CCB Commissioning Data, Oct 2007
- system reinstalled with 8 instead of 16 channels; better balanced receiver
- try SR NODs
I tried nodding the subreflector with 5 seconds spent in each phase (simple A/B/A/B pattern). This appeared to work well, and my analysis scripts were fairly easily modified to deal with the data.
Here is a regular (primary motion) NOD (A/B/B/A -- ie "symmetric nod")) on a bright calibrator:
Error: (3) can't find brightFitReg.png in Main
white is differenced data, and green is the fit. The fit is simply a linear least squares fit for the amplitude of a template, where the template was computed by calculating where each beam was looking on the sky, and where the source of interest was in relation to those beams, assuming a beamwidth vs frequency and beamthrow.
Here is a Subreflector Nod on the same source:
Error: (3) can't find brightFitSr.png in Main
The fitted amplitudes agree to within a few percent and the signs are right, which is good.
The twist appears when we look at a faint
source. Here is a regular (primary motion) symmetric NOD on a faint sources (it's a few mJy at 1.4 GHz and about this or less at 30 GHz)
Error: (3) can't find faintFitReg.png in Main
Here is the SR NOD on that same source:
Error: (3) can't find faintFitSr.png in Main
The fitted amplitude is about 20 mJy and has the opposite sign! This fitted amplitude is obtained for all SR NOD observations within 10-15 percent
indicating a spurious signal is coming in to the Sub reflector NODs at this level.
No signal brighter than a couple of mJy is seen in any
of the regular nods on this same set of sources (57 observations of half a dozen sources collected over several hours)
Here is the time series of regular NOD observations (there are a couple of outliers needing looking into; also, this series contains signal, not just noise; different CCB channels are different colors)
Error: (3) can't find ccbNodTimeSeries.png in Main
Here is the time series of SR Nods on the same set of sources, in the same order-- note the -20 mJy offset in the mean!!
Error: (3) can't find ccbSrNodTimeSeries.png in Main
Here are the SR Nod means for those two channels plotted versus elevation
Error: (3) can't find ccbSrNodElev.png in Main
Error: (3) can't find ccbSrNodAz.png in Main
My guess is that we are seeing is modulation of ground spillover with secondary motion at the 30 mK/300K=0.01 percent level.
From the 57 regular nods on faint sources collected during the run I've computed noise levels--
|| MedAbsDev/0.68 [mJy]
I've used the median absolute deviation, rescaled to give the same answer as the RMS for a Gaussian distribution -- you can just read the last column as "RMS", but it's less sensitive to outliers.
The scatter in measurements between channels is highly correlated, suggesting that the noise is dominated by residual fluctuations in sky emission. One might have expected SR nodding to help but...
- 10 Oct 2007