Line-Spread Functions

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. Early on orbit internal lamp observations confirm the excellent optical performance of STIS (see e.g., STIS ISR 98-04). Data from external targets on the line spread functions are just becoming available, so these models have not yet been confirmed in detail. Nevertheless they are ample for proposal planning.

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 Spectral Purity, Order Confusion, and Peculiarities. 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.