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With a cross-dispersed spectrograph, there is a tradeoff between full spatial coverage along the slit and the requirement that the two spectral orders should not overlap, to ensure for example that the first order image of the sky does not contaminate the second order image of the object. Various dekkers are provided to allow you to make your choice. Alternatively, a colour filter can be used to suppress the second order light.

Tue Aug 15 16:42:46 BST 1995