Pitch, roll, focus correction at multiple frequency on 3C48, CTA21  (feb01)


     The  main reflector surface (along the main support cables) was measured and the panels adjusted in jan01 (see reflector adjustments jan01).  The gain improvement is shown in: sband , cband.  The remaining losses are from the unadjusted panels and the pitch, roll, and focus collimation errors. Doing multi frequency measurements you can estimate the surface rms using the ruze formula. This assumes that the dish errors are not correlated over large areas, and that the other losses (za spill over, collimation errors) have been corrected.

    Using a theodolite and tilt sensors we've created a model of the pitch, roll, and focus errors of the dish.  We can use the tiedowns to compensate for these errors (over a limited range) while we track a source.  This correction is currently limited by:

  •  The pitch, roll, and focus is not measured very well above 16 degrees za.
  • The pitch and roll errors above 10 degrees za require a tiedown throw larger than available. This requires us to try and reshim the elevation rails yet again.
  •     On 18feb01 we tracked 3C48 (J0137+331) and CTA21 (J0318+164) with pitch, roll, focus (PRF) enabled. The standard 4 strip calibration technique was used. We alternated between lband, sband, and cband every pattern. We wanted to find out :
    1. What is the gain with PRF correction enabled.
    2. Can we estimate the surface rms (all panels) assuming the PRF correction is done correctly.
    3. How accurate is  the PRF model that we are using.
    When interpreting the data it is important to keep in mind sources of possible error. These include:
      1. The correctness of the cal values affects the gain and ruze formula computation. The 5400 Mhz cal value is probably in error.
      2. The PRF model uses the temperature as one of the inputs to determine the platform height. Experimenting before the run showed that the maximum  gain may have been occurring about .5 inches lower (for cband) than what was used.
      3. The fluxes need to be known at the different frequencies. Errors in them will be errors in the gain.
    The figures show the results of the measurements: