A fast new cadioptric design for fiber-fed spectrographs

摘要

The next generation of massively multiplexed multi-object spectrographs (DESpec, SUMIRE, BigBOSS, 4MOST,HECTOR) demand fast, efficient and affordable spectrographs, with higher resolutions (R = 3000-5000) than currentdesigns. Beam-size is a (relatively) free parameter in the design, but the properties of VPH gratings are such that, forfixed resolution and wavelength coverage, the effect on beam-size on overall VPH efficiency is very small. For alltransmissivecameras, this suggests modest beam-sizes (say 80-150mm) to minimize costs; while for cadioptric(Schmidt-type) cameras, much larger beam-sizes (say 250mm+) are preferred to improve image quality and to minimizeobstruction losses. Schmidt designs have benefits in terms of image quality, camera speed and scattered lightperformance, and recent advances such as MRF technology mean that the required aspherics are no longer a prohibitivecost or risk.The main objections to traditional Schmidt designs are the inaccessibility of the detector package, and the loss inthroughput caused by it being in the beam. With expected count rates and current read-noise technology, the gain incamera speed allowed by Schmidt optics largely compensates for the additional obstruction losses. However, futureadvances in readout technology may erase most of this compensation.A new Schmidt/Maksutov-derived design is presented, which differs from previous designs in having the detectorpackage outside the camera, and adjacent to the spectrograph pupil. The telescope pupil already contains a hole at itscenter, because of the obstruction from the telescope top-end. With a 250mm beam, it is possible to largely hide a 6cm ×6cm detector package and its dewar within this hole. This means that the design achieves a very high efficiency,competitive with transmissive designs. The optics are excellent, as least as good as classic Schmidt designs, allowingF/1.25 or even faster cameras. The principal hardware has been costed at $300K per arm, making the design affordable.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.