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Telescope focus

See section 9.4 for how to focus the telescope when using TAURUS in Fabry-Perot mode with the CCD.

The process of focussing the telescope onto the IPCS is as follows:

1. Find a suitable star for focussing. This should produce an acceptable level of illumination on the detector without the need to use a neutral density filter in the A&G unit, since use of these neutral density filters will result in a shift of the telescope focus.

2. Set the telescope focus to the lowest value to be covered in the focus run. Use the FOCUS command on the TCS to do this.

3. Start an exposure using the EXPOSE IPCS command.

4. Once you have adequate signal-to-noise, close the IPCS shutter using the IPC command, increment the telescope focus, and offset the telescope using the TCS handset by about 5 arcsec.

5. Repeat until the telescope has clearly gone through focus.

6. Finish the IPCS exposure. Measure the FWHM of each image as described in section 9.4, and determine the position of best focus

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. Table 12 lists the thicknesses of the interference filters normally used as order-sorting filters with TAURUS.