The Autoguider Focus at Prime

  The autoguider focus at Prime appears to be stable, but should be checked occasionally. There are no autoguider (colour) filters at Prime, and also the autoguider does not look through the same filter as the CCD. However, it is most unlikely that the autoguider will be detectably out of focus, provided that it is initially set up at the nominal value for the (CCD) filter in use. Nevertheless, if the images look very extended and in particular if they have holes in their middles, then the focus should be adjusted.

The Prime autoguider focus is determined by performing an interactive `focus run' under ADAM control. To do this, first focus the telescope with no filter. Subsequently, acquire a star on the autoguider and step the autoguider focus through a range of values around the nominal one using the GDFOCPOS command:

GDFOCPOS value

where value is the test focus position in mm. Recommended values are from -3.3 to +3.3 mm. The autoguider focus drives much more quickly than the telescope focus, so you need not worry about starting an exposure with it out of position. After each step, press the FIELD button followed by WAIT on the Autoguider Panel and inspect the image. Select the focus position where the profiles produced (of a point source) are narrowest. Do not leave the autoguider integrating, otherwise the images will get mixed up. When you have discovered the correct focus position type:

SETGDFOCUS value

where value is your best focus position in mm. This command enables the automatic focus compensation, such that the autoguider focus is changed automatically on change of (CCD) filter. (If you do not type this command at the beginning of an observing session, the software will assume that the autoguider focus in the absence of a filter is +2.7 mm).