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Telescope and autoguider focus

It is necessary to check the focus of the telescope at the start of the night. The various means of viewing the field in order to do this are described in section v; there are three fibre light guides, one in a fixed position viewing the central part of the slit, and two which can be moved to view stellar images in any part of the field directly. In addition, an ISEC TV can be used to view most of the field reflected off the slitjaws, but at a rather coarse scale.

The focus of the ISEC TV and the slitviewer probe can only be adjusted manually, and should not need to be checked. However, the focus of the coarse and fine autoguider probes can be changed under computer control, and should be checked.

The first thing to do is focus the telescope. The ISEC TV provides too coarse a scale for this to be done accurately, and so it will be necessary to do this on one of the three guide probes. The focus of the telescope and the focus of the offset guide probes are interdependent, so until the focus of the offset guide probes has been determined it will be necessary to focus on the slitviewing fibre.

To focus on the slitviewing fibre, acquire a star onto the slit, and start the autoguider continuously reading out the slitviewer CCD window (section 25.1). Close the slit right down and move the dekker out of the way (type USLITWIDTH 20 MICRON and USLITLEN 10 ARCSEC at the ICL prompt). Use the TCS handset to move the star slightly off the slit, and then focus the he focus of the offset guide probes has been determined it will be necessary to focus on the slitviewing fibre.

To focus on the slitviewing fibre, acquire a star onto the slit, and start the autoguider continuously reading out the slitviewer CCD window (section 25.1). Close the slit right down and move the dekker out of the way (type USLITWIDTH 20 MICRON and USLITLEN 10 ARCSEC at the ICL prompt). Use the TCS handset to move the star slightly off the slit, and then focus the telescope. This will be difficult, since the field viewed by this fibre is extremely small !

A more reliable focus can be obtained by acquiring a bright star and taking quick exposures at a number of telescope positions. The FWHM of the stellar spectrum can be measured using either the DMS or IRAF on the SPARCstation. By plotting FWHM against telescope focus, the best focus can be determined.

It should be noted that the telescope focus is expected to be different to that obtained at Cass. It will also be different with and without the derotation optics mounted. Inserting the derotation optics requires the telescope to be refocussed by moving the secondary mirror downwards by 3.82 mm (see La Palma Technical Note 9). See the telescope log or the quick reference section of this manual for previously used focus settings.

Once the telescope is focussed, focus the autoguider by acquiring a star onto the coarse guide probe, and starting the autoguider continuously reading out the coarse guide probe CCD window (section 25.1). Adjust the autoguider focus using the UAGFOC command. The autoguider filters all have the same thickness (including the clear position, which is actually fused silica), so the focus is independent of which filter is selected.

Once the autoguider focus has been checked, at the start of an observing run, it should be fairly stable from night to night. It should then be possible to check the telescope focus each night by acquiring a star onto the coarse guide probe, and continuously reading out the CCD window whilst adjusting the focus using the TCS handset.

N.B. Remember-the WHT focus (and consequently the autoguider focus) will need to be adjusted to compensate for any change in the thickness of main-beam filters used (see sec. 16.3).