2008-12-10 ALMA Calibration Group Telecon Agenda and Minutes


ALMA Calibration Group Telecon
  • Date: December 11, 2008 (Thursday)
  • Time: 16:30 UT (NOTE: Non-standard time)
  • Duration: 1.0 hr
  • USA Toll Free Number: 877-940-1273
  • International: +1-517-466-6602
  • Passcode: 5 6 6 4 7 9 4
  • Leader: JeffMangum

Table of Contents:



Agenda and Minutes

NOTE: Discussion from the telecon is shown in italic.


Attendees

  1. Richard Hills
  2. RobertLucas
  3. Bojan Nikolic
  4. JeffMangum
  5. Morita
  6. Dick Sramek
  7. Gie Han Tan
  8. LeonardoTesti
  9. Jack Welch
  10. Wolfgang Wild
  11. AlWootten
  12. Nick Whyborn
  13. Pavel Yagoubov


Amplitude Calibration Device Development Status

Richard will lead a discussion on the status of the amplitude calibration device (ACD). The starting point is the study by Axel Murk which is in edm at Second Murk Report. A note that REH wrote in response to that will be found in a reply to that edm entry.

Pavel's note sent around via email is as follows:

Dear all,

Please find below a short summary of ACD loads development as an input 
for tomorrow's telecon.

ACD-1 was delivered to Chile and is already installed. Hot load has a 
problem with heater, currently disabled, will be repaired beginning next 
year.

ACD-2 will be shipped to Chile next week, is fully functional.

ACD-1 and ACD-2 have TK tiles as ambient load w/o baffles and shrouds. 
Hot loads are RAL casted pyramidal loads. Practically no shroud is used, 
it is Case 1 design described in the thermal report.

Prototype loads have been fabricated. The ambient load is based on TK 
tiles with a conical shroud; the output aperture diameter is 90 mm. The 
hot load is RAL pyramidal load with a conical shroud; the output 
aperture diameter is 72 mm. This is not exactly the geometry which has 
been simulated.  However, the current design of the hot load shroud 
appears to be more favorable than Case 2 regarding the thermal issue. As 
to electromagnetic analysis, the current design lies in between the two 
conical shrouds analysed, closer to Cone 1.

Measurements of the prototype ACDs including shrouds are currently 
underway at the University of Bern. Preliminary results for the return 
loss at W-band are available. Return loss will be tested up to 700 GHz. 
Radiometric tests for the total scatter are planned at the W-band, where 
new hardware has recently become available.

The radiometric tests are best to be performed with ALMA receivers to 
improve S/N and also to have representative optics. It can be done in 
collaboration with cartridge groups, but ideally these measurements 
should be performed with ALMA cryostat to cover the full frequency range.

When the measurements at the UB are finished (Jan. 2009), the results 
will be reviewed and a conclusion will be expected on whether the loads 
performance is sufficient to go in production. If not, alternative 
designs will be investigated and tested, including bi-conical target.

Further improvements of the thermal properties are expected with 
decreasing an input aperture of the hot load. This change should not 
degrade return loss or total scatter, rather improve those.

In the mean time, the ACD-3 through ACD-7 will be equipped with ambient 
loads only. Those will be copies of the ambient loads installed at ACD-1 
and ACD-2. Still to decide whether we leave the hot loads mounting 
places empty or install extra ambient loads there.


Best regards,
Pavel.

Discussion:
  • Gie Han noted that there have been some tests of tilt-dependence of load temperature. Apparently there is a report on these measurements.
  • Richard asked if the plan to deliver ACD-3 through ACD-7 without hot loads was a good idea. There were some objections, related to non-uniformity of calibration systems and extended use of systems which do not meet ALMA requirements. Conclusion was that this was not acceptable and that extra prototype or interim loads should be procured.
  • Richard asked why we are pursuing a pyramid+shroud design rather than a cone+shroud design which, based on the initial tests, performed better. Richard suggested that we pursue a simple cone+shroud design.
  • Pavel noted that:
    • The reason that the testing of the pyramid+shroud design is being done is to allow validation of EM and thermal simulation models.
    • Even though the conical+shroud prototype design performed better in initial tests, it does not currently fit in the widget space.
  • Richard suggested that Pavel produce a report comparing the pyramidal and cone+shroud options which also spells out the path forward for production design. Pavel noted that this report will include pyramidal load measurements currently being made by Murk at University of Bern.
  • Bojan pointed out that at the edges of bands 1 and especially 2 the one-load method may not work satisfactory due to the significant oxygen opacity of the sky. At 67GHz opacity is about 0.4 and at 70GHz about 0.12. By 75 GHz it is 0.05.

The wrap of ATM I used to compute above figures is available at: http://www.mrao.cam.ac.uk/~bn204/alma/atmomodel.html


Ancilliary Measurement Devices

Weather Station (P,T,RH,Ws,Wd) Instrumentation

In November 2008 the first set of weather station instrumentation (RH, P, T, and wind) was delivered to the OSF. AIV personnel installed these devices in a test setup in the OSF TB. JeffMangum has tested this system and verified that it is working properly. AIV has taken over the installation of this set of weather station monitors on holography tower number 2.


Date of Next Phone Meeting

Tentative date for next ALMA Calibration Group telecon will be 2009, February 12th 15:00 UT.

-- JeffMangum - 10 Dec 2008

* Calibration_Loads-20070810.pdf: R. Hills' memo with approximate flowdown of return loss requirement for hot load
Topic attachments
I Attachment Action Size Date Who CommentSorted ascending
Calibration_Loads-20070810.pdfpdf Calibration_Loads-20070810.pdf manage 146 K 2008-12-12 - 04:58 BojanNikolic R. Hills' memo with flowdown of return loss calculation
Topic revision: r7 - 2008-12-12, BojanNikolic
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