Finding the best focus at Prime

There is no TV camera at Prime, so interactive focussing is not possible. The telescope is focussed by making a multiple exposure on the CCD (for a full description see section ), stepping both focus and telescope between exposures.

1. If not done already, startup ADAM
2. Move the rotator to a cardinal point, preferably 180, with

   ROTPOS 180

3. Move to a not very bright star and take a GLANCE exposure of between 5 and 10 seconds. After the image appears on the LEXIDATA display go into PHOTOM and make sure that the star image is not saturated. If it is, select another object and repeat.
4. Check whether the star is in the correct position on the chip: with the rotator at 180 the orientation on the LEXIDATA display is: North up, and East to the right. The telescope will step with 20between exposures with a double step between the first and second exposure. Is there enough space on the chip for your intended number of steps and stepping direction? If not, move the star to a position where it is and confirm with another GLANCE.
5. Start the multiple focus exposure with:

   FOCUS

   and follow the prompts. Use 7 exposures for a first setup, 5 for subsequent fine-tuning; provide a negative number of exposures if you want to use a negative increment; use an exposure time between 5 and 10 seconds. The focus is stepped by 0.1 mm between exposures; the current focus is in the middle of the range.

6. After the FOCUS procedure has finished, the multiple image will appear on the LEXIDATA display. It will normally be obvious which images are closest to focus and these should be measured using the PROFILE option of PHOTOM and the seeing determinated. If you find yourself far from focus, then make an educated guess about its approximate value and move the focus to that value via the User Interface, followed by a repeat of the FOCUS procedure.
7. Log the current value of the Seeing.

Beware of starting an observation until the focus has stopped driving and the red focus stop lamp on the Control Desk is lit.