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Collimator Focus

The final focus is achieved by moving the ISIS collimators, in practice this is all that most users will have to do. The grating setting, set by the ICL procedure CENWAVE, and the CCD window, set by the ICL procedure WINDOW, should be appropriate for the observations of the coming night.

The collimator position can be set from the ICL interface by typing:--

BCOLL <position> where the position is in the range 0-52000
RCOLL <position> where the position is in the range 0-30000

The spectrograph works best with the collimators near 8000, except if the blue camera is in use with the folding prism, in which case the focus position should be near 30000. If focus is obtained far from these values the spectrograph will be astigmatic, and although the comparison lines may be in focus the resolution along the slit will be degraded. In this case it will be necessary to rotate the three capstans by the same amount to bring the CCD into approximate focus near the optimum value of the collimator position. Movement of the capstans is described in section 8.3.2. One complete turn of each capstan will move the CCD with respect to the focal plane of the spectrograph by 0.5 mm, which will cause a Hartmann shift of 3.5 CCD pixels. This is equivalent to approximately 10000 units of collimator movement. A clockwise rotation of the capstans will be compensated by a negative collimator movement on either arm.