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 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:

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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).

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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”). 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 “STIS Calibration” chapter in the STIS Data Handbook) to fully exploit the highres data.

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For MAMA echelle observations, consider using the very small 0.1x0.03 aperture. When used with very 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).

•	Use of a higher resolution grating mode in which the intrinsic line width of the source is well sampled.