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Scattered Light

Scattered light in ISIS is minimised by the use of optimised anti-reflection coatings, and if scattered light would be a serious problem for a particular observation it is important to exclude light of wavelengths other than those required, particularly wavelengths outside the range for which the coatings are optimised, from the optics by use of appropriate colour filters.

Diffuse scattered light has be shown to be below 2% by observations during commissioning of Quasar absorption lines known to be completely black.

Ghost images are caused by stray reflections within the spectrograph, and can either be in-focus images or images of the pupil. Pupil images take the form of the telescope pupil with the central obstruction, even if the illumination is from the comparison lamp system, because the illumination from the integrating sphere is designed to mimic exactly that of the telescope. This is not true of the comparison lamps for the fibre optic system.

There are a number of known ghosts in the ISIS system, these are listed below:--



Previous: Stability and radial velocities
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Fri Jan 7 15:34:48 GMT 1994