multi-object spectrograph

A multi-object spectrograph is a spectrograph designed to collect spectrographic data from multiple targets simultaneously. The term is commonly used for spectrographs connected to the focal plane by multiple optical fibers: at the focal plane, the fiber end is located at the position of the image of the target source, and at the spectrograph, the fiber ends are in a line along the spectrograph's slit. Such instruments have been built that can handle as many as five hundred targets simultaneously. The Sloan Digital Sky Survey (SDSS) used such a spectrograph, in its initial phases, using pre-created plates with holes so at the time of observation, the fibers could be quickly and accurately positioned to catch the target sources. Automated fiber positioners are now used for advanced models, which position the fibers based upon the targets' coordinate data. Both the spectrograph and fiber positioner are complicated and expensive, often requiring years to design, build, and commission. However, the efficiency of gathering hundreds of spectra at a time is a significant advancement for astronomy. Telescopes/instruments using this:

• ESO VLT's VIMOS (Visible multi-object spectrograph)
• Subaru's FMOS (Fibre multi-object spectrograph)

The term multi-object spectrography indicates observation with such instruments.

Among older strategies (which predate the common use of the term multi-object spectrograph) is use of a slitless spectrograph, which could produce a number of small "rainbows" on a photographic plate, a strategy was used in some earlier star surveys. Color photography was not used: by inspecting the line produced by each star, some spectral characteristics could be discerned. Another strategy has been to use a mask that has slits positioned for a number of individual targets rather than a single slit.