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It will be neccassary to determine the tilt of the CCD camera and adjust the CCD dewar on the kinematic mount so that it lies coplanar. The CCD dewar tilt can be adjusted by means of the three capstans; A, B and C.
In order to determine the degree of tilt a Hartmann test must first be performed. This involves moving the Hartmann unit into the light path and then taking two arc exposures with the Hartmann shutters in the up and down positions. Measuring the Hartmann shifts at the edges of the detector then gives the degree of tilt. For maximum accuracy the spectrograph must be roughly in focus before carrying out these tests. Hence it may be neccessary carry out the collimator focus as described in the following section before correcting for tilt.
The following commands are required at the ICL prompt in order to take the exposures to measure tilt, issue the following commands at the ICL console (here it is assumed that the echelle has been configured to the desired central wavelength and that the slit length has been set):
The Hartmann shifts in the positions of the arc lines taken at the edges of the CCD frames should now be determined. At present the easiest way to mesure these shifts is to extract portions of individual orders of the arc from each of the corners of the image and one from the centre. Then collapse these down to form 1-D spectra and cross correlate the corresponding spectra from the `UP' and `DOWN' images.
(N.B. At a future date FIGARO and/or iraf routines will be available to perform these cross-correlations automatically: please check with your Support Astronomer if these routines are available.)
Depending on the degree and direction of the tilt it will be neccessary to adjust the CCD dewar capstans. Below the effects of moving the capstans are indicated: