On the possibility of increasing the throughput of astronomical spectrographs by overfilling the dispersing element

This paper describes a technique which has been applied to the new AAT coude echelle spectrograph, under construction at UCL, to increase its throughput without degrading resolution. The effect of illuminating the slit with stellar images and extended objects is considered and a method of increasing the flexibility of the design is presented. The telescope focal ratio is modified by the addition of a lens in front of the slit. The degree of modification can be adjusted to optimize the spectrograph’s light throughput under difficult seeing conditions or for different applications. The technique may also be relevant to other spectrographs being designed and could even find applications in some existing instruments.