Hubble Space Telescope Primer for Cycle 17

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 1.3) for details.

5.1.1 ACS, COS & STIS

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

The MAMA detectors in the ACS/SBC, COS (NUV) and STIS, and the COS/XDL (FUV) detector can be damaged by excessive levels of illumination and are therefore protected by hardware safety mechanisms. In order to avoid triggering these safety mechanisms, successful proposers using the above detectors are required to check their targets and fields in detail for excessively bright sources by the Phase II deadline. This can be accomplished using tools available within APT. An exception to this is moving target fields which must be cleared after the scheduling windows have been established.

The bright object count-rate limits are mode dependent. Specific values are given in the 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 target and configuration combination exceed the global or local countrate screening limits.

For the SBC, the V magnitude screening limits are quoted in Section 7.2 of the ACS Instrument Handbook. For the STIS MAMAs, these limits are given in Section 7.7 of the STIS Instrument Handbook. For the COS XDL and MAMA, screening limits are given in Section 11.5 of the COS Instrument Handbook. For objects with strong UV fluxes (e.g., early-type stars), the screening limit can be as faint as V=19. Therefore, proposers using any of these health and safety instrument modes must refer to the relevant instrument handbooks 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). Both the targets and other objects in the FOV will have to be cleared during Phase II, but if the field is particularly crowded or if any object in the FOV is known to pose a brightness concern, observers are asked to explain in the Description of Observations section of the Phase I program how they will propose to clear them during Phase II.

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 (see Section 8.15 of the Call for Proposals) and the safety discussion.

In the case of aperiodic variables that are either known to undergo unpredictable outbursts, or belong to classes of objects that are subject to outbursts, the proposer must determine whether the target will violate the bright object limits during outburst. If a violation is possible, the proposer must outline a strategy that will ensure that the target is safe to observe with COS, the ACS/SBC or the STIS/MAMA. The observing strategy might include additional observations, obtained over a timescale appropriate to the particular type of variable object, with either HST or ground-based telescopes. If HST data 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. The general policies described there apply to the STIS/MAMA and COS detectors as well, with suitable scaling for the differences in the exact BOP limits for each detector and mode. These limits are described in the individual instrument handbooks.

5.1.2 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).

5.1.3 NICMOS

There are no safety-related brightness limits for NICMOS. Observers should, however, be aware that saturating the detectors may lead to significant persistent signal in subsequent exposures.

5.1.4 WFC3

There are no safety-related brightness limits for WFC3. Furthermore, over-exposure of UVIS images does not leave persistent signals in subsequent exposures. Heavily saturated IR images do persist, but decay to less than dark current within 15 minutes.