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Rotation alignment

Data reduction is greatly simplified if the dispersion direction is along one CCD column. A check for rotation of the CCD can be performed as follows. It is best to use a high dispersion (600 or 1200 lines) grating set for the red (6000-8000 Å ).

IDS OPEN        (camera shutter open)
     DEKKER 1        (narrow dekker) with IPCS dekker; DEK 3 with FOS dekker
     TUNG ON
     COMPMIRROR IN   (light from calibration lamps through the IDS)
     SLIT 180
     GLANCE 10       (a 10 sec trial exposure)
     PHOTOM          (to take a horizontal profile)

Use PHOT to take horizontal profiles (figure 4.9) at (a) and (b), i.e. near top and bottom of detector. The effective line centres are printed out. It is best to take the profiles over only about 10 elements and perhaps to sample in one or two places at top and bottom. It is possible to align the rotation to within 0.1 or 0.2 pixel over the length of the CCD. This method gives good setup of rotation; it assumes that the slit is normal to the dispersion direction.

Rotation adjustment if necessary, is achieved by manual movement of the cryostat; the mounting ring has six slotted holes for this purpose (figure 4.8). The 6 bolts should be slightly released and the cryostat should be rotated out of position so as to approach the ideal position in one direction only. The micrometer D should be used only to measure rotation position (not to move the cryostat) although a small degree of backlash/flop can be experienced. An increase of 0.35 mm in the micrometer reading will rotate a dispersion column by about 1 pixel in 500 anticlockwise. The rotation normally only needs setting when the CCD is first installed on the camera, and this will be done by a DT or a SA. Observers should not attempt to make these adjustments themselves.