Background

DEIMOS masks can occasionally jam or get cross threaded and someone needs to manually set things right. There are multiple ways things can go wrong here. The description below is for the case of cross threaded masks. Certain other problems may pose a danger so proceed carefully and consult with the staff astronomer as you go.

Procedure

  1. Have the staff astronomer move the grabber arm out of the way (if possible) by hitting the Retrack arm button on the slitmask monitoring GUI, which can be launched from the Actions menu on the DEIMOS instrument control GUI.
  2. Rotate the instrument remotely to -90° from the DEIMOS rotator GUI. Alternatively, use the hand paddle: set the paddle to manual, elect mode 2, select position 4, and push and release either the FWD or REV button.
  3. The slit mask access door should now be on the same side as the hand paddle (right side when viewing from back of instrument, looking toward telescope).
  4. Safety first! Set the band brake. See figure 1.
  5. Open the access door with a flat blade screwdriver, pulling the handle out and pushing door down. Screwdriver should be on tether beside the handpaddle.
  6. Figure 2 shows the slit mask mechanism. Undo the two red knobs (captured) and flip the little slit mask door down.
  7. Pause for a moment. Is the grabber arm all the way to one side allowing any or all masks to be removed? If not, it could move suddenly. Be careful.
  8. You should now be able to look down the lengths of the masks and see if any are cross threaded such that two share the same slot in the back.
  9. You may be facing a problem other than a simple cross thread. Describe what you see to the support astronomer. If it is a simple cross thread, pull the offending mask out a bit and try to wiggle it into its correct slot in the back.
  10. If the grabber arm is stuck somewhere other than all the way to one side so that masks can be pulled out do not proceed further without instructions. If an attempt is to be made to free the grabber arm, instrument air should be released for safety reasons. See air pressure reset procedure.
  11. If for some reason a bent mask is suspected you may be asked to remove one or more masks and examine them. If so, the best way to proceed is to look at one mask at a time. Pull it out, look at it, put it back in its original slot.
  12. After closing the access panel, release band brake and return hand paddle to computer mode.

Figure 1. DEIMOS band brake

Image showing DEIMOS band brake

Band brake. Push lever down to engage. Pull up to release.

Figure 2. Slitmask mechanism

Image showing slitmask door

Door to release for slitmask removal.