Accidental application of on-line SelfCal

In 2006 the command "selfcal on/off" was added to CAOBS to allow selfcalibration of the array in tied array mode (for NASA tracking). We recently noticed that this mode could be switched on accidentally by typing e.g. "sel aa" (a VIS command) in the CAOBS window. Going back through the logs we've found that 5 observations have been affected by this. The CAOBS problem has now been fixed and a miriad task "atscfix" has been written to correct the data.

The effect of the on-line selfcal is visible on calibrators, the phases gets calibrated to near zero every 2nd integration cycle. For weak sources the phases have 'random' offsets applied every 2 cycles. Note that only the first frequency band of dual frequency observations is affected.

The following date ranges are affected by this issue:

• 17-DEC-2007 04:37:49 to 17-DEC-2007 15:02:51, C1621 Norris
• 21-JUL-2008 22:09:21 to 23-JUL-2008 06:22:41, C1865 Testi, C1727 Macquart
• 07-SEP-2008 10:49:51 to 07-SEP-2008 22:30:41 C1851 Norris, C1654 Feain

To correct the data follow this recipe:

• Reload the data using atlod, selecting the first frequency band with ifsel=1
• Copy the file with selfcal corrections from the list below to your working directory (name it selfcal.txt or similar)
• Run the atscfix program using the following command:
  atscfix vis=mydataf1.uv out=mydataf1-fix.uv scphase=selfcal.txt offset=10
• Use the cycle period (integration time of visibilities) of your observation for the offset parameter.

This should restore the phases to a continuous sequence. Depending on where in the data you did your cacal, a constant phase offset may be introduced, but this should calibrate out with normal offline calibration.

Selfcal correction files:

• selfcal-07dec17.txt
• selfcal-08jul21.txt
• selfcal-08sep07.txt

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