Efficient and affordable catadioptric spectrograph designs for 4MOST and Hector

TL;DR

This paper presents innovative catadioptric spectrograph designs that reduce costs and improve throughput, especially in the blue spectrum, suitable for large multiplexed fiber-based instruments like 4MOST and Hector.

Contribution

It introduces a novel catadioptric design with the detector near the pupil, reducing obstruction losses and operational difficulties compared to classic Schmidt designs.

Findings

Throughput is competitive with transmissive designs.

Design is more compact and easier to operate.

Improved blue spectrum performance.

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.