If you are nodding along the slit using either the Slitmove widget or the Echelle Format Simulator and you notice that the star does not land on the slit, there are two possible causes.

First, if this is right after a telescope slew, check to see that you have reselected your slit PA after the slew. The NIRSPEC rotator is supposed to maintain physical angle during a slew, which means that after the slew it just begins tracking at whatever sky PA it lands on for that physical angle. You must reselect your sky PA by clicking Set under the "Slit PA" or "Scam PA" columns on the rotator GUI. See this description for more information.

Second, if you've already reset the the sky PA and you still find nodding inaccurate, the most likely remaining cause is that the image rotator has dropped some motor steps. The cryogenic mechanisms have no encoders, so the software must rely upon counting motor steps sent to the mechanisms since the last initialization. The rotator is known to drop steps occasionally, and if that happens then all further slit nodding will be inaccurate, because the rotator is not where the software thinks it is.

Fortunately, the solution is simple: re-initialize the image rotator. It is not necessary shut down any of the rotator software or turn off "rotator tracking." 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).

Slit nodding should now be much more accurate.

Dropped rotator motor steps are most common during slewing. You can avoid unnecessary rotator slewing by making sure that the PRESROT keyword in the DCS software is always set to 1 or phys. See this note for more information.