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Data Reduction Case #10

Focus Data

Focus Data is reduced in the following steps:

  1. Calibrate the total power data
  2. Average the polarizations (optional)
  3. Flag any data if necessary
  4. Subtract the second beam (which records atmospheric contributions) if applicable
  5. Fit a curve (may be Gaussian or quadratic) to the focus profile, to find the peak
  6. Apply the measured peak offset as the new LFC

Focus data is taken in continuum total power mode. Each scan can first be calibrated to determine antenna temperature as a focus position, using the standard calibration:

T_{ant}(z,p) = T_{cal}(p) \times \frac{calon(z,p) + caloff(z,p)}{calon(z,p) - caloff(z,p)} \times \frac{1}{2} - \frac{{T_{cal}}(p)}{2}

Here, p is an index for each of the two polarizations, and z is the focus position.

The polarizations may either be averaged, or handled separately.

If there are two beams, the antenna temperature of the secondary beam should be subtracted from that of the first, thereby removing any atmospheric fluctuations in the data.

At this point the observer has data which should show a curve with a peak near the center. Depending on the scanned focus range, this data may or may not include a baseline region. If there is a baseline, some fraction of the data must be marked to fit it. Which part of the scan are used for the baseline depends on the extent of the focus range observed, but it should be selected to exclude the central Gaussian feature itself. The user can then fit a polynomial function to the baseline region, subtract the polynomial fit, and fit a Gaussian function to the source. These two functions (polynomial and Gaussian) may also be fit simultaneously if it is convenient.

If the data do not include a baseline region, it may be advantageous to find the peak in the focus curve by fitting a quadratic instead of a Gaussian.

The best focus is then revealed by the peak in the curve. The result can be sent back to the M&C system to apply the focus correction.

-- JimBraatz - 04 Aug 2004

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Revision r1.1 - 04 Aug 2004 - 20:54 GMT - JimBraatz
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