Scattered light and ghosting

As is the case for many spectrographs, WYFFOS suffers from diffuse scattered light 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 for the R158B, R300B, R600B, R1200B and H2400B gratings.