Below we provide model line spread functions (i.e., instrumental
profiles) for point source observations through a sample of supported slits in first-order and echelle modes as a function of observing wavelength. Internal lamp observations and on-orbit data for external targets have been used to confirm and revise the original pre-flight models of the STIS LSFs.
These plots and the derived FWHM presented below in tabular
form show that increasing the slit width for point source observations has very little effect on the FWHM of the observed lines but does broaden the wings of the LSFs. This occurs because a wider aperture allows more of the wings of the HST Optical Telescope Assembly PSF to enter but does not change the shape of the inner part of the PSF which is quite narrow. The implications of the LSF wings on line profile and equivalent width measures are discussed in more detail in Section 13.7
. For diffuse source observations, don’t forget that widening the slit, while it allows more light to enter, also will degrade the spectral resolution obtained.