On the XNIRSPEC GUI, left-click on the "Irot" button, then left-click on the "IROT Tracking On" option. This must be done at the start of the night. Then, on the Image Rotator GUI, select an angle and a mode in the lower half of the GUI: fixed physical angle on the lower left, fixed slit PA on the sky on the lower middle, or fixed SCAM PA on the sky on the lower right. Type the desired angle in the appropriate text entry box, then click the "SET" button under that box. In the upper half of the GUI you should see a red frame around the display for that mode while the rotator is moving. The frame should turn white when the rotator has acquired the requested angle.

If the red frame never turns white, even after 30-60 seconds, try left-clicking on the "IROT Tracking Off" option under the "Irot" menu button, waiting 5-10 seconds, then left-clicking on "IROT Tracking On" and click "SET" in the Rotator GUI again.

If the red box still does not turn white, most likely, the rotator software is hung and needs a restart. To do this, right-click in the background and select "Restart Image Rotator" from the "NIRSPEC Control Menu". This will kill and restart the rotator software (called watch_keywords).

At the start of the night, before running MIRA, it is best to be in stationary mode at physical angle 0.0 degrees. So if you are starting up and you've just left-clicked on "IROT Tracking On", type 0.0 in the lower left of the Rotator GUI, the left-click the "SET" button to set that physical angle.

PLEASE NOTE: The semantics here are unfortunately somewhat confusing. Left-clicking on "IROT Tracking On" does not actually make the rotator track a sky PA. The first time it is issued, it starts a task called irot_wrapper that must be running for the rotator to work and sets the NIRSPEC keyword "rottrack" to 1. Subsequent cycles of IROT tracking On/Off merely change the "rottrack" keyword to 1/0 respectively.

If the rotator becomes unresponsive, the rotator server process called watch_keywords may need to be restarted or the rotator may have hit a limit. To restart the rotator server, right-click in the background and select "Restart Image Rotator" from the "NIRSPEC Control Menu". This will kill and restart the rotator software (called watch_keywords). If this does not solve your problem or if FACSUM indicates that the rotator is "Off", you may need to re-initialize the rotator. Simply re-initialize, as follows:

• Tell the OA that you need to re-initialize the rotator, and that the OA should stop guiding.
• On the top XNIRSPEC panel, left-click on the button labelled Engineering. You'll see a popup menu.
• On the resulting popup menu, left-click on Motors. You'll get another popup menu.
• On this second popup menu, left-click on Init. You'll get a grey popup window.
• On this popup window, left-click on Image Rotator, then Init, then Dismiss. The popup window will disappear, and the rotator will begin initializing.
• Watch the status messages on the top panel of XNIRSPEC. When the rotator is successfully initialized, enter your desired PA in the Image Rotator GUI, and click the "Set" button under your desired mode.
• Ask the OA to re-acquire your object if necessary, and to restart guiding.
• Ask the OA to report the problem in a nightlog ticket, and give the OA your estimate of the amount of time lost to the problem (including any inefficiency or lost exposures before you fixed it by re-initializing).

The DCS keyword PRESROT defines what the rotator will be asked to do during a telescope slew. The possible values for this Boolean keyword are 0=sky, which means "preserve the PA on the sky during the slew," and 1=phys, which means "preserve the physical angle you're at right now during the slew." PRESROT should always have a value of 1 or phys for NIRSPEC, because of the limited range of physical motion of the NIRSPEC image rotator (-90 degrees to +90 degrees). If PRESROT = sky for NIRSPEC, the rotator will work very hard during telescope slews and probably slew into a limit, requiring a rotator intialization.

One consequence of keeping PRESROT = phys is that the user MUST reset their desired PA after every slew. If, for example, the user is tracking a science object at SlitPA = 0 degrees, then slews to a calibration star 15 degrees away, the rotator will maintain the physical angle it had on the science object, then just begin tracking on the cal star from that same physical angle, which will of course yield a different SlitPA on the sky for the cal star. There are also reports that after long slews, the rotator is so far from the last requested PA that the telescope pointing and slit nodding become inaccurate, because the demanded PA and actual PA are so different.

To get the same SlitPA on the cal star, after the slew, the user would have to enter 0 degrees and left-click "SET" on lower middle panel of the Rotator GUI. PLEASE NOTE also that this resetting of the desired SlitPA should be done after the slew is complete. Resetting while the slew is in progress will cause the reset to be inaccurate, as it will be based on the instantaneous telescope position when you click "SET" on the Rotator GUI.

In normal operations, the image rotator tracks in different directions depending on the location of the object in the sky:

• Tracks CLOCKWISE if the object is SOUTH of Mauna Kea (i.e., Declinations SOUTH of about 19 deg 50 min)
• Tracks COUNTERCLOCKWISE if the object is NORTH of Mauna Kea (i.e., Declinations NORTH of about 19 deg 50 min)

Pay attention to the current physical position of the rotator when selecting a new PA. As you type numbers into the SlitPA or SCAMPA text entry boxes, the lower left Physical panel will preview the physical angle of the rotator at the current telescope pointing and your new PA. If there is a chance that the rotator might track into a limit, then choose instead the 180-degree complement of your desired angle and check the preview to see that this gives you a safe amount of tracking range.

There is an arrow in the upper left "Physical - Actual" part of the Rotator GUI that attempts to represent the tracking direction of the rotator.