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

    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.

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.

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.

• Figure 1 shows the gain versus zenith angle for all of the measurements. 3C48 is a + while CTA21 is a *. The color encodes the different frequencies. The solid line is source rising and the dotted line is source setting. Since 3C48 is at 33 degrees declination it never came lower than 15 degrees za. The first point of each source (solid line rightmost point) was done with the PRF correction turned off. You can see the improvement the PRF correction gives. For CTA21 the first point is a little lower, while the first point for 3C48 is much lower at all frequencies. There is no zenith angle gain correction included here. It should start at 15 degrees za and drop off about 1 db at 20 degrees za. So the lband improvement 1st -> 2nd point is probably the za change. The sband, cband improvement is much more.

 
• Figure 2 plots the gain versus frequency for all the points (the first point with no PRF correction has been left out). The lband gains for 3C48 and CTA21 vary by about 10%. The cband gains for the two sources also have an offset approaching 10% while the sband gains for the two sources are very close. This variation is not inaccuracies in the cal values since we used the same cal value for each source. The discrepancy could be the zenith angle difference or an error in the spectral index of the source fluxes.

    The solid lines are the computed gains for a system with 10.5 K/Jy and a surface rms of 3 mm, 4 mm, 5 mm, and 6 mm (using the ruze formula). The data points don't line up very well with any of the curves (plus the constant should probably have been 12 K/Jy for a perfect dish). This discrepancy could come from:
1. the cal values used are not correct at the various frequencies
2. The fluxes are off
3. We aren't doing the PRF correction correctly or there is a larger zenith angle gain dependence than we think.
4. The unmeasured panels have some large scale correlation's in their errors.
5. The different horn illumination patterns is affecting the gain.
If you look at CTA21 a 4 mm surface rms is reasonable guess of the surface rms.
 
• Figure 3 shows the improvement in gain at cband for the PRF correction on 3C48. The solid black line was measured on 02feb01 after the surface was adjusted and with no PRF correction. The red line is the gain measured on 18feb01 with the PRF correction enabled. The rightmost red point had no PRF correction so it should have fallen on the black line (it's pretty close). The green line shows what the PRF model predicts if the 02feb01 data was corrected for PRF errors.  The two measured  points about transit (red) are close to the predicted (green) values. The predicted  points (green) below 160 deg az and above 210 deg az are not realistic.

    The lower graph has the pitch and roll errors (.1 degree units) and the focus error in inches. The dashed line is the correction factor that is being applied for the pitch and roll combined (blue dash line) and the focus error (red dash  line).  The large focus error correction is causing the gain values below 160 az to shoot up. We probably need to have a better estimate of the pitch, roll, and focus errors before we contemplate reshimming the elevation rails at high zenith angle.

processing: x101/010218/plotit.pro

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