6.1. Cassegrain guiding

At Cassegrain focus, the CCD has all of its image area available. In order to maximise the loop time, a small window of default 15 x 15 pixels is read out each time round the guide loop. This window can track the guide star around the CCD when the guide loop is open, and the error signal will bring the guide star back to its original position as the telescope corrects its motion with the loop closed. The tracking window is shown on the display with a black border so the movement over the CCD with the guide loop closed can be seen with a trail of black square edges. If all is well, the window should not move as the telescope can follow the guide star to sub pixel accuracy.

To start the autoguider outputting errors, first acquire and select a guide star as described above, then start the guide loop running with GUIDE ON . The display will show the small window round the guide star which will be updated regularly. The statistics given at the right hand side of the display will be updated one at a time. To stop the autoguider outputting errors, enter GUIDE OFF . If you wish to use a different star from the starlog to guide with, enter GUIDE ON n to select the nth star and start guiding with it. This will save having to use the select command explicitly.

The guiding integration time can be changed with the GUIINT command. All time values at the engineering terminal are in milliseconds. At Cassegrain, the limits are from 100 mS to 50 seconds for the guide integration time ( for a 15 x 15 window ). This command may take some time to operate

If the seeing is bad, the size of the tracking window can be changed with the GUISIZE command. This is useful when the guide star is too large for the tracking window, or when the guide star is dancing around. Remember that as the window size increases, the minimum integration time also increases so use this command with care. The system will not crash, but it may drop out of the guide loop unexpectedly if the tracking window size is increased unreasonably. To get an idea of how close to the wind you are sailing, enter TIMING and look for the time left over after displaying the CCD window.