MOS - Multi-Object Spectroscopy with movable-slits

Multi-object spectroscopy (MOS) is one of the fundamental observing modes of FORS2.

Specification of the standard FORS MOS Mode

FORS2 can observe up to 19 objects simultaneously. The chosen design principle is to produce the MOS slits by pairs of slitlets which are individually driven by a motor via a precision spindle, controlled by an incremental encoder.

[FORSMOS unit]

the MOS unit as seen from above.

[FORSMOS slits]

MOS slit jaws in more detail


Specifications and performance of the FORS MOS mode are:

  • field of view: 6.83' x 6.83'
  • slit width: larger than 0.3'', no upper limit
    • setting accuracy of slit width: +/- 5% or +/- 10 µm, whichever is greater
    • straightness of the slit edges: +/- 0.5 µm (measured)
    • parallelism of the slit edges: +/- 5 µm (spec)
  • slit length: 22.5''
  • setup time for new slit pattern: less than 25 seconds
  • setting accuracy of slit centre position: +/- 13 µm or +/- 0.025'' in the focal plane

The slitlets also serve as a strip mask for imaging polarimetry in the standard resolution mode: alternating slits are closed and opened to form the strips.