A DUAL-BEAM NASMYTH SPECTROGRAPH

A recently-constructed double-beam spectrograph mounted on the Nasmyth focus of the Australian National University 2.3-m reflector is described. The spectrograph features two 150-mm collimator beams, with high throughput optics in each arm. Spectral resolutions range from 40 to 240 km s-1^. Flexure in rotation about the Nasmyth axis is elastic and has an amplitude of ± 16 microns in the camera focal planes. With S-20 and NEA Ga-As cathodes in large- format photon-counting arrays, the system detects 1 photoelectron A-1^ sec-1^ from stars with B = 15.0 at 4000 A and R = 15.1 at Hα. Some precepts from the optical configuration of this spectrograph may be applicable to the design of Nasmyth spectrographs for the large altitude-azimuth telescopes to come.