HERCULES: a high-resolution spectrograph for small- to medium-sized telescopes

The High Efficiency and Resolution Canterbury University Large Echelle Spectrograph (HERCULES) is a fibre-fed echelle spectrograph that has been in operation at the Mount John University Observatory for just over one year. HERCULES is used in conjunction with the 1-m McLellan telescope, and can capture the spectrum from 380 nm to 880 nm on a single 50-mm square CCD. Resolving powers of up to 70000 are possible when using 50-μm fibres, and a resolving power of 35000 is possible with a 100-μm fibre. Wavelength calibration is done using sequential exposures of a thorium-argon emission lamp. The spectrograph is designed to achieve high efficiency when the seeing is well matched to the image scale on the fibre input (up to 20% in 1 arcsec seeing), and high stability is achieved by having the spectrograph installed inside a vacuum tank in a thermally isolated environment. Initial indications are that radial velocities with a precision of ≤ 10 ms -1 are routinely possible in the short-term.