Managing Autofocus Vibrations

The telescope autofocus system updates the secondary position at intervals based on elevation and temperature using a lookup table. The problem for MagAO-X is that this imparts high-frequency vibrations (significant on lambda/D scales) on the telescope. For the best residual tip/tilt performance we can operate with autofocus off with the following procedure.

Clay TCS display showing the status of autofocus

The Clay TCS display. Circled in red is the autofocus status, here shown off. When on, the word “Focus” turns green.

Important Things

  • The telescope operator has to manually disable/enable autofocus.

  • When autofocus is disabled, we can not dump focus to the telescope. This is the key caveat to using this procedure.

  • Turning autofocus back on after a period of no corrections will throw the star off the pyramid, breaking the HO loop.

  • Be sure to re-enable autofocus before slewing to a new target.

  • Plan to perform this procedure 30 minutes before transit even if the threshold hasn’t been reached.

Procedure

  1. TO slews to the target with autofocus on as usual.

  2. Perform MagAO-X acquisition and close the loop

  3. Once the loop is closed, wait about 10 sec, then dump focus to the telescope

    • If the amplitude is very large (>~50), wait another 10 sec and dump again

  4. Now ask the TO to disable autofocus

  5. Begin taking data

  6. Monitor the focus build-up on the woofer

    • You can press the focus dump button and observe the amount that would be sent, even though the command will have no effect.

  7. Once the amplitude gets large (>~ 75) it is time to fix it. Do the following:

    1. Make the science cameras EM gain safe, i.e. close the shutters (this is a good chance to take a few darks)

    2. Open the loop.

    3. Ask the TO to enable autofocus. This will likely throw the star off the pyramid.

    4. put in flipacq and turn on camacq

    5. take a camwfs dark because it’s a good time to do it

    6. Move the telescope to re-acquire the star on the pyramid

    7. remove flipacq and turn off camacq

    8. close the loop

    9. ask the TO to disable autofocus

    10. return to you regularly scheduled data taking

  8. Repeat step 6 and 7 until you are done with the observation

  9. Open the loop

  10. Ask the TO to enable autofocus

  11. Goto step 1.