Hectospec, the MMT’s 300 Optical Fiber‐Fed Spectrograph

TLDR

Hectospec, a 300‑fiber optical spectrograph commissioned at the MMT in 2004, uses a radial “fisherman on the pond” fiber layout held by magnets and has become a workhorse instrument, with Hectochelle as a high‑dispersion complement, occupying about one‑third of MMT nights. A pair of high‑speed six‑axis robots reposition the 300 fiber buttons within ~300 s to ~25 µm accuracy, and 26‑m fibers feed the bench spectrograph operating at R≈1000–2000. The system achieves a peak throughput of ~10% (≈17% after correcting for aperture losses), close to the predicted 20%, and has produced about 60,000 reduced spectra for 16 programs in its first year.

Abstract

The Hectospec is a 300 optical fiber fed spectrograph commissioned at the MMT in the spring of 2004. In the configuration pioneered by the Autofib instrument at the Anglo‐Australian Telescope, Hectospec's fiber probes are arranged in a radial "fisherman on the pond" geometry and held in position with small magnets. A pair of high‐speed, six‐axis robots move the 300 fiber buttons between observing configurations within ∼300 s, and to an accuracy of ∼25 μm. The optical fibers run for 26 m between the MMT's focal surface and the bench spectrograph, operating at R∼1000–2000. Hectochelle, another high‐dispersion bench spectrograph offering R∼35,000, is also available. The system throughput, including all losses in the telescope optics, fibers, and spectrograph, peaks at ∼10% at the grating blaze in 1'' FWHM seeing. Correcting for aperture losses at the 15 diameter fiber entrance aperture, the system throughput peaks at ∼17%, close to our prediction of 20%. Hectospec has proven to be a workhorse instrument at the MMT. Together, Hectospec and Hectochelle have been scheduled for 1/3 of the available nights since its commissioning. Hectospec has returned approximately 60,000 reduced spectra for 16 scientific programs during its first year of operation.