4.6 SBC Bright-Object Limits

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STScI has responsibility to ensure that the MAMA detectors are not damaged through over-illumination. Consequently, we have developed procedures and rules to protect the MAMA. We ask all potential users to share in this responsibility by reading and taking note of the information in this section, and designing observing programs which operate in the safe regime for these detectors. The safety of all proposed SBC targets and fields must be discussed in the Phase I proposal, so that their feasibility can be assessed by the TAC and STScI.

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 and address this issue in your Phase I proposal.

4.6.2 Observational Limits

To ensure the safety of the SBC detector and the robustness of the observing timeline, we have established observational limits on the incident count rates. Observations which exceed the allowed limits will not be scheduled. The allowed limits are given in Table 7.3, which includes separate limits for nonvariable and irregularly-variable sources. The limits for irregular variable sources are a factor 2.5 more conservative than for sources with predictable fluxes. Predictable variables are treated as nonvariable for this purpose. Examples of sources whose variability is predictable are Cepheids or eclipsing binaries. Irregularly variable sources are, for instance, cataclysmic variables or AGN.

SBC observations of targets subject to large, unpredictable outbursts must be preceded by groundbased monitoring within the previous 24 hours, or by an HRC NUV observation a couple of orbits in advance. In the latter case, the HRC observation must be included in the Phase I orbit request.

For further information about checking for SBC bright object limits while planning your observations, please refer to Section 7.2.