5.1 Bright-Object Constraints

5.1.1 ACS, COS & STIS

5.1.2 FGS

5.1.3 WFC3

Some scientific instruments must be protected against over-illumination. Observations that violate these protections cannot be executed and should not be proposed. The constraints discussed below are safety constraints.

Data may also be affected by bright objects at limits that are substantially fainter than the safety limits discussed in the following sections. Such bright object-related effects include non-linearity, saturation, and residual image effects. Please consult the relevant HST Instrument Handbooks (see Section 1.3) for details.

5.1.1 ACS, COS & STIS

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

The MAMA detectors in the ACS/SBC, COS (NUV), and STIS, as well as 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 their safety mechanisms, successful proposers using these detectors are required to perform a detailed check of the field surrounding their targets for excessively bright sources (this must be done by the Phase II deadline). Bright object checks can be done using tools in APT. Exceptions to Phase II bright object checks are moving targets; these must be cleared after scheduling windows have been established.

Bright object countrate 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 instrument configuration combination exceeds the global or local countrate screening limits.

Objects with strong UV fluxes (e.g., early-type stars) can have a screening limit 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 propose to clear them during Phase II.

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 Chapter 9 of the COS Instrument Handbook. For the SBC, the V magnitude screening limits are quoted in Section 7.2 of the ACS Instrument Handbook. 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 irregular 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, STIS/MAMA or ACS/SBC. 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). STScI reserves the right to limit the number of visits requiring quiescence verification observations within 20 days or less of an HST observation to no more than 12 such visits per Cycle. 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 Bright Object Protection (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 WFC3

There are no safety-related brightness limits for WFC3. Furthermore, over-exposure of UVIS images does not leave persistent signals in subsequent exposures. Signals in IR images that are greater than approximately half full-well do persist and have significant implications for design of observing strategies and data analysis. See the WFC3 Instrument Handbook for more information.