Since an SSA spectrum is the best resolution we can obtain with GHRS, it is useful to describe the LSA LSF in terms of the SSA PSF. Consequently, we have measured the LSA-SSA differential LSF for a number of gratings and at a sample of wavelengths. This differential LSF satisfies the relationship: LSF * SSA = LSA; i.e., the differential LSF is the LSF that when convolved with an observed SSA spectrum produces the best match to an identical spectrum obtained through the LSA. By combining the intrinsic LSF for the SSA with the empirical differential LSF we can obtain the intrinsic LSF for the LSA. Since the SSA and differential LSF are Gaussians, we obtain an LSA LSF that is also a Gaussian with a FWHM that is slightly greater than that of the SSA.
The pre-COSTAR LSF of the GHRS was characterized by a Gaussian core nearly twice as broad as that provided by the instrumental resolution limit, provided by the SSA, with extended non-Gaussian wings. The post-COSTAR LSF for the LSA is only 19-51% broader in a Gaussian core than spectra from the SSA and the extended wings are absent. Additional information about the GHRS LSF can be found in GHRS ISR 063. Table 38.1 summaries the post-COSTAR LSF for the SSA and the LSA-SSA differential LSF.
GHRS Post-COSTAR Differential LSF
Deconvolution of GHRS spectra was investigated after the spherical aberration was found in the primary mirror. With COSTAR, the need for deconvolution has become less pressing, however, for the best spectral resolution, it is possible to deconvolve LSA spectra to the level of SSA spectra. See, The Restoration of HST Images and Spectra, (proceedings of the HST Calibration Workshop at STScI), STScI, 1990. The STSDAS task, lucy, can be used to deconvolve GHRS spectra.