Next: 2 MECHANICAL DESIGN
Up: Versatile multi object spectroscopy
Previous: Versatile multi object spectroscopy
Multi object spectroscopy (MOS) is a simple way for increasing the observational efficiency of a telescope and instrument. It was from the beginning one of the motivating forces of the FORS concept. It was designed to meet the following requirements:
The number of 19 slits that will be realized in FORS is given by the overall field of view of the instrument (see Seifert et al., these proceedings) and by the length of the individual slits. The latter in turn is determined by (1) the need for a reasonable amount of sky area in the slit needed for accurate sky subtraction which may limit the obtainable spectrophotometric precision, (2) by the mechanical properties of the components used.
At the beginning of the FORS project a study was performed to compare various multi slit concepts. All competing concepts (like star plates or opaque thin films cut mechanically or with a laser) had to be abandoned because they could not fulfill one or more of the above requirements. Especially the fast and reliable operation and the freedom from operator intervention appeared to be best satisfied by the concept of individually movable slitlets which was finally chosen.
The main constraints of the slitlet concept in terms of slit number are imposed by the physical size of commercial motors, encoders and spindles for the drives and the mechanical stiffness of the slitlets. A slit length of 12 mm turned out to be the best compromise. The most powerful asset of the slitlet concept is however the fact that all slitlets can be moved individually and simultaneously. This results in a great variety of observational possibilities which also help to fulfill the functional requirements.
All slits can be configured independently of the others, in particular it is possible to set different widths for different slits or to close single slits during an ongoing exposure. It is also possible to do a test exposure through the slightly opened slits, without grism, to check the exact centering of the objects in the slits.
To minimize defocussing, adjacent slitlets are slightly offset in height so
that they move along a cylinder with the radius of the telescope focal
plane (
mm).
In the imaging modes a single slitlet (or more if necessary) can be moved into the field to mask objects which would otherwise saturate the detector.
An additional function is to establish a strip mask for imaging polarimetry in the standard resolution mode. According to the concept described in [2] the splitting angle of the Wollaston prism and the apparent width of the slitlets were chosen to match each other. Since the image scale is different in the high resolution mode, a separate strip mask is necessary which will be inserted under remote control.
For high precision longslit spectroscopy an additional mask will
be used. It contains 9 slits with fixed widths in steps of
any of which can be selected by a movable decker.
Both polarization and long slit masks will be cylinder shaped to adapt to the curvature of the telescope focal plane.
Next: 2 MECHANICAL DESIGN
Up: Versatile multi object spectroscopy
Previous: Versatile multi object spectroscopy