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, which 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.
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. The calculator will evaluate the global and per pixel count rates, and will warn you if your exposure exceeds the absolute bright-object limits.
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
; you may consult with an ACS Instrument Scientist via the Help Desk if needed. 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.
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 magnitude, which, combined with conservative spectral-type vs. color relationships, 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.
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 available only with GSC2 and GALEX), including the STScI Multimission Archive (MAST)
, which contains the IUE and GALEX in addition to the HST
data. An existing UV spectrum 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 stars later than the Sun, since they lack chromospheric emission lines; actual UV data must be used for them. 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. (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 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.) Bessell & Brett 1988, PASP, 100, 1134
, 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).
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.
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 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.
is the area of the target in arcsecond2
. This surface brightness and the diameter of the target in arc seconds 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.