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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 here. See also the ACS
Instrument Handbook, Section 7.2, including Tables 7.3 and 7.4, which give
the applicable countrate and V-magnitude screening limits, respectively.

STScI has developed bright object tools (BOT) to conduct detailed field
checking prior to SBC program implementation. These tools are 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 mag, which, combined with conservative spectral-type
vs. color relationships, supports determinations derived from the Exposure
Time Calculator (ETC) 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.

STScI will check all targets and fields before any SBC observations are
cleared. However, by policy 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. While the procedures may appear complex on first exposure,
their convenience and straightforward application rapidly become apparent.
All SBC proposers must conduct BOP reviews of their targets and fields in
conjunction with their Phase II preparations. Thus, 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 as necessary and
requested during the Phase II preparations, and through program execution.

Briefly, for a single default SBC pointing with unconstrained orientation,
a field of 70 arcseconds in diameter must be cleared. The APT/BOT
automatically reports on all GSC2 stars 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 (but
beware of moving back to the bad rows or the edges of the detector). 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.

An SBC GO must send his/her CS, by the Phase II deadline, 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 in fact 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 currently available only with GSC2), including the STScI
Multimission Archive (MAST), which contains the IUE and GALEX data in addition
to the HST data. An existing UV spectrogram of the target or class may be
imported directly into the ETC; IUE data must be low resolution, large
aperture for BOP. If model spectra are used, the original Kurucz (not
Castelli & Kurucz) set should be used for early-type stars. None of the
provided models is adequate for late-type stars, since they lack chromospheric
emission lines; actual UV data must be used for them. In worst cases, new
groundbased 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. (The older Kurucz O5 model with higher Teff in the ETC should be
used for BOP purposes.) If one color is available, it may be processed as
a reddened O5 (which will always have a greater UV flux than an unreddened
star of the same color). If two colors are available, then the actual
(main-sequence) spectral type and reddening can be estimated separately.
The APT/BOT now automatically clears faint stars with only one GSC2
magnitude available, on the unreddened O5 assumption, greatly reducing
the number of stars reported as "unknown". Any remaining unknowns must be
cleared explicitly.

In some cases, the 2MASS JHK may 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 countrates
in the ETC. F. Martins & B. Plez, A&A, 457, 637 (2006), 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.) M.S. Bessell & J.M. Brett, PASP, 100, 1134
(1988), Appendix B, give relationships between the NIR reddenings and E(B-V).
These data determine the necessary parameters. Note that the ETC also
supports direct entry of observed J, 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 in the judgement of the CS will not
be observed. It is possible that equivalent alternative targets may 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 06-04.