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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:--

• A ghost spectrum parallel to the primary spectrum which is seen in blue arm observations when a dichroic filter is used. It is caused by light reflected off the back surface instead of the front surface of the dichroic. It is strongest at wavelengths in the crossover region. The offset on the detector from the primary spectrum is 0.2 mm for the older (thin) dichroics; and 0.6 mm for the new (thick) dichroics.
• A Narcissus ghost pupil image which is caused by reflection between the surface of the CCD or its surrounds and the cryostat window. This appears strongest when caused by strong spectral features just off of the CCD, for example when looking at a Copper-Argon source at blue wavelengths at low dispersion, when the ghost is caused by reflection of the strong red lines from the surrounds of the CCD. If it is due to a continuum source the pupil image will be smeared in wavelength, and may be difficult to recognise as such. The intensity of this ghost is at the level of 10 of the primary source.
• A grating ghost caused by reflection between the grating and the aspheric plate of the camera when using grating R158R in the red camera. The ghosts images of all wavelengths add, so this ghost can appear quite strong.
• An in-focus ghost caused by a reflection from the folding prism, in either the red arm of ISIS or in FOS.
• Rowland ghosts, which are caused by periodic errors in the ruling of the gratings. These appear around strong emission or comparison lines, as satellite lines at the level of >10 of the primary.