CDS has six slits available for use. Their sizes are:
1) 2"x2" - the standard for GIS use
2) 4"x4" - the alternative for GIS use
3) 8"x51" - not normally used for science observations
4) 2"x240" - the standard for NIS use
5) 4"x240" - the alternative for NIS use. More counts and little loss of spectral resolution over 2"x240"
6) 90"x240" - the wide (non-spectral) mode slit. Used for 'movies'.
To reduce the areal coverage of an NIS raster it is also possible to use only part of the data from the long slits (numbers 4-6). This is done during planning by the routine mk_raster .
All the slits are machined into the one mechanical assembly. This assembly can be scanned (in a Solar N/S direction) to allow rastering with GIS. The planning software does not allow slit movement for NIS observations.
Ideally the home position for each slit should be identical with reference to the optical path. There does, however, appear to be a slight offset which should be taken into account when comparing data taken with different slits.