FGS2r: Deadtime, Background, Photon Aging, Sensitivity Calibrations, and Noise Properties

Details FGS2r: Deadtime, Background, Photon Aging, Sensitivity Calibrations, and Noise Properties FGS2r: Deadtime, Background, Photon Aging, Sensitivity Calibrations, and Noise Properties

Apr 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009tel..rept....1g&link_type=abstract

Instrument Science Report TEL 2009-01, 24 pages

Physics

Hubble Space Telescope, Hst, Space Telescope Science Institute, Fgs2R

Scientific paper

Execution of a large science program using FGS2r on a very bright target for which accurate knowledge of deadtime corrections was needed has led to this and closely related calibrations. FGS2r had not previously been used for science; and, thus, accurate calibrations had not been needed in the past. Using calibration observations obtained in support of the large science program we will derive accurate values for the deadtime corrections for each of the four PMTs that support relative correction accuracies of -0.7% to +0.6% at the count level of the science target motivating this study. Typical background levels resulting from detector noise and sky are a necessary input for the deadtime calibration. A determination of sensitivity (count rate as function of V ) is readily available and quoted. The large science program counted some 1.0 × 10^12 photons over 10 days; those observations, and separate before and after observations of the FGS standard Upgren 69 quantify the small resulting sensitivity loss. Deadtime corrections across the 4 PMTs in FGS2r differ by nearly 50%, suggesting that the single canonical value usually assumed does not suffice for accurate work at high count rates on any of the FGSs without similar on-orbit calibrations.

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