7.2 SBC Bright Object Protection

7.2.1 Bright Object Violation: SBC Exposures

High global and local count rates can cause catastrophic damage to the SBC detector. Therefore, targets should be checked to verify that their fluxes do not exceed the defined SBC safety limits. As a first step, you can check your source V magnitude and peak flux against the bright object screening magnitudes in Table 7.4 for your chosen observing configuration (ACS ISR 2019-10). In many cases, your source properties will be much fainter than these limits, and you need not worry further.

However, the magnitudes in this table are hard screening limits that correspond to the count rate limits in Table 7.3 and the output of the ETC. In earlier editions of this Handbook, these magnitudes were made fainter by arbitrary 1 or 2 magnitude pads, depending on the spectral type range, but these have now been removed. If your target is near these limits (within 2 magnitudes or a factor of 6.3 of the flux limits), then you need to carefully consider whether your source will be observable in the chosen configuration. Remember that the limits in these tables assume zero extinction. Thus, you will want to correct the limits appropriately for your source reddening due to interstellar dust.

Table 7.3: Absolute SBC count rate screening limits for nonvariable and variable objects.

^a The limits for irregularly variable sources are a factor 2.5 more conservative than for sources with predictable fluxes. Predictable variables are treated as nonvariable for this purpose. Examples of sources for which variability is predictable are Cepheids or eclipsing binaries. Irregularly variable sources are, for instance, cataclysmic variables or AGN.
^b While this is the health limit, the detector becomes non-linear at ~22 counts/second/pixel.
^c Applies to the brightest state of the target.

You can use the information presented in Section 9.2 to calculate peak and global count rates. You can also use the ETC to calculate the expected count rate from your source. The ETC has a host of template stellar spectra. If you have a spectrum of your source (e.g., from IUE, FOS, GHRS, or STIS) you can also use it directly in the calculator. As implied by the footnotes to Table 7.4, the model spectra in the ETC cannot be used for bright object checking at the solar spectral type and later; the UV spectra of such stars are dominated by emission lines and continua not reproduced by the models. For these types, more realistic theoretical or observational input spectra (e.g., from the IUE or HST archives) must be used (ACS ISR 2019-10). The ETC will evaluate the global and per pixel count rates, and will warn you if your exposure exceeds the absolute bright object limits.

7.2.2 Policy and Observers' Responsibility in Phase I and Phase II 

It is your responsibility to ensure that you have checked your planned observations against the brightness limits prior to proposing for Phase I. To conduct this check, use the bright object tool (BOT) in APT. The APT training page contains detailed instructions on running the BOT. If your proposal is accepted and we (or you) subsequently determine in Phase II that your source violates the absolute limits, then you will either have to change the configuration or target, if allowed, or lose the granted observing time. We request that you address the safety of your SBC targets by means of the ACS ETC, and if needed, consult with an ACS Instrument Scientist via the Help Desk. For SBC target-of-opportunity proposals, please include in your Phase I proposal an explanation of how you will ensure that your target can be safely observed.

In Phase II, proposers of SBC observations are required to check their targets and fields in detail for excessively bright sources by the Phase II deadline. The relevant policies and procedures are described in more detail in the Phase II Proposal Instructions.

The BOT conducts detailed field checking prior to SBC program implementation. It is based on automated analysis of the fields by means of data from the second Guide Star Catalogue (GSC2) and displays of the Digital Sky Survey (DSS). GSC2 provides two magnitudes (photographic J and F), hence one color, for most fields down to about 22nd magnitude. Combined with conservative spectral-type vs. color relationships, this color supports determinations of safety or otherwise for individual objects. In the best cases, these procedures allow expeditious safety clearing, but in some cases the GSC2 is inadequate because of crowding or absence of one of the filters, for instance. Then supplementary information must be provided by the proposers to support the bright object protection (BOP) process. The target should always be checked directly in the ETC with the more detailed information generally available for it, rather than relying on its field report data.

Subsequently, automated GALEX screening has been added as a selectable option in the BOT. GALEX All-sky Imaging Survey (AIS) sources are screened as unreddened O5 stars and reported as either safe or unsafe. This is a powerful tool because it is based directly on UV fluxes; e.g., previously unknown hot companions to late-type stars will be revealed.

The target should still be checked by entering the appropriate GALEX magnitude into the ETC. Unsafe objects require further investigation; the GALEX fluxes are upper limits in crowded regions because of the relatively low spatial resolution, or the source may clear with more specific parameter information.

