Note: NIRIM is
mounted so that the nominal “home” position is at -90 degrees. One issue this
creates is that with NIRIM, the WNIR rotator must be limited in its range of motion, not the full +/-180d that is normal.
Another issue is that when the rotator is initialized, you must bring WNIR back
to -90d as soon as the initialization is
finished! The reason is so that the cryogens are not dumped on the floor
when tracking an object. We estimate that JUST during the
initialization process the dewar loses 0.5+ liters on LN2 (out of 7
total liters).
To limit this, the following command is
used:
·
cli% wnir limit set
<angle-a>, <angle-b>
where angle-a and angle-b are the - and
+ limits, (typically -179d and 179d)
For NIRIM, this should be -175d, -25d. You can set it to lower
values to be extra conservative.
Of course, this will create a tracking issue in most cases as the WNIR will want to be at some angle beyond the spill limits. To get around this, you have to set the WNIR OFFSET angle such that the rotator tracks and will track through the length of exposures. YOU'VE GOT TO KEEP A CLOSE EYE ON THIS. Please round the offset angle to 0 or 90 such that the orientation of the detector on sky is oriented favorably, otherwise it could be nearly impossible for the observers to determine what the orientation of the imager was when they are reducing their data. We suggest that you include a comment in the observing procedure logfile. Note: The offset angle may change from 0 to 90 if the same object is observed for a long time. NOTE IT!
NOTE: YOU'LL HAVE TO SET ROTATOR LIMIT EACH AND EVERY NIGHT, or after the "startup.tcl" script has been sourced.