10.5 Policies and Procedures

Bright Object Protection and target variability

All COS exposures, whether target-acquisition, spectroscopic, or imaging, must be checked for bright objects. Any COS targets or fields that cannot be demonstrated to be safe to a reasonable level of certainty in the judgment of the Contact Scientist (CS) will not be observed. In that case, it is possible that equivalent alternative targets may be approved upon request. Any observations that trigger the onboard safety mechanisms will not be rescheduled.

COS GOs must enter the ETC calculation number for each discrete exposure into the appropriate field of the APT when preparing their Phase II material. A GO must  also report any unsafe or unknown stars from APT/BOT for each field, and either show that the observations are safe or document any unresolved issues. (An exception is moving-target fields, which cannot be cleared until the scheduling windows have been established.) It is not expected that all such issues will be resolved by the Phase II deadline, but they should at least be identified and have planned resolutions by then.

Light from a bright nearby source could scatter into the PSA. For example, a target that is safe for the BOA may scatter enough light into the PSA to violate our screening limits. The region of concern is an annulus extending 5 to 15 arcsec from the center of the PSA. Any field object falling in this annulus may not produce a global count rate in excess of 1 × 105 count s-1 per segment for the FUV channel or 2 × 105 count s-1 for the NUV, or a local count rate over 3.3 count s-1 pixel-1 in the FUV or 250 count s-1 pixel-1 in the NUV. At present, the APT/BOT does not search for such objects, so they must be checked by hand. In such cases, count rates must be estimated using the ETC as though the source were at the center of the PSA.

In worst cases, new ground-based data or HST CCD UV exposures may be required to clear the fields for BOP; in general, the latter must be covered by the existing Phase I time allocation.

For unsafe targets, one solution is to change to a less sensitive instrument configuration: one could use the BOA, MIRRORB, or both (though the BOA is Available-but-Unsupported for science, see Section 5.13), a higher-resolution grating, or a less-sensitive wavelength setting. Note that the medium-resolution gratings actually have higher throughput than G140L when the data are rebinned, but are subject to brighter limits.

Use of ORIENT to reduce risk from nearby objects

For unsafe field objects that threaten to fall into the non-target aperture, an orientation restriction (ORIENT) may be used to constrain the spacecraft roll angle and thus the position of the non-target aperture, but such constraints will limit the scheduling of the observation and need to be requested under "Special Requirements" in the Phase I proposal document.

GOs planning COS observations of unpredictably variable targets should be aware of the special BOP procedures in effect for such cases. Observations of flare stars are allowed with COS (and STIS) only if the Contact Scientist is convinced that the targets would not violate BOP limits even in their brightest states. COS ISR 2017-01 provides specific guidance for evaluating the suitability of M dwarfs for observation.

"STOP" procedure for highly variable targets

Targets that are known to vary on short timescales of days or more, such as cataclysmic variables, may require the Safe Target Offset Procedure (STOP). This procedure is described in detail in ACS ISR 2006-04, and applies to all HST detectors subject to BOP; here we summarize it. In STOP visits, the proposers are required to provide photometric monitoring for at least seven consecutive nights leading up to the HST observation, with the latest observation happening within 24 hours of the target being observed by HST. The photometric monitoring may be done from the ground using optical bands, or may be coordinated with other space observatories. If the need to coordinate the photometric monitoring with other observatories imposes scheduling constraints, these constraints must be mentioned and justified in the Phase I proposal as a special requirement. 

The Contact Scientist will make the final decision to observe the target if the photometric monitoring indicates that the target is stable and there is no danger to the health of the COS detectors. STOP visits are initially set up by STScI staff with a small offset that points the telescope at blank sky. If the Contact Scientist clears the visit to proceed, a command will be sent to remove the offset, and the observation will be executed normally. If the observation is not cleared, the observation will still be executed pointing at blank sky, since the offset will not be removed, and no light from the science target will enter the COS apertures.

It is expected that targets for which the STOP procedure is applied will usually be safe to observe. The intent of the procedure is to catch rare exceptions. If photometric monitoring indicates that the target is not safe to observe, the STOP observation will not be rescheduled, and the orbits for that observation will be charged against the program. Users who wish to observe targets that have a significant chance of being in an unsafe state should consider proposing those targets as Targets of Opportunity.

STScI reserves the right to limit the number of visits requiring quiescence observations within 20 days or less of an HST observation to no more than 12 such visits per Cycle, including all HST programs.

A pointing or configuration change after the targets and fields have been cleared by the STScI BOP review must be approved by the COS Team on the basis of a specific scientific justification and a new BOP review by the GO, which may be submitted via the CS.

Observers are expected to follow policies pertaining to values of BUFFER-TIME when proposals require a STOP procedure. For targets of variable brightness, the BUFFER-TIME should correspond to the maximum safe flux scenario, rather than the more likely scenario of a quiescent observation.