GHRS Instrument Handbook
Target Acquisition Reference Information
We have calculated GHRS target acquisition count rates for the spectra of a subset of the 175 stars contained in the Bruzual-Persson-Gunn-Stryker (BPGS) Library of Stellar Spectra by using calcphot, a task in the synphot package in STSDAS. GHRS target acquisition count rates for objects similar to those in the BPGS catalog can be predicted by following the procedures described here. Constraints on the value of STEP-TIME are discussed. Please note that the values tabulated are the total count rate for a star, and that the count rate for a particular diode will depend on that portion of the Point Spread Function that strikes it. That can influence the degree, for example, to which the paired-pulse correction applies. However, the acquisition procedure sums the counts over the eight science diodes upon which the LSA is imaged, so for most objects these values may be used with confidence.
Do not forget to reduce these values by a factor of 0.3 if the focus diodes are being used for an IMAGE; this is because of the reduced area of the focus diodes compared to using eight normal diodes for an acquisition. This factor applies to when a focus diode is centered on a point source.
The flux distributions in the BPGS catalog include ultraviolet wavelengths and can be used for planning GHRS target acquisitions. Each spectrum in the catalog was dereddened and scaled to V0 = 0.0. The calcphot task in the synphot package of STSDAS was used to convolve the catalog flux distributions with the effective areas of the acquisition mirrors. Table 7.1 on page 92 contains columns giving the BPGS catalog object name, spectral type, (B-V)0, count rate for the acquisition mirror with no reddening, and scale factors (per unit magnitude) indicating the relative count rate observed at given amounts of reddening compared to the count rate with no reddening.
To use Table 7.1 on page 92 to predict target acquisition count rates:
- Determine the intrinsic color, (B - V)0, and intrinsic magnitude, V0, of your object as well as its color excess, E(B - V).
- Find an entry in Table 7.1 that has similar spectral characteristics to your object [by spectral type or (B - V)0 and note that luminosity class is important for the coolest stars]. The table is sorted by increasing (B - V)0. Make sure you pick from the column corresponding to the acquisition mirror that you plan to use.
- Scale the predicted count rate found in the previous step by the ratio of apparent brightness of your object to an object of magnitude zero, i.e., multiply by
. - To obtain the scale factor by which the unreddened count rate will be reduced for an amount of reddening appropriate to your object (the reddening reduction factor), multiply the count rate from the previous step by this factor:
- The GHRS detectors are nonlinear at high count rates: this phenomenon is referred to as the "dead-time" or "paired-pulse" effect. Consequently, the predicted count rate from the previous step must be reduced to yield the actual count rate that GHRS will measure. Multiply the count rate you just determined by the "fraction detected" value determined from Figure 7.2 on page 89 to obtain the final predicted count rate.
- This final value should be reliable to within a factor of two, which is adequate for acquisition purposes in almost all instances.