16jan05 spider scan beam maps on 3c138


    Spider scans (heiles calibration scans) were taken with alfa on 16jan05 using the calibrator 3C138 (J0521+166, 9.1 Jy at 1420Mhz).The setup was 100 Mhz bandwidth centered at 1420 Mhz. Each pattern consisted of 4 60 second strips through the source (each strip rotated by 45 degrees). Each strip covered 6 beam widths.  With overhead,  a single pattern took about 6 minutes.  separate spider scan was done for each beam (centering that beam on the pattern). In all it took 43 minutes for the 7 spider scans. During this time the  za went from 7 deg before transit to + 3 degrees after transit. The rotation angle of the floor was kept constant at 0 deg.

Processing the data:

    The processing steps for the calibration are:
  1. Fit the main beam and sidelobes for each strip using 1d gaussians (allow coma for the main beam).
  2. Remove the sidelobe fit from each strip then do a 2d gaussian fit to the main beam
  3. model the 8 sidelobe measurements with a fft of 8 terms in the sidelobe height, position, and width.
  Beamaps were then made evaluating the fits of the main beam and sidelobes(the sidelobe was recreated using all 8 terms of the sidelobe fit).  Each beammap was then converted to db's below the peak and contoured with 3db steps out to -21db. The horizontal and vertical axis are great circle offsets from the center of the beam (in the az,za directions).
processing: usr/a2010/bmmap_gc.pro