LUCIFER-MOS is a liquid nitrogen cooled near IR multi object spectrograph imaging 20 freely selectable sub-fields of about 2.2 inch by 1.8 inch and 6 by 4 image elements each on the entrance slit of the LUCIFER spectrograph. The image elements are re-arranged by 480 fused silica fibers of 50 micrometers core diameter and 100 micrometers total diameter with integrated, hexagonal lenslets of 0.6 mm width corresponding to a 0.3 inch field. The pre-optics magnifies the telescope image by a factor 3.3, thus adapting the telescope plate scale to the lenslet scale, and additionally providing a cold stop. The post-optics converts the f/3 fiber output beam to the f/15 beam accepted by the spectrograph. Each of the 20 6 by 4 fiber arrays together with its pre-optics is mounted in a spider arm which can be freely positioned within the 200 mm diameter field of view by a cryogenic robot. The robot performs three rotational movements to position the spider arms and is driven by cold stepper motors. The spider arms are locked in their positions by two permanent magnets each. Their magnetic field can be compensated by coils to unlock the arms and move them across the field of view.
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