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

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

The MAMA detector in the ACS/SBC can be damaged by excessive levels of illumination and is 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 SBC. Observers are responsible for proposing safe observations and must provide accurate information to support the STScI screening process.

The MAMA count-rate limits are given in the ACS Instrument Handbook, which also includes sample magnitude screening limits for astronomical objects observed in the various SBC modes. In addition, the Exposure Time Calculators (ETCs), accessible from the ACS Software Tools Web page, can be used to determine if a particular target and configuration exceed the global or local countrate screening limit of the SBC.

The V magnitude screening limits are quoted in Section 7.2 of the ACS Instrument Handbook. For objects with strong UV fluxes (e.g., early-type stars), the screening limit can be as faint as V=16-19. Therefore, ACS/SBC proposers must refer to the ACS Instrument Handbook for instructions on performing a detailed analysis for their specific sources, and discuss the results in the Description of the Observations section of the Phase I proposal (see Section 9.2 of the Call for Proposals), for both the targets and other objects in the FOV. Note that for SBC prism spectroscopy, a direct image must be added manually to provide the wavelength calibration, and it will drive the safety issue since the direct filters are more sensitive than the prisms. This image must be included in the Observing Summary and the safety discussion.

For ACS/SBC observations of aperiodic variable objects with known properties, the maximum flux values must be applied for the bright object protection (BOP) screening. For objects either known to be subject to unpredictable outbursts on a timescale less than 100 years that would exceed the screening limit of the specified configuration, or belonging to a class of such objects, either external or HST data confirming quiescence within about 24 hours of the SBC observation must be provided by the proposers, or the SBC observation will be cancelled. The choice between external and HST confirmatory data is at the discretion of the proposers. If HST data, generally ACS/HRC, are to be used for this purpose, the required orbits must be requested in Phase I (see Section 4.1.3 of the Call for Proposals). Further details about these procedures are presented in ACS ISR 06-04.

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 of 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).