5.1 Bright-Object Constraints

Some of the SIs must be protected against over-illumination; observations that violate these protections cannot be executed, and should not be proposed. We emphasize that the constraints discussed below are safety constraints; data become affected by bright objects at substantially fainter limits than the safety limits discussed in the following sections. Bright-object related effects include non-linearity, saturation and residual-image effects. Please consult the HST Instrument Handbooks (see Section 1.2 ) for details.

5.1.1 NICMOS & WFPC2

There are no safety-related brightness limits for NICMOS and WFPC2.

5.1.2 ACS & STIS

The CCDs on ACS and STIS have no safety-related brightness limits.

The MAMA detectors on ACS/SBC and STIS can be damaged by excessive levels of illumination and are therefore protected by hardware safety mechanisms. In order to avoid triggering these safety mechanisms, STScI will screen all proposals to enforce absolute limits on the brightest targets that can be observed by the MAMAs. Observers must provide accurate information to assist in this screening process.

The MAMA bright object count-rate limits are mode dependent. Specific values are given in the ACS and STIS Instrument Handbooks, including example magnitude screening limits for astronomical objects observed in the most commonly used modes. In addition, the Exposure Time Calculators (ETCs) accessible from the HST Instruments Web page can be used to determine if a particular instrument/target combination exceeds the screening limit.

5.1.3 FGS

Objects as bright as V=3.0 may be observed if the 5-magnitude neutral-density filter (F5ND) is used. Observations on all objects brighter than V=8.0 should be performed with this filter. A hardware limitation prevents the FGS target acquisition from succeeding for any target brighter than V=8.0 (3.0 with F5ND).