Space Telescope Science Institute   4.6 SBC Bright-Object Limits  4.6.2 Observational Limits

4.6.1 Overview


The SBC detector is subject to catastrophic damage at high global and local count rates, and cannot be used to observe sources which exceed the defined safety limits. The potential detector damage mechanisms include over-extraction of charge from the microchannel plates causing permanent reduction of response, ion feedback from the microchannel plates causing damage to the photocathode, and release of gas which can overpressure the tube.

To safeguard the detector, checks of the global (over the whole detector) and local (per pixel) illumination rates are automatically performed in flight for all SBC exposures. The global illumination rate is monitored continuously; if the global rate approaches the level where the detector can be damaged, the high voltage on the detector is automatically turned off. This event can result in the loss of all observations scheduled to be taken with that detector for the remainder of the calendar (~1 week). The peak local illumination rate is measured over the SBC field at the start of each new exposure. If the local rate approaches the damage level, the SBC filter wheel will be used to block the light, since there is no "shutter". Also, all subsequent SBC exposures in the obset will be lost until a new filter is requested.

Sources that would over-illuminate the SBC detector cannot be observed. It is the responsibility of the observer to avoid specifying observations that exceed the limits described below. Please refer to Section 7.2 for more information.


 4.6 SBC Bright-Object Limits  4.6.2 Observational Limits
Space Telescope Science Institute
http://www.stsci.edu
Voice: (410) 338-1082
help@stsci.edu