Instrument Performance

Detector Background

The detector background arises primarily from thermal electrons at the first photocathode and high energy particles. In the 600~km altitude, 28xfb inclination orbit of HST, substantial fluxes of magnetospheric electrons and protons are encountered in the South Atlantic Anomaly (SAA). The more energetic of these particles are capable of generating intense flashes of C^erenkov radiation in the MgF2 faceplate of the FOC intensifiers. Since this noise source originates as photons at the very front end of the detector, the Video Processing Unit of the FOC is not able to distinguish between real celestial photons striking the cathode and C^erenkov generated photons.

The threshold energy for C^erenkov radiation in MgF2 is E>220~keV for electrons and E>400~MeV for protons. Shielding of 4 mm aluminum or more was built into the design of the FOC in order to prevent electrons of energies E < 3-5~MeV from reaching the detectors from any direction.

The effects of the SAA on the FOC were extensively mapped during the commissioning phase. The FOC turned out to be considerably less sensitive to SAA electrons than had been feared. This is presumably due to the additional shielding to electrons provided by the rest of the HST spacecraft. The response of the FOC to SAA protons on the other hand is in good qualitative agreement with the expectations-although the sensitivity of the two FOC detectors differs somewhat.

The highest background rates (0.2~counts pixel-1 s-1 in the F/48 during nominal operations and 0.02 counts pixel-1 s-1 in the F/96) are encountered over South America within the peak of the SAA proton density distribution. Since these rates are not high enough to cause damage to the FOC detectors, the FOC is kept fully operational during SAA passages. However, such high background rates do exclude useful scientific observations. A ground-track contour delineating the observed region of high background has been installed within the HST ground system in order that FOC observations not be scheduled within it. Users of the FOC need therefore not concern themselves with avoiding the SAA under normal circumstances (i.e., periods not having unusually high solar activity). The typical detector background rates experienced well outside the SAA are 7 10-4 counts pixel-1 s-1 in the detector for the F/96 relay and 10-2 counts pixel-1 s-1 in the detector for the F/48 relay (see also "Current F/48 Performance" on page 91 for current F/48 performance). Upward fluctuations of a factor xbb 3 from these minimum values are, however, seen throughout the HST orbit. The minimum in-orbit background rates are, respectively, factors of xbb 5 and xbb 3 higher than the background rates measured during ground testing implying that the bulk of detector background counts are particle induced.

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