9 Instrument set-up (staff)

9.1 Spectrograph alignment

9.1.1 Principles

The alignment of MOS or SIS involves three separate adjustments:

A properly aligned MOS or SIS means that:

We can note that since MOS and SIS share the same set of grism wheels and, once the grisms are aligned for MOS, the grism wheel can be installed on SIS. The SIS alignment will only consist of a CCD rotation until spectra run along the CCD columns. There is then no need to align grisms on SIS, if they were properly aligned on MOS.

Of course, this is not valid for grisms installed in the MOS/ARGUS wheel, since the orientation here is at 90 degrees to that expected in MOS or SIS.

9.1.2 Alignment procedure:

To properly align SIS, proceed as follows:

Once SIS is aligned, and if you need to align MOS:

9.2 Spectrograph focussing

The internal focussing of the spectrograph is done by moving the SIS camera focus. Each filter in the filter wheel will have a specific camera focus; this is due to the fact that the filters are not accurately plane.

To focus the camera, proceed as follows for each filter:

Note that the values are significantly different for different CCDs (for instance, close to 1000 for Loral 3 and close to 3000 for Lick2).

9.3 CAF set-up:

Once satisfied with the internal focussing, the zero point for the Computer Aided Focus is ready to be computed. CAF is normally set with a zero point which indicates the difference Y between the two images produced by the Hartmann focussing device to be taken out of the CAF computation when the telescope is exactly in focus. Inversely, when you know that the spectrograph is in focus, (such as after focussing the camera) setting zero point = 0 will force CAF to compute Y, hence giving the zero point. This comes from the linear formula which transforms Y into telescope encoder motion:

Proceed as follows:

9.4 OFFSET set-up:

The fine offset calibration is done on the sky with the ``autocal'' tool. Over several runs we found that a new calibration was not needed for each new run, provided that the alignment procedure as defined above is thoroughly executed. However, if you need to calibrate the fine offset, proceed as follows:

The calibration values are then entered into the respective ``offset'' fields. The ``autocal'' programme maps telescope coordinates versus CCD coordinates and allows very accurate offsetting.

9.5 Checking the LAser MAchine:

The set-up procedures should include the following:

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