Table 7.4: Bright limit V-band magnitudes for observations with the SBC filters and prisms (no reddening) (ACS ISR 2019-10).

^a O5V through F8V values, not including named individual stars, are based on Castelli & Kurucz models.
^b The magnitudes listed for these stars are generated from combining IUE data and model spectra for the stellar types and renormalizing the combined spectrum (using pysynphot) to the magnitude that corresponds to 50 counts/sec in the brightest pixel. The F122M magnitudes are not listed because the Ly-α line in the IUE spectra may be contaminated by Earth's geocoronal emission. The F115LP filter also contains the Ly-α line in its passband, and therefore the F115LP magnitudes for these stars are more uncertain than in the other filters.
^c System made of a late-type star with an O5 companion contributing 20% to the total light in the V band. In the UV, the O5 component dominates and sets the same magnitude limit for companion types A to M.

STScI will check all targets and fields before any SBC observations are cleared. However, it is STScI's policy that GOs must provide screened, safe targets for SBC programs, and supplementary data as needed to verify target and field safety. The APT/BOT, including an Aladin interface, makes the BOP procedures accessible for GO use. Extensive help files and training movies are available on the HST Training Materials webpage.

All SBC proposers must conduct BOP reviews of their targets and fields in conjunction with their Phase II preparations. With this information, they will become aware of any problems earlier, such as the need for supplementary data, which may otherwise entail lengthy implementation delays following the Phase II deadline. (An exception is moving target fields, which must be cleared after the scheduling windows have been established.) To assist with these procedures, a Contact Scientist (CS) who is an SBC/BOP specialist will be assigned to each SBC program to interact with the GO during the Phase II preparations and through program execution.

To briefly summarize the BOP process, a field of 70 arcseconds in diameter must be cleared for a single default SBC pointing with an unconstrained orientation. The APT/BOT automatically reports on all GSC2 stars or GALEX sources within that field. If any displacements from the default pointing (e.g., POS TARGs, patterns, or mosaics) are specified, the field to be cleared increases commensurately. POS TARG vectors and the enlarged, rotated field circles are conveniently displayed in APT/Aladin. No unsafe or unknown star may lie within 5 arcseconds of the detector edge at any orientation. Conversely, POS TARGs and orientation restrictions may be introduced to avoid bright objects in the fields. In case a single guide star implementation becomes necessary, the field to be cleared increases to 140 arcseconds in diameter, but usually that will not become known until scheduling is attempted after the Phase II deadline.

By the Phase II deadline, a SBC GO must send their CS ETC calculations for each discrete target and reports on any unsafe or unknown stars from APT/BOT for each field, either showing that the observations are safe or documenting any unresolved issues. In the latter case, including inadequacy of BOT/GSC2 to clear the observations, other photometric or spectroscopic data sources must be sought by the GO to clear the fields. Many of these are available directly through the APT/Aladin interface (although automatic BOP calculations are available only with GSC2 and GALEX), including the Mikulski Archive for Space Telescopes (MAST), which contains IUE and GALEX data in addition to HST data. An existing UV spectrum of the target or class may be imported directly into the ETC. IUE data must be low resolution and large aperture for BOP. None of the provided models are adequate for stars later than the Sun because they lack chromospheric emission lines; actual UV data must be used for them. As shown in Table 7.4, some F stars also do not match expectations from the models, and therefore, checking actual UV data for those stars is highly recommended as well (ACS ISR 2019-10). 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.

If a given star has only a V magnitude, it must be treated as an unreddened O5 star. If one color is available, it may be processed as a reddened O5 (which will always have a lower UV flux than an unreddened star of the same color). If two colors are available, then the actual spectral type and reddening can be estimated separately. The APT/BOT now automatically clears stars with only a single GSC2 magnitude, if they are safe on the unreddened O5 assumption. Any other "unknowns" must be cleared explicitly.

In some cases, the 2MASS JHK might be the only photometry available for an otherwise "unknown" star. It is possible to estimate V and E(B–V) from those data on the assumption of a reddened O5 star, and thus determine its count rates in the ETC. Martins & Plez 2006, A&A, 457, 637 derive (J–H)₀ = –0.11 for all O stars and (V–J)₀ = –0.67, (V–H)₀ = –0.79 for early O types. The K band should be avoided for BOP because of various instrumental and astrophysical complications. In Bessell & Brett 1988, PASP, 100, 1134, Appendix B gives relationships between NIR reddenings and E(B–V). These data determine the necessary parameters. Note that the ETC also supports direct entry of observed J and H magnitudes with E(B–V).

