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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:
- 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.
- 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.
- Start an exposure using the EXPOSE IPCS command.
- 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.
- Repeat until the telescope has clearly gone through focus.
- 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.