Noise Source Passband Calibration

The noise source is only present in the LSB and can also be used to bandpass calibrate narrow calibrator modes. Only for data since early December 2006 has the signal of the Noise Source been sufficiently amplified to be useful for this calibration mode.

The following procedure uses the phases of a wide band signal (in Window 2) and applies them to a narrow band signal, in order to check phase transfer:

  # bwsel can select out pieces of a track with the same BW settings

  % selfcal vis=ct010.500_500_500.2006dec01.1.miriad select='source(3c279),win(2)' refant=9 interval=20
  % uvcat   vis=ct010.500_8_500.2006dec01.1.miriad   out=3c279.8mhz.1dec select='source(3C279)'
  % gpcopy  vis=ct010.500_500_500.2006dec01.1.miriad out=3c279.8mhz.1dec options=nopass

The phases in the 3c279.8mhz.1dec data can now be compared to that of the noise source, and will still show offsets compared to that of the noise source.

The amplified noise source can effectively remove any passband variations. For example, to apply an mfcal solution on the narrow band of the noise source (skipping the first channel):

  % mfcal vis=ct010.500_31_500.2006dec01.1.miriad interval=999 line=channel,62,2,1,1 refant=9 tol=0.001 \
    select='source(noise),win(2)'

If the signal of interest is in the USB, where there is no noise source, the data will have to be conjugated into the USB and headers faked in order for mfcal to apply the correction, after a slight manual copying of important header variables:

  % uvcat vis=ct010.500_31_500.2006dec01.1.miriad out=noise.lsb select='source(noise),win(2)
  % uvcat vis=ct010.500_31_500.2006dec01.1.miriad out=source.usb select='source(3C279),win(5)'
  % uvcal vis=noise.lsb options=conjugate  out=noise.usb
  
  # look at the parameters for the spectra in USB and LSB
  % uvlist vis=source.usb options=spec
  % uvlist vis=noise.lsb options=spec

  # cheat and copy two important variables accross
  # note sfreq varies with time, sdf does not
  # see bandcal.csh for more automated methods
  % uvputhd vis=noise.usb hdvar=sfreq varval=96.99336 out=noise2.usb
  % rm -rf noise.usb
  % uvputhd vis=noise2.usb hdvar=sdf varval=0.00049 out=noise.usb

  # now calibrate USB
  % mfcal vis=noise.usb interval=9999 line=channel,62,2,1,1 refant=9 tol=0.001 
  % gpcopy vis=noise.usb out=source.usb options=nocal