6.5 m telescope for multi-object spectroscopy over a 3° field of view

Abstract

Multi-object spectroscopy via independently positioned optical fibers is of growing importance in many research areas in astronomy. Currently the most powerful instrument of this type is the Dark Energy Spectroscopic Instrument (DESI), now being commissioned at the 3.8 m Mayall telescope. It has a 3.2° field of view where spectra of 5000 different objects may be recorded simultaneously. Here we present an optical design for a two-mirror 6.5 m telescope with 3.0° field of view, for an etendue 2.6 times larger than for DESI. The images are at f/3.7 for fiber matching, and -averaged over the field and elevations down to 40°-have a diameter of 0.53 arcsec for 80% encircled energy. We outline methods capable of polishing and testing the 1.56 m diameter gull-wing lens of the wide field corrector. If a 2 m diameter lens could be made, the same design could be scaled up to an 8.4 m primary for a 4.3-fold etendue advantage.