The FAST Spectrograph for the Tillinghast Telescope

Abstract

We describe a high-throughput optical spectrograph that has been in operation at the Cassegrain focus of the 1.5-m Tillinghast reflector since 1994 January. FAST has a 3'-long slit and is typically operated at resolutions between 1 and 6 Å. With a collimated beam diameter of ~100 mm, FAST (with a 300 lines mm^-1 grating and a 1.5"-wide slit) offers 4000 Å of spectral coverage at 3 Å resolution. FAST's optics are primarily reflective, are adequately sized to prevent vignetting, and use high-performance coatings. The high measured system peak efficiency of 26% (fraction of light incident on the primary detected at the CCD) demonstrates that the throughput of reflective optics can be quite competitive with that of refractive optics. FAST's structure is constructed from graphite-epoxy composite panels, which have an excellent stiffness-to-weight ratio and low thermal expansion, resulting in low flexure and excellent focus stability.