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:
- Find a star in Table 7.1 on page 92 with a spectral energy distribution like that of your object, or consult the IUE Atlas of Star-Forming Galaxies, by Kinney et al. (1993, ApJS, 86, 5). This publication provides representative spectra of many classes of galaxies and compares their shapes to ones of stellar spectra (whence the "spectral types" listed).
- Estimate the flux that will fall within the LSA from either Table 7.1 on page 92 or Table 7.2 on page 96, as appropriate.
- Calculate STEP-TIME in the same manner as for stars. If necessary, you may use mirror N2 on Side 2 for your acquisition, even if Side 1 is being used to observe, but doing so will add about 40 minutes of time in order to switch from one Side to the other.
- If your object does not fall within these categories, please consult us.