Beam scanning telecon


  • 1430 EDT = 1830 UT on Friday October 10
  • Attendees: Todd, Geoff, Antonio, Charles C., Darrel, Richard


  1. Summarize hardware changes and other findings since Richard's visit. These have been summarized by Geoff.
  2. Discuss the measurements of the spare holography horn and pyramidal horn using the room temperature receiver. Describe the outcome from suggestions made at John Webber's Tuesday meeting on this topic.
  3. Plans for the coming week.
  4. Update on Fred Schwab's Mathematica tool for processing NSI raw data into proper efficiencies.


  1. Comparison of holography horn pattern with GB farfield measurements
    • The co-polar is just about acceptable, there are differences at the -25dB level. Is there a theoretical pattern for this horn? Yes, Bill Shillue designed it in Mathcad, and will search for his files.
    • ACTION ITEM for Josh: compute the integrated power in the cross-pol pattern and compare it to the theoretical value, and to the ALMA 23dB spec.
    • Darrel: Does Sri have a 2D measurement? No.
    • In this document, was the cross-pol taken with two Z settings? Need to check with Josh.
    • Are Figure 6 green peaks really 30dB down?
    • A remaining, related question: what is the polarization purity of our open-ended waveguides?
    • Conclusion: if theoretical pattern has cross-pol features significantly below the measured pattern, then we should work more to understand/improve the scanner system.
  2. Coming week
    • have switched back to the ALMA cryostat on the scanner. Hopefully band 6 OEW will arrive and scans can begin.
  3. Fred's stuff
    • He has streamlined his program to read the NSI files and headers and run a group of scans. He also built in the flat-topped Gaussian
    • Richard: the important detail is the calculation of efficiencies w.r.t. the nominal subreflector position.
    • On scan 103, Fred gets slightly higher z values (305mm vs. 303-303). Richard: this could be due to approximation in the spreadsheet in the parabolic wavefront assumption.
  4. Review of Richard's points
    • 1d: on defocus errors and beam squint: awaiting Richard's response
    • 2a: of the three options given by NSI (pre-loaded bearing, larger bearing, dual-bridge system), the first two won't solve the problem.
    • 2c: How much attenuation is in the IF? there is 20-30dB in the IF 4-channel processor.
  5. Future work
    • Need to pull on the green cable to see how much the phase changes in the new mounting scheme.
    • Richard will send revised spreadsheet with sign change to Josh.
    • Need to run Band 3 scans with proper alignment to see if it can be mechanically achieved. Find how far we have to move it from the nominal position. If > 5mm, then the cartridge may be out of spec.

-- ToddHunter - 09 Oct 2008
Topic revision: r2 - 2008-10-14, ToddHunter
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