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Telescope focus

Once TAURUS itself has been focussed, the next stage is to focus the telescope onto the detector. The procedure is simply to take a series of CCD exposures with different values of the telescope focus, and measure the FWHM for each one. Then use pencil and graph paper to determine the optimum focus.

For each exposure use the FOCUS command to set the telescope focus. Then take an exposure using the EXPOSE command. Finally measure the FWHM of a star in the field using the DMS. The DMS functions X--FIND and Y--FIND can be used for this purpose.

The nominal telescope focus is at 97.45 mm. A typical focus run might involve taking a series of CCD exposures over a range of +-0.3mm (i.e. from 97.15 to 97.75 mm) stepping the focus by about 0.1 mm between exposures. A smaller stepsize of about 0.03mm might be appropriate in conditions of very good seeing.

An example set of commands to carry out this procedure might be as follows. All the commands are issued from ICL, except for FOCUS which is normally issued from the TCS and X--FIND which is issued from the DMS keyboard. Note that the actual procedure used will depend, for example, on the name of the detector being used.

Remember that the telescope focus will depend on which filter is being used in the focal plane filter wheel. Table 7 gives the offset in telescope focus required for different filter thicknesses for both interference (n=2.1) and glass (n=1.5) filters. The standard BVRI Harris glass filters used with TAURUS all have a thickness of 4mm, so should all provide the same focus. Table 12 lists the thicknesses of the interference filters normally used with TAURUS.

Once the telescope is focussed on the detector, it is necesary to check that the autoguider and field--viewing TV are also in focus. The procedure is as before to adjust the focus of these mechanisms, using the AUTOFOCUS and TVFOCUS commands, until the FWHM of images seen on the autoguider and on the TV system is minimised. The nominal values of AUTOFOCUS and TVFOCUS (as of 17/5/91) are 3000 AND 15000 respectively.

Thu Apr 7 00:29:52 BST 1994