Scattered light and ghosting
As is the case for many spectrographs, WYFFOS suffers from diffuse
and from ghosting. The diffuse scattered light has been reduced by locating baffling
at strategic locations in the spectrograph, but the ghosting is inherent
to the design of the spectrograph, and results from undispersed light
from the slit unit reflected off the "upstream' optical surfaces of the
double-pass collimator, some of which is subsequently imaged on the detector
along with the spectra.
The nature of the ghosting is similar with all gratings, namely
a large "spot" of undispersed light in the blue (i.e. lower) part of the
detector, although its precise nature depends on which fibres are deployed.
It generally amounts to about 5% of total flux,
but can be significantly more than this relative to the spectra for
configurations involving the higher resolution gratings.
We are currently investigating the optimum choice of
blocking filters to attenuate the ghost relative to the dispersed
light for these gratings.
Example ghost images for tungsten-lamp exposures
with all fibres
configured in a circle, but with no blocking filters deployed, are shown