a1552 cordrift mapping lbw, galaxies



    A1552 made lband maps using cordrift (drift scans).  The time to drift through the lbw beam was about 15 seconds. To increase the sensitivity up to 21 separate drift maps were done of the same region. One map had a relatively strong galaxy present. When the strip that contained the galaxy was bandpass corrected, a negative image of the galaxy was present for the entire strip.
    The processing steps used for each individual map were:
  1. input a single map using corinpscan
  2. Do a linear fit to the cal deflections. Use this rather than the actual cal deflections. This data was taken when the cal deflection was taken at the end of the drift scan rather than the beginning (I think..). The telescope was stationary and the sky was drifting while the cal was turned on/off. If a continuum source drifted through the beam during this time, the cal deflection would be contaminated. The linear fit (throwing away outliers) would be an attempt at fixing this problem.
  3. scale the data to kelvins using the cal fits.
  4. Do the band pass correction with cormapbc(). Various methods were tried for the bandpass correction:
    1. do a linear fit by channel along the strip direction throwing out outliers. Use the mean value of the fit for along each channel as the bandpass value for that channel.
    2. compute the median by channel over the strip  and use that for the bandpass correction.
  5. Remove Tsys (also in the routine cormapbc()). For each spectra tsys was computed by taking the median over the channels, or a robust average (throwing out outliers). This was done after the bandpass correction. The computation did not use 10% of the channels on each edge of the bandpass.
  6. The map was then converted to Jy use the gain curve.
  7. This process was then repeated for the 21 different maps.
    Strip number 16 (counting from 0) contained the galaxy. The first image shows the dynamic spectra of this strip using the median for bandpass correction and robust average for tsys removal.     The next plot shows the total power along strip 16 computed over the frequency channels that contain the galaxy (.ps) (.pdf)  (408.5 to 410 Mhz).     The plot shows why the median bandpass is not going to work. The plot shows that over have of the samples have either source, sidelobe, or galaxy. So the median is not going to do a good job. The negative image of the galaxy in the "other" samples is caused by the bandpass correction containing some of the power from the galaxy. A robust average was also used but it did not do any better at removing the galaxy from the bandpass correction. This is probably because the noise in a single channel is large enough to not exclude the galaxy samples.

    The problem is that there are not enough uncontaminated samples to compute a good bandpass correction. You could take more samples (a longer strip) or remove the continuum sources before computing the bandpass correction. This removal is a bit tricky (especially if there is absorption in the continuum source spectra).

    To solve this problem mary putman suggested breaking the strip of 120 samples up into smaller sections, computing the median of each of these sections, and then taking the minimum value of these sections for the bandpass correction.
    The next image shows the band pass correction breaking the strip up into 3 sections and computing the median on each section.

Breaking the strip up into 3 sections does not leave a negative image of the galaxy in the "other" samples. There is still a trace of the continuum source in the image. The bandpass correction does not contain any of the continuum source. After the bandpass correction, a single number per spectra is used to remove the system temperature. Any slope in the continuum flux would not be removed by the bandpass correction. The average over the resulting spectra would then leave a slope. Looking at the amount of the slope, the spectral index would have to be a little larger than .75 to explain all of the slope.