THE FAST SPECTROGRAPH FOR THE TILLINGHAST TELESCOPE

We describe a high-throughput optical spectrograph that has been in op eration at the Cassegrain focus of the 1.5-m Tillinghast reflector sin ce 1994 January. FAST has a 3'-long slit and is typically operated at resolutions between 1 and 6 Angstrom. With a collimated beam diameter of similar to 100 mm, FAST (with a 300 lines mm(-1) grating and a 1.'' 5-wide slit) offers 4000 Angstrom of spectral coverage at 3 Angstrom resolution. FAST's optics are primarily reflective, are adequately siz ed 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 r eflective optics can be quite competitive with that of refractive opti cs. FAST's structure is constructed from graphite-epoxy composite pane ls, which have an excellent stiffness-to-weight ratio and low thermal expansion, resulting in low flexure and excellent focus stability.