MXU - Multi-object spectroscopy with exchangable masks
In order to increase the number of spectra taken with one exposure it was decided to build a Mask Exchange Unit (MXU) into the top section of FORS2. This MXU was commissioned in August/December 2000 and holds pre-prepared masks with slits for a large number of objects, distributed over a useful field of view of 6.8'x5.7'. The masks are cut by the VIMOS mask manufacturing unit and are loaded by hand during the day. Ten masks can be stored simultaneously in the MXU and are available during the night.
The length (and hence their total number), shape and width of the slits can be specified by the observer. Details are given in the FORS User Manual.
An artist's view of the MXU inside the FORS top section.
An artist's view of the MXU. The magazine is filled with masks; one mask is in the focal plane observing position inside the MOS box.
An artist's view of the MXU. Ten masks are loaded, the lowermost is in observing position. The drive seen at left is the mask drive moving the mask into and out of the focal plane. The belt drive in the foreground serves to raise and lower the magazine for the selection of the proper mask (for loading/unloading as well as observation).