The DEIMOS internal focus can change significantly on a daily basis; hence, it's advisable to refocus the spectrograph each afternoon. Since the offsets between various filters appear to be stable over time, the standard procedure is to focus the instrument in direct imaging mode using the R filter and dome illumination. Once the focus in this fiducial filter is measured and set, the instrument's automated filter focus discipline software will adjust the internal focus as needed to account for the changes in the filter. This document describes the procedure for measuring the fiducial focus.

Acquisition

Focus is measured using dome illumination through the grid of holes masks in direct imaging mode with the R-band filter. Follow these steps to acquire a focus sequence in this mode:

• Once the telescope has been released on the Instrument Activity Schedule, verify on FACSUM that the telescope is pointed to the "dome flat position" (i.e., telescope elevation is 45° and the telescope azimuth value is 90° different from the dome azimuth). It's not necessary for you to turn on any dome lamps, since the script will turn on the appropriate flatfield lamp for your filter, and turn it off when finished.
• Ensure that the rotator is unlocked by clicking the Rotation button on the Dashboard to view the Rotator subpanel. If there is a button reading UNLOCK in red lettering on a black background, click it to unlock the rotator.
• From the OpenWindows pulldown menu, select the item DEIMOS Utilities > Run Focusloop (R+dome). This will configure the spectrograph appropriately for R-band dome images and acquire a sequence of 7 focus images every 1000 focus units from -5000 to -11000.
• Verify that 7 images are written to disk.

Measurement

The images are analyzed in IDL using locally-developed software. Follow these steps to analyze the focus data:

• From the OpenWindows pulldown menu, select the item DEIMOS Utilities > Analyze Focusloop. This will locate the 7 latest focus images and analyze them to derive the best global focus. The analysis takes about 10 minutes to completed.
• When finished, the script produces a plot and displays it on your screen in a ghostview window. Inspect the plot to verify that the overall morphology of the contours is similar to that seen in this sample plot. When a mask buckles, it looks like this instead. If the plot looks good, adopt the last value shown on the page (labeled Array focus) as the nominal focus value. If it looks bad, try the following:
  1. Rotate DEIMOS to a different rotator position (DEIMOS PA=-30 is most likely to succeed).
  2. Click the Retract Arm button on the slitmask panel to remove the GOH mask.
  3. Re-select the GOH mask from the main Dashboard panel.
  4. Take a single image using the internal lamps and check for incredibly out of focus images in the slitmask regions which look unreasonable on the plot.
  5. Once the image appears to be in reasonable focus across the entire mask, re-run the focusloop procedure.

Setting the focus

• On the Dashboard GUI, enter the nominal focus value in the Focus entry blank in the filter box.
• Double-click in the filter box to raise the filter detail panel. Change the filter from R to anything else and verify that the value of FOCUSOFF is non-zero and that FFD Status is Ready.

Return to the Afternoon Checklist.

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Last modified: Wed Dec 15 21:21:02 HST 2004