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Other considerations

FOS slightly undersamples its spectrum. 1 GEC pixel (22 micron= 10.7Å in first order) is equivalent to 1.3 arcsec at the slit. The optimum resolution for a narrow slit is observed to be 1.15 pixels fwhm. In principle to match a 2 pixel fwhm requires a 2.1 arcsec slit, but the resolution is then approximately 21Å fwhm. Narrower slits will produce sharper features which is generally desirable, since 21Å resolution is often inadequate for absorption line work. The online software does not resample the spectrum, so this is an important point. In good seeing, a 1.0 arcsec slit gives 15Å resolution.

With such a wide spectral coverage, another important factor which is affected by the slit width is light loss due to atmospheric dispersion. There are two ways to overcome this problem:

1) align the slit along the parallactic angle - this is shown on the telescope status screen, or

2) widen your slit to compensate. As a guide, the following displacements occur across the orders of FOS at various zenith distances,

Tue Apr 12 04:13:28 BST 1994