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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 been 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.
Next: Wood's Anomalies in
Up: ISIS
Previous: Stability and radial
Tue Aug 15 16:42:46 BST 1995