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Acquisition Count Rates for Extended Objects

GHRS Instrument Handbook

Target Acquisition Reference Information

Chapter 4 mentions acquisition methodologies for extended objects, and, in particular, the use of the EXTENDED Optional Parameter. In this section we provide some guidance on predicting the count rates to be expected during an acquisition of an extended object, especially one beyond our own Galaxy. The renewed availability of Side 1 of the GHRS and its G140L grating make it possible to get high-quality spectra of faint objects efficiently.

Whenever possible, we recommend that faint objects be acquired by offsetting from a nearby and brighter point source. If accurate coordinates are used, this method should be reliable. However, such objects are not always present next to targets of astrophysical interest. Also, obtaining a good astrometric position of an extended source can be prevented by its large saturated area on the photographic plates upon which the Guide Star Catalog is based. In these cases a direct target acquisition will need to be attempted, and it should succeed if the object provides enough ultraviolet photons. The procedure closely follows that for point sources just described: