The default `BLUE' and `RED' positions need to be set within GODOT using the `define red/blue position' commands in the GIRAF menu.
The nominal positions are:
Blue = 260 lambda = 3768--5560 Angstroms n = 103--152
To change from `BLUE' to `RED', or vice versa, the following motions occur, successively:
1. the prism rotates to the new position
2. the whole detector unit, carrying the Canon camera lens and the CCD dewar, rotates (`coarse position' movement)
3. the CCD dewar pivots a small amount with respect to the Canon lens, tilting the focal plane (`fine position' movement).
Moving from one position to another takes a total of 80 sec. Experience to date indicates that positions relocate to a pixel or better, although there are sometimes larger shifts on the first move. Of course it is important that arc exposures be taken between successive moves to keep track of such shifts.
It is possible to get intermediate wavelength ranges by changing the angle of the `RED' prism from its minimum deviation angle, pushing it bluewards of its nominal short wavelength limit. For example, setting the default `RED' position to 46200 shifts the whole wavelength interval bluewards, covering the wavelength range 4400-6700Å (n 86-129).