12.6 Improving the Sampling of the Line Spread Function

In most configurations the point source spectral line FWHM is slightly less than two detector pixels (see Section 13.6). Most observations should not be affected, but if one is observing lines which are near to being unresolved in the grating of interest and requires accurate line profiles, the following techniques may be useful:

  • 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 Section 13.7).
  • For MAMA observations, highres pixels may provide twice the sampling; however, flat-field variability may make it difficult to realize the benefit in resolution (see Highres Section in 3.2). Note that all STIS data are taken by default in highres format, and then binned in calibration on the ground, so all one needs to do is to recalibrate the data, changing the switch settings (see "STIS Calibration" chapter in the STIS Data Handbook) to fully exploit the highres data.
  • For MAMA echelle observations, one can use the very small 0.1X0.03 aperture.  When used with high signal-to-noise data in the highres format and with specialized data reduction techniques, resolutions as high as 200,000 are possible with the E140H and E230H echelle gratings (Jenkins & Tripp 2001, ApJS, 137, 297).  See Section 3.1 on the smallest STIS apertures (with caveat detailed in the footnote) and details about throughput losses given in ISR 2017-01. The STIS Throughput Curves are available on the STIS Performance website.
  • Use of a higher resolution grating mode in which the intrinsic line width of the source is well sampled.