Follow the steps below to focus the telescope for ALFOSC.

  1. Set the focus to the default value for ALFOSC by typing foc-pos 23230 on the TCS. This is also done by the script tcs.setup-tel-alfosc The value is automatically set to zero for foc-del when setting foc-pos.

  2. Load the focus field catalogue by using the command read alphot.cat in the TCS.
  3. Point the telescope to a focus field.
  4. Start guiding. It is recommended to use autoguiding.
  5. Setup ALFOSC for focusing with the pyramid. This can be done with the script focuspyr-setup [-filter]
    where the optional argument -filter leaves filter wheels as they have been setup by you,
    while the default option without this argument sets all filter wheels to OPEN. The setup can also be done manually:

  6. Take an exposure of at least 10 seconds by typing expose 12 in the sequencer window. For the focus fields typical exposure times are 10 seconds without a filter. In case the filter offset has to be measured, 15-30 seconds for broad band filters, and significantly longer exposure times for narrow band filters are required. The focus image will be shown in the DS9 window.
  7. Type focuspyr-auto in a sequencer window. The focus pyramid images will be analyzed automatically, and the calculated telescope focus offset will be displayed in the talker and sequencer windows. If you wish to analyse the focus pyramid image interactively, do the following step:
  8. Type focuspyr in a sequencer window. (This will automatically run focuspyr-auto in the image as well so offsets can be compared). Put the cursor on the upper one of the four images of one focus star in the display. Type a followed by q to measure the focus offset. Repeat this sequence on several stars to get a better estimate of the offset. With the cursor in the DS9 display, type q to escape from the focus measurement mode. The number of steps to move the telescope focus will be shown.
  9. Change telescope focus by typing foc-pos <new-value> on the TCS, or tcs.focus-position <new-value> in the sequencer.
  10. Repeat the focusing procedure at least once until a stable focus has been found.
  11. When finished: