Troubleshooting the SCAM Guider

This page describes errors that the OA will see, not the observer. However, the SCAM is a unique guiding system that is unlike anything the OAs are used to, and it generates error messages that they can't readily interpret because it is so different. Here are a couple of the most common ones.

1. Can't open FITS file
2. Other "CAM Failure" messages
3. XGuide can't get a centroid on a visible star

1. Can't open FITS file

   The SCAM guider camera server always subtracts off a "sky" frame before displaying an image through the OA's XGuide. This makes the SCAM guider much more useful in the IR where the sky is typically much brighter than many objects of interest. This "sky" subtraction is handled very simple-mindedly: there is a copy of a SCAM image stored on the NIRSPEC host computer waimea as:

   /tmp/TEMPSKY2

   At the start of the night, this is usually the 1 sec test dark that is taken automatically when the instrument software is started. The OA can refresh this at any time SCAM guiding is working by clicking the button labelled "Make Sky" on the OA's NIRSPEC eavesdrop GUI. Clicking this button simply overwrites the current /tmp/TEMPSKY2 with the most recently exposed SCAM test frame, which is the most recent guider frame in SCAM guiding mode.

   It sometimes happens that the file /tmp/TEMPSKY2 gets inadvertently deleted, most commonly by a reboot of the waimea computer (reboots wipe /tmp clean). If this happens, the OA will get an error message in their XGuide window, saying something like, "CAM failure: can't open FITS file." This problem can be fixed by

   • Having the OA stop SCAM guiding
   • Checking that there is a scratch file by taking a short TEST image with SCAM
   • Manually creating a new "sky" frame by issuing the following command in any terminal window with a waimea> prompt:

     waimea> cp /tmp/TEMPFITS2 /tmp/TEMPSKY2

   • Having the OA restart SCAM guiding.

2. Other "CAM Failure" Messages

   The camera server process for SCAM guiding actually runs on the NIRSPEC host computer waimea. Sometimes the OA's XGuide software will not be able to start the camera server properly on waimea. The reasons for this are not clear, but the problem seems to be cured by the OA killing and restarting XGuide. This can sometimes take a few minutes, but it seems to work.

3. XGuide can't get a centroid on a visible star

   Sometimes XGuide will complain that it can't get a centroid on a star on the SCAM guider, even though the star looks perfectly bright enough and not saturated. This seems to happen more often with the SCAM guider than with the normal CCD guiders. The most common cause of this centroiding failure is an uneven background in the area around the guide box. You can usually correct this background problem by "making" a new sky frame:

   • Offset the telescope in any direction, far enough to put the star off of the SCAM field of view. SCAM's FOV is 46 arcseconds wide, so a 30 arcsec offset should be enough if the star was near the center (i.e., on REF) when you started).
   • Click "Make Sky" on your NIRSPEC Eavesdrop GUI.
   • Undo the offset to return the star to its original position in the SCAM guider.

   Now try the centroiding again.

   The sky brightness varies rapidly, on timescales of minutes, in the near-IR wavelengths that SCAM sees. Also, the sky brightness and response of the SCAM detector vary with wavelength. The guider software does a simple sky subtraction, but it continues using the same "sky" frame over and over until the OA clicks "Make Sky". Either the passage of time or a change of filters in NIRSPEC can make the guider's sky subtraction invalid. So, if you haven't done a "make sky" in 30 min or longer, or if the observers have changed NIRSPEC filters, doing a "make sky" is probably necessary.