[Top] [Prev] [Next] [Bottom]

38.2 The GHRS Line Spread Function

The line spread functions (LSFs) for the GHRS gratings describe the instrumental broadening for a delta-function spectral feature. Knowledge of such a blurring function is necessary for quantitative studies of GHRS spectral line profiles. The resolution element (one diode) for GHRS was matched to the width of the SSA. By using substepping strategies, it is possible to get properly sampled spectra, obtaining the minimum of two sample points per resolution element. A delta-function spectra line observed through the SSA can be described by a Gaussian with a FWHM of about 0.925 diodes which is about 3.7 quarter-stepped pixels. (See Gilliland 1992, PASP, 104, 367).

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. The LSF results are presented in GHRS ISR 063. The differential degradation of the LSF due to the LSA with respect to the LSF of the SSA was derived. A spectral line of a point source with an infinitesimal line width (delta function) measured with the SSA has an LSF with a full width at half maximum (FWHM) of 0.925 diodes. This is independent of wavelength and grating. Measurements of the differential LSF of the LSA were performed for the five wavelength/grating combinations listed in Table 38.1. Column 3 of this table gives the measured FWHM of the differential LSF. Adding this value in quadrature to the FWHM of the SSA LSF (=0.925 diodes) leads to the FWHM of the LSA LSF, given in column 4 of the table. Plots showing the differential LSFs are in GHRS ISR 063. No reliable measurements of the LSF for the G140L grating are available. It is very likely that the G140L LSF is similar to those of other gratings.



GHRS Post-COSTAR Differential LSF

Grating

Wavelength

(Å)

Differential LSF of the LSA (diodes)

Relative FWHM (LSA/SSA)

G160M

1360

0.60

1.10

G160M

1900

0.72

1.18

G200M

1900

0.60

1.10

ECH-B

1900

0.82

1.24

ECH-B

2680

0.60

1.10

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.



[Top] [Prev] [Next] [Bottom]

stevens@stsci.edu
Copyright © 1997, Association of Universities for Research in Astronomy. All rights reserved. Last updated: 06/30/98 11:12:21