Efficient and affordable catadioptric spectrograph designs for 4MOST and Hector

摘要

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. The fast cameras mean that with 15micron pixels, the PSF sampling is close to the Nyquist limit; this minimises the effects of read-noise, which for sky-limited observations, far outweighs any difference in throughput. It does introduce pixellation penalties; these are investigated and found to be modest. For 4MOST, low and high resolution designs are presented, with 300mm beams, 3 arms with f/1.3 cameras, and standard 61mm × 61mm detectors. Coverage is 380-930nm at R=5000-7000, or R～20000 in three smaller ranges. A switchable design is also presented. For Hector, a design is presented with 2 arms, 380-930nm coverage, and R=3000-4500; a 4-armed design with smaller beam-size and detectors is also presented. The designs are costed, and appear to represent excellent value.