RESOLUTION AND NOISE PROPERTIES OF THE GODDARD HIGH-RESOLUTION SPECTROGRAPH

The Goddard High-Resolution Spectrograph (GHRS) on the Hubble Space Telescope (HST) provides an unprecedented capability for ultraviolet spectroscopy of a wide range of astronomical targets. The combination of high resolution (to R is similar to 100,000), good sensitivity (maximum total system quantum efficiency of approximately 1%), low background noise (order 0.01 counts s-1 diode-1), coupled with the fast photon-counting detector system makes the GHRS a very flexible instrument. Unfortunately, the presence of spherical aberration in the primary mirror of HST complicates issues of both observing strategy and data analysis procedures, since the throughput and resolving-power losses of the two available GHRS apertures are often offsetting. We discuss several characteristics of GHRS observations, and data-analysis procedures, including deconvolution, with a particular emphasis on the detection of weak absorption lines in low-signal-to-noise observations. We find that observations with the GHRS large aperture generally provide greater sensitivity for the detection of weak, isolated spectral features than does use of the small aperture for equal observing times. At low signal-to-noise levels a Poisson distribution well represents the noise characteristics. Analysis of the UV spectrum of 3C 273 serves as the motivation for this study. Deconvolution of small-aperture first-order grating spectra taken at high signal to noise may be preferred to less efficient echelle spectra in some cases. We argue that GHRS observations should be routinely structured with use of comb-addition, quarter-substepping, FP-SPLITS, and multiple repeats to allow proper discrimination against noise events that may mimic spectral features.