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The cryostat may be mounted on the 235 or 500 mm cameras; in both cases it is fitted in the same orientation - with its fill tube uppermost above the horizonal axis. A circular mounting ring is bolted onto the spectrograph - this clamps the cryostat with a three-point kinematic location; it allows a tilt/focus adjustment as well as rotation of the cryostat (figure 4.8).
Rough focus, rotation and tilt should only need adjustment when the camera head is changed. This is a difficult process which is performed by local technical support. Fine focus (with collimator) should be checked each night and when extreme grating angle changes are made. It is possible to remove the CCD cryostat and then replace it on the same spectrograph camera without adjustment.
The appropriate CCD field-flattener lenses for each camera must be used. On the 235 camera only an additional 3mm aluminium spacer ring is used. When fitting the mounting ring to the spectrograph take care not to adjust any spectrograph mirror screws!
For the recommended micrometer settings, see the CCD Log-Book in the control room. The micrometers read to 0.01mm, with 0.50mm per revolution. In order to adjust any of the three support points it is important to back off all three micrometers to avoid damaging them, as they are for reading the position only. The tightening knobs should be slackened slightly to allow the three capstan screws to jack the cryostat in or out. One revolution of the capstan = 0.63 mm. (The micrometers are shortly to be replaced by an electronic sensor).
Clockwise rotation of the capstan screws moves the cryostat outwards and results in a reduced micrometer reading. This corresponds to a positive Hartmann shift if measured Left (HARTMANN 0 1 or OUT IN) then Right (HARTMANN 1 0 or IN OUT) The cryostat is only retained in place by the three clamping knobs so take care to support it when they are loosened.