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. Another possible resolution is a change to a less sensitive SBC configuration. Any SBC targets or fields that cannot be demonstrated to be safe to a reasonable level of certainty as judged by the CS will not be observed. It is possible that equivalent alternative targets can be approved upon request in that case, but any observations that trigger the onboard safety mechanisms will not be replaced.

A related issue is SBC pointing specification changes after the targets and fields have been cleared by the STScI BOP review. Any such changes must be approved by the ACS Team on the basis of a specific scientific justification and a new BOP review by the GO, which may be submitted via the CS if absolutely necessary. However, in general such requests should be avoided by ensuring that submitted SBC specifications are final, to prevent a need for multiple BOP reviews.

GOs planning SBC observations of unpredictably variable targets, such as cataclysmic variables, are reminded of the special BOP procedures in effect for them, which are detailed in ACS ISR 2006-04.

Policy on Observations Which Fail Because they Exceed Bright-Object Limits

If your source passes screening, but causes the automatic flight checking to shutter your exposures or shut down the detector voltage causing the loss of your observing time, then that lost time will not be returned to you. It is the observer's responsibility to ensure that observations do not exceed the bright-object limits.

7.2.3 Bright-Object Protection for Solar System Observations

Observations of planets with the SBC require particularly careful planning due to very stringent overlight limits. In principle, Table 7.3 and Table 7.4 can be used to determine if a particular observation of a solar-system target exceeds the safety limit. In practice, the simplest and most straightforward method of checking the bright object limits for a particular observation is to use the ETC. With a user-supplied input spectrum, or assumptions about the spectral energy distribution of the target, the ETC will determine whether a specified observation violates any bright object limits.

Generally speaking, for small (< ~0.5 to 1 arcseconds) solar system objects, the local count rate limit is the more restrictive constraint, while for large objects (> ~1 to 2 arcseconds), the global limit is more restrictive.

As a first approximation, small solar system targets can be regarded as point sources with a solar (G2 V) spectrum, and if the V magnitude is known, Table 7.3 and Table 7.4 can be used to estimate whether an observation with a particular SBC prism or filter is near the bright-object limits. V magnitudes for the most common solar system targets (all planets and satellites, and the principal minor planets) can be found in the Astronomical Almanac. This approximation should provide a conservative estimate, particularly for the local limit, because it is equivalent to assuming that all of the flux from the target falls on a single pixel, which is an overestimate, and because the albedos of solar system objects in the UV are almost always significantly less than their values in the visible part of the spectrum. A very conservative estimate of the global count rate can be obtained by estimating the peak (local) count rate assuming all the flux falls on one pixel, and then multiplying by the number of pixels subtended by the target. If these simple estimates produce numbers near the bright object limits, more sophisticated estimates may be required to provide assurance that the object is not too bright to observe in a particular configuration.

For large solar system targets, checking of the bright object limits is most conveniently done by converting the integrated V magnitude (V₀, which can be found in the Astronomical Almanac) to V magnitude/arcsec² as follows:

where area is the area of the target in arcsec². The surface brightness and the diameter of the target in arcseconds can then be input into the ETC (choose the solar template spectrum for the spectral energy distribution) to test whether the bright object limits are satisfied.

7.2.4 Prime and Parallel Observing with the SBC

STScI will perform screening of all SBC exposures prior to scheduling. Targets not established as safe for the configuration in which they are being observed will not be scheduled. Observations that pass screening but are lost in orbit due to a bright object violation will not be rescheduled. Observers are responsible for ensuring that their observations do not violate the SBC count rate limits.

To ensure that STScI can adequately screen observations, special constraints are imposed on parallel observing with the SBC (see Section 7.9 for details on parallel observations). In particular:

• No pure or coordinated parallels are allowed using the SBC
• SNAPSHOT observations with the SBC are not allowed

Table 7.5: Bright object protection policy for SBC observations.

Targets that are one magnitude or more fainter than the magnitude limits in the screening tables generally automatically pass screening. For a target that is within one magnitude of the screening limits, observers must provide a calibrated spectrum of the source at the intended observing wavelength. If such a spectrum is not available, the prospective GO must request an orbit in Phase I for a pre-qualification exposure, during which the target spectrum must be determined by observation in an allowed configuration.

Please note that if you are proposing SBC target-of-opportunity observations, we ask you to provide an explanation in your Phase I proposal of how you will ensure that your target can be safely observed.