COS Instrument Handbook for Cycle 24

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10.5	Tools for Bright-Object Screening

10.5.1 The Bright Object Tool (BOT)

10.5.2 The Exposure Time Calculator (ETC)

10.5.1 The Bright Object Tool (BOT)

STScI has developed a Bright Object Tool (BOT) to facilitate field checking prior to COS program implementation. The BOT is implemented within APT (the Astronomer’s Proposal Tool), using the Aladin interface, and reads target and exposure information from the Phase II proposal. Help files and training movies are available within APT. The BOT is based on displays of the Digital Sky Survey (DSS) and on automated analysis of the field using data from two catalogs: the second Guide Star Catalog (GSC2) and the GALEX catalog.

GSC2 provides two magnitudes (photographic J and F), hence one color, for most fields down to about magnitude 22, which, combined with conservative assumptions about spectral type vs. color, allow expeditious target and field checks. In some cases, the GSC2 is inadequate because of crowding or the absence of one filter. The APT/BOT automatically clears stars with only a single GSC2 magnitude if they are safe based on the assumption that the target is an unreddened O5V star. Any other unknown targets must be cleared by hand.

Automated GALEX screening is now available within the APT/BOT. The AIS (all-sky) sources are screened as unreddened O5V stars and reported as either safe, unsafe, or unknown. Because it is based directly on UV fluxes, GALEX screening can reveal, for example, previously unknown hot companions to late-type stars. If the field passes the BOT check, it is safe; unsafe and unknown objects require further investigation. Caveats: (1) The GALEX catalog does not cover the whole sky, so PIs must check that their COS field is fully covered by GALEX. (2) GALEX fluxes represent upper limits in crowded regions because of the instrument’s relatively low spatial resolution. (3) The GALEX detectors suffer local non-linear effects at high count rates. The fluxes and magnitudes in the current version of the GALEX catalog are not corrected for these effects and may be underestimated for the brightest stars. A preliminary correction is presented in Morrissey et al. (2007, ApJS, 173, 682). The BOT now applies this correction to the GALEX catalog. As a result, it may report GALEX magnitudes that are brighter than those given in the GALEX catalog itself.

In general, a COS pointing with unconstrained telescope orientation requires the clearance of a field 43 arcsec in diameter. If any displacements from the default pointing (e.g., acquisition scans, POS TARGs, patterns, or mosaics) are specified, then the field to be cleared increases commensurately. Because both the PSA and BOA are exposed to the sky at all times, no unsafe or unknown star may fall within 7 arcsec of either aperture at any allowed orientation. The BOT automatically accounts for the reduced throughput of the BOA and MIRRORB. The BOT automatically creates a macroaperture covering the expected field of view for an observation that accounts for a given combination of scans, POS-TARGs, patterns or mosaics. It also can account for the proper motion of the science target.

Note: Always check the science target with the ETC, rather than relying on the BOT field report. When constructing your Phase II proposal within APT, include the ETC exposure ID number to document your work and to facilitate the Phase II review.

10.5.2 The Exposure Time Calculator (ETC)

Should the tools available within the BOT be insufficient to clear a field object, its safety may be confirmed using the ETC.

An existing UV spectrogram of the target or class may be imported directly into the ETC. When using IUE data as input spectra in the ETC, consider only low-resolution spectra taken through the large aperture. Note that the ETC does not convolve imported spectra to the COS resolution. To be conservative, one must assume that the entire flux of an emission line falls within a single COS resolution element. The ETC can also have emission lines of a given intensity and width included in a calculation.

If model spectra are used in the ETC, the original Kurucz (not Castelli & Kurucz) models should be used for early-type stars. None of the provided models is adequate for late-type stars since the models lack chromospheric emission lines. Actual UV data must be used for late-type stars when possible. If a given star has only a V magnitude, it must be treated as an unreddened O5V star. If one color is available it may be processed as a reddened O5V star (which will always have a greater UV flux than an unreddened star of the same color).

If two colors are available, then the spectral type and reddening can be estimated separately. In some cases, the 2MASS JHK may be the only photometry available for an otherwise unknown star. The ETC supports direct entry of observed J and H magnitudes with EB−V. It is also possible to estimate V and EB−V from those data on the assumption of a reddened O5V 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 and (V−H)0 = −0.79 for early O types. (The K band should be avoided for BOP purposes because of various instrumental and astrophysical complications.) Bessell & Brett (1988, PASP, 100, 1134), Appendix B, give relationships between the NIR reddenings and EB−V.