scientific paper
Feb 2012
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012acs..rept....1b&link_type=abstract
Instrument Science Report ACS 2012-01, 28 pages
Other
Hubble Telescope, Hst, Advanced Camera For Surveys, Acs, Stsci
Scientific paper
ISRs I-III in this series define the charge transfer efficiency (CTE) for the CCD detectors, the encircled energy fractions, and the optical throughput degradation. This fourth ISR establishes the absolute flux calibration for the photometry, as corrected with the algorithms specified in I-III. Synthetic photometry from the SEDs of three primary hot 30,000-60,000K WDs define these sensitivity calibrations for the standard HRC and WFC filters. Even though the external uncertainty for the absolute flux of the three primary WD flux standards is ∼1%, the internal consistency of the sensitivities is ∼0.3%. However, when these WD sensitivities are applied to the cooler F, G, and K stars, there are internal inconsistencies of 1-2% for the F775W, F814W, and F850LP filters. For the same cooler stars, the other broadband filters are internally consistent to better than the 1% ACS flux calibration goal after applying the corrections and calibrations specified here. One filter F435W requires a small shift of the long wavelength edge of the bandpass by 18 °A toward longer wavelengths in order to reduce the cool star residuals from as much as 2% to <0.5%; but wavelength shifts cannot remedy the flux discrepancies for the F, G, and K stars in the three long wavelength filters. For HRC, the sensitivities change by 0.5-2% as a monotonic function of wavelength because of improvements in data processing. Additional residual corrections for individual filters are typically <0.5% for wide filters in both cameras. For the medium and narrow filters, these residuals are as large as ∼3% for the WFC F550M and ∼4% for HRC F344N. After implementing these throughput updates, the synthetic predictions of the WFC and HRC count rates for the average of the three primary WD standard stars agree with the observations to 0.1% for every filter.
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