Variation in VEGAS system temperatures and line intensities between spectral windows
VEGAS was released for shared risk commissioning in February 2014. During the commissioning process, we identified and fixed a number of low-level errors (it is still possible that additional, subtle errors remain). However, there is one issue that has still not been fully resolved. This is that the system temperatures, and line intensities, as measured by different spectral windows / polarizations appear to vary within an individual scan, and slowly as a function of time, with an rms between windows of the order of 5%. At least for modest line intensities, the ratio of line to system temperatures appears constant. We have ruled out all obvious potential causes of this problem (for example, ADC quantization or clipping), and are actively working at high priority, along with staff from UC Berkeley, to identify the root cause and resolve this problem.
For absolute intensity measurements, the magnitude of this error is within the normal calibration accuracy range we advertise for the telescope. However we recognize that it should be possible for VEGAS to achieve better calibration accuracy than its current performance. At the present time, we urge observers to follow the “best practices” calibration strategy, as described in the observer’s guide. In particular, observers should observe a continuum flux density calibration source at least once during an observing session. If you have any further questions, please contact the “friend” of your observing project.
Further information will be made available as soon as we have an update on the situation.
