SAAO - South African Astronomical Observatory

Spectrograph Fibre Positioning




Wavelength Range

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
Red = 45400 lambda = 5207--10413 Angstroms n = 55--110


To change from `BLUE' to `RED', or vice versa, the following motions occur, successively:


1. the prism rotates tex2html_wrap_inline533 to the new position


2. the whole detector unit, carrying the Canon camera lens and the CCD dewar, rotates tex2html_wrap_inline399 tex2html_wrap_inline477 (`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 tex2html_wrap_inline399 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 tex2html_wrap_inline541 4400-6700Å (n tex2html_wrap_inline541 86-129).