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Proposed agenda:

• Reporting on the 'action' items from last week's meeting
  • Attempt at improving the statistics that go into the Tatm model
  • Residuals from the Tatm model
  • Variable Air Mass statistics vs. our error budget
• Strategies for determining opacities
  • Tippings
    • Why we need multi-frequency tips
    • DCR vs. DCR-AFR vs. Spectrometer (vs. CCB?)
  • Sampling and slew rates
  • Best elevation ranges for determining either Tau or Trcvr or both
  • Azimuths to avoid
  • Differential Air Mass
  • Revised maximum errors in Tatm

Minutes:

Reporting on the 'action' items from last week's meeting:

• Attempt at improving the statistics that go into the Tatm model

Was able to add in 2x as much statistically uncorrelated data as before. The results are not statistically very different than before.

• Residuals from the Tatm model

Presented residual plots that show no strong frequency or temperature shapes. The rms of the residuals was 2.5 K, just below out error budget of 3 K for a 1% calibration uncertainty.

• Variable Air Mass statistics vs. our error budget

Presented plots for a full year, not just 4 months as before, that show an insignificant annual trend. There are only a few hours per year when the error in Air Mass introduced by variable weather will exceed our 1% error budget.

Strategies for determining opacities

• Tippings
  • Why we need multi-frequency tips
  • DCR vs. DCR-AFR vs. Spectrometer (vs. CCB?)

Because of wide bandwidths and multi-IF work, tippings need to be done at multiple frequencies. Even within an 800 MHz band at Q- and K-band, opacities will differ by more than our 1% error budget from one end of the band to the other. Suggests we need to use the AFR-DCR detectors for tips or perform multiple IFRAck-DCR tips. The AFR-DCR detectors are not calibrated for DC offsets and linearity.

Tip error budgets

Presented a table of how the uncertainties in the factors going into a tip result in an uncertainty in opacity. At low frequencies (1 GHz), eta_spill dominates the error budget. At high frequencies, the error in Tcal and, thereby, Tsys dominates. At intermediate frequencies, Tcal/Tsys errors and Tatm errors dominate the error budget.

Errors in Tcal get you twice. If Tcal's are high, so will Tsys and, the, so will the tau derived from a tip. If Tcal is high, Ta is also high.

In all, the tau derived from a typical tipping will introduce a 2% error in calibration. To reduce that error will require obtaining opacities from something other than tips. This prompted a discussion of various alternative methods, including weather modeling and using Tsys vs. Elev from ones actual observations instead of a separate tip. Having a table of accurate Trcvrs will be necessary.

Sampling and slew rates, Best elevation ranges for determining either Tau or Trcvr or both

Discussed what are the optimum elevations as a function of opacities. Slew rates can be as fast as the telescope allows. Recommended elevation limits are:

Afterwards: Differential Air Mass

Since the top and bottom of the dish are at two heights, the atmosphere rays from the top and bottom of the dish will be affected by different amounts of refraction. Thus, rays from the top and bottom of the dish will go through different air masses. Because of variable weather, this differential refraction will also be time variable. Showed plots that estimate the variability of differential air mass for 1 year for an observing elevation of 5 deg. Not only can differential air mass range from -1 t0 +0.5 air mass. The positive values come about when inversion layers cause the ray at the bottom of the dish to cross with that from the top.

-- RonMaddalena - 20 Apr 2006