4.6 SBC Bright-Object Limits

STScI is responsible for ensuring that the MAMA detectors are not damaged by over-illumination. Consequently, procedures and rules have been developed to protect the MAMA. We ask all users to share this responsibility by reading and taking note of the information in this section, and designing observing programs that 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 that 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. For more information, see ACS ISRs 1998-03 and 2019-10.

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 observation set will be lost until a new filter is requested.

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 irregularly-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 ground-based monitoring within the previous 24 hours, or by a WFC3/UVIS observation a couple of orbits in advance. In the latter case, the 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.