2.7 SBC Considerations

It is essential that observers proposing to use the SBC review and abide by the procedures and rules described in Section 4.6 and Section 7.2. Designing safe observing programs will protect the MAMA detectors in the SBC from being permanently damaged by over-illumination. We therefore ask all users to share the responsibility with STScI to ensure the safety of the MAMA detectors. 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.

The SBC experiences elevated DARK rates upon reaching a temperature of 25.5°C, which takes approximately two hours after the SBC is turned on. Because of this issue, there is a typical limit on the length of some SBC observations. A special SBC-LODARK aperture is available on a section of the detector which does not show elevated DARK levels after long periods, and is recommended for small targets. See Section 4.5 and ACS-ISR 2024-04 for further details. 

In ACS ISR 2025-04, it was shown that the SBC backgrounds in the ACS ETC have been historically overestimated by average factors of 4–160, for filters at wavelengths ≤ 1400 Å. As of ETC v34.1, users may specify SBC background percentiles determined through empirical analysis.