5.1. Cassegrain Acquisition.

At Cassegrain focus, the Autoguider has the full field of view of its CCD available for guiding, and several guide stars would be expected to fall upon it. The process of selecting a guide star is reasonably simple, in principle, though having several stars in the Field of view can cause problems. A very bright star may cause the CCD to saturate, even with small exposure times, and so it may be necessary to choose a star on the CCD image which is not the brightest one there. The stars are entered into a log as they are found in order of brightness, and any of the stars in the log can be selected to guide on. If there are too many stars to get the desired one in the log, or one is so bright that much of the CCD area is saturated, then the search area can be restricted to a small rectangular window on the CCD.

None of these methods is usually needed, as the brightest star on the image is usually acceptable for guiding with. The easiest solution is to use the AUTOGUIDE command which does an acquisition, looks for the brightest star on the field and immediately starts tracking it. If this command fails to lock onto a star, it will exit gracefully, with the error message:

Less than n stars in window !

and one of the other methods may be tried. If you can see some images on the screen then just increasing the intergration time with ACQINT n may do the trick. Otherwise, check that there is not a ND filter in the Autoguider light path.

If you know that there will be several stars on the field suitable for guiding, enter FIELD 8 to acquire a full frame image and search for the 8 brightest stars. The star log will be displayed on the terminal. The brightest one will have been selected, an arrow will appear beside its entry in the log and a cursor will be placed over the centre of the star on the display. To select the nth star in the log, enter SELECT STAR n, the log will be re-displayed with the arrow beside star n and it will be marked with a cursor. Any of the stars in the log can be selected in this fashion. The autoguider should now be ready to start guiding.