2.6 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 of 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.

In 2019, the zeropoints for the SBC were updated to correct a long-standing ~30% discrepancy in the absolute flux calibration of the imaging modes.  The error was found to be caused by inaccuracies in the filter and detector throughput tables used to derive the SBC zeropoints. The discrepancy is such that the SBC is actually ~30% more sensitive than previously estimated: a source of a given astronomical flux produces a ~30% larger SBC count-rate. Conversely, prior conversions of observed SBC count-rate to flux have overestimated the astronomical flux by ~30%. The throughput tables have now been corrected, and new zeropoints have been derived for the relevant imaging modes. Details are provided in ACS ISR 2019-05.