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

1. Verify that the telescope is released on the Instrument Activity Schedule. Telescope status should be set to
   • "Ready"
   • with green background color
2. Confirm telescope pointing. On FACSUM verify that:
   • telescope elevation is 45°
   • telescope azimuth value is 90° different from the dome azimuth
3. Ensure that the rotator is unlocked:
   • click 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.
4. From the window manager pulldown menu, select the item

   	DEIMOS Utilities > Run/analyze focusloop (R+dome)

   This will configure the spectrograph appropriately for R-band dome images, acquire a sequence of 7 focus images every 1000 focus units from -5000 to -11000, analyze the results, and display a focus map and best focus value.

When finished, the script produces a plot and displays it on your screen in a ghostview window.

1. Inspect the plot to verify that the overall morphology of the contours is similar to that seen in this sample plot.
2. If the plot looks good, adopt the last value shown on the page or in the xterm output (labeled Array focus) as the nominal focus value. Proceed to Setting the Focus below.
3. If it looks bad (see example of a buckled mask resulting in a poor focus like this), 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.
4. Proceed to Setting the Focus below

Setting the focus

1. On the Dashboard GUI, enter the nominal focus value in the Focus entry blank in the filter box.
2. Double-click in the filter box to raise the filter detail panel.
3. Change the filter from R to anything else
4. Verify that the values of:
   1. FOCUSOFF is non-zero
   2. FFD Status is Ready.

Return to the Afternoon Checklist.