Improving the Sampling of the Line-Spread Function

In some configurations the point-source spectral-line FWHM is modeled to be less than two detector pixels (see "Line-Spread Functions" on page 350). The realized in-flight line-spread functions (LSFs) are still being evaluated as of the publication of this Handbook (see also STIS ISR 98-04) and updates will be posted on the STIS web page. Most observations should not be affected, but if you are observing lines which are near to being unresolved in the grating of interest and require accurate line profiles, you can consider the following technique:

• Stepping of the target in the dispersion direction in a wide slit or slitless aperture to subsample the line-spread function by displacing the spectrogram. This technique can also be used to increase the signal-to-noise (see above). Note that in employing this strategy one will have to trade off the benefits of the sampling with the negative impact of increased wings in the line-spread function with a wide slit, particularly for MAMA observations (see "Spectral Purity, Order Confusion, and Peculiarities" on page 355).
• For MAMA observations, use of highres sampling may provide ~15-30% better sampling; however, flat-field variability and rate dependence may make it difficult to realize the benefit in resolution (see "Highres" on page 218). Note that all STIS data are taken by default in highres format, and then binned in calibration on the ground so all you need to do is recalibrate your data, changing the switch settings (see Chapter 21 in the HST Data Handbook) to fully exploit the highres data.
• Use of a higher resolution grating mode in which your LSF is fully sampled.