Slit and dekker assembly

The slit and dekker slide are mounted from inside the main casing as a single unit. A shutter mechanism is adjacent to the slit.

The dekker has four positions defined at follows:

1. Slide fully ``in'': whole slit covered.
2. Inner detented position: star slot uncovered in which the central 2,mm of slit is exposed.
3. Outer detented position: arc slots uncovered in which two 2mm portions of the slit each side of the central 2,mm are exposed.
4. Fully ``out'': the full length of the slit is exposed being some 20,mm. This position would normally only be used for acquisition purposes.

The dimensions and location of star and arc slots are arranged so that their respective spectra do not overlap. 20,mm slit lengths project to approximately 0.5,mm wide spectra at the camera focus.

The slit width is fixed but the whole unit can be replaced for a unit having a different width. Initially two units have been provided widths of 70,m and 100,m corresponding, to I and 1.5 arc seconds projected onto-the sky. During the commissioning period it was determined that slits of equivalent width less than 1 arcsecond would find less use than a 1.5 arc second slit. Additional slit units may be available in future, depending on demand.

The slit to detector reduction factor of 0.28 means that a slit width of 1 arc second projects to 20,m at the detector (neglecting foreshortening effects). Geometrically the optics should produce images of width less than 10,m for a zero width slit object.