FORS

FORS (for Focal Reducer and Low Velocity dispersion Spectrograph) is the name of two ESO VLT instruments, called FORS1 and FORS2. They are general purpose instruments supporting imaging, modest-resolution spectrography, and multi-object spectrography. The focal reducer function is to lower the focal length seen by the instrument's sensor. This is done with lenses within the instrument rather than mirrors.

They were conceived as identical, but the latter of the pair (FORS2) had some differences. FORS1 was one of the initial instruments on UT1 (the first of the four VLT telescopes), which began operation in 1999, and FORS2 was on UT2 when it began operation in 2000. FORS1 was retired in 2009 to make room for a different, newer instrument. FORS2 is to be replaced by FORS3, which is essentially FORS2 with some upgrades (the upgrades termed FORS-Up). However, FORS3 will actually be created from the retired FORS1: FORS2 will remain in operation until sometime near FORS3's completion, after which those desired FORS2 features that FORS1 lacks will be incorporated in it.