Design and construction of an echelle spectrograph: HERCULES

A fiber-fed spectrograph (HERCULES) has recently been installed at the Mt. John Observatory. This spectrograph achieves resolving powers of 35,000 and 70,000 using 100 micrometers and 50 micrometers fibers. The spectrograph is designed to capture the visual spectrum from 380 nm to 880 nm in a single exposure on a 50 mm square CCD. As the spectrograph is enclosed in a vacuum tank and is thermally isolated it is expected that high precision radial velocity measurements will be obtainable. Overall instrument efficiency is predicted to peak at 23% in 1' seeing. Preliminary design ideas for the 11 m Southern African Large Telescope (SALT) high resolution spectrograph are also discussed. While it is hoped that a variant of the HERCULES design can be adopted for this instrument, as with all spectrographs designed for large telescopes, the design presents significant challenges.