Efficient and affordable catadioptric spectrograph designs for 4MOST and Hector

Abstract

Spectrograph costs have become the limiting factor in multiplexed fiber-based spectroscopic instruments, because tens of millions of resolution elements (spectral x spatial) are now required. Catadioptric (Schmidt-like) designs allow faster cameras and hence reduced detector costs, and recent advances in aspheric lens production make the overall optics costs competitive with transmissive designs. Classic Schmidt designs suffer from obstruction losses caused by the detector being within the beam. A new catadioptric design puts the detector close to the spectrograph pupil, and hence largely in the shadow of the telescope top-end obstruction. The throughput is competitive with the best transmissive designs, and much better in the Blue, where it is usually most valuable. The design also has milder aspheres and is more compact than classic Schmidts, and avoids most of their operational difficulties.