2009-07-09 ALMA Calibration Group Telecon Agenda and Minutes

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ALMA Calibration Group Telecon Date: July 9, 2009 (Thursday) Time: 14:00 UT 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:

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Agenda and Minutes

NOTE: Discussion from the telecon is shown in italic.

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Attendees

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Amplitude Calibration Device Development Status

See the February 12, 2009 meeting and [[CalAgendaMinutes20090611][June 11, 2009 meeting minutes for current discussion on this subject.

ACD Development Update

To augment this discussion Pavel provided the following draft reports on 2009-06-10:

• ALMA Calibration Target (ACT) Analysis Report (Eder): Analysis report of tests performance on ACT with the goal of optimizing the sizing and thermo-mechanical design.
• Conical Calibration Target Simulations and Measurements (Murk): Calibration target simulation results.
• Calibration Load Drawing
• Calibration Load Development Plan 2009-05: Updated plan.

Discussion:

• Design appears to be converging toward the requirements.
• Pavel noted that with the current design encounters some problems with current airflow properties of the widget space on each antenna. For example, NAOJ airflow is likely a problem as there appears to be quite a high volume of air being cycled through the area around the ACD. Vertex antenna is not thought to be a problem, and design of AEM antenna looks to be not a problem either.
• RH will discuss NAOJ airflow issues with NAOJ staff.
• Some apprehension expressed regarding the complexity of the design. Mel Wright, in particular, suggested that perhaps a simple partially-illuminated ambient load might suffice.

Solar Filter Prototype Development

On 2009-06-11 Ferdinand Patt reported that there is a test report for the prototype solar filter available. Ferdinand reports that there are some issues that need to be discussed about the solar filter specification. Specifically:

• Could the in band attenuation be relaxed by lets say 2 dB, from range 13 to 16 dB to range 12 to 17 dB?

This question has been pointed to the solar radio astronomer community. See SolarObserving.

Ferdinand also notes that ESO is planning to send out a CfT for the solar filter production in preparation for the ESO FC by early July 2009. Hopefully we will have received feedback on the design question above by then.

Receiver Gain Compression

During our June 11, 2009 telecon the general issue of receiver gain compression, and its relationship to the ACD design, was discussed. From that discussion:

RH brought up the issue of non-linearity in receiver response and how we deal with it (which is follow-on to some questions asked by Pavel on this subject). On this point PY notes that:

• Recall that there was a thread about the problem of front-end compression and absolute calibration accuracy (see my original note attached). I couldn't find any conclusion on that and also whether or not FE IPT is expected to mitigate this error by front-end calibration or tuning.
• To strengthen an importance of this issue, please note that this original note is based on the FE specification for the compression, "5% large signal compression @ 373 K".
• Cartridge specifications have adopted a different definition: "The large signal gain compression caused by the changing the RF load temperatures from 77 K to 373 K shall be less than 5 %". Because of this different definition, the large signal compression @ 373K translates to about 8%. And the calibration error of the Front-ends will become even larger.
• In practice the cartridge groups verify the compression using the 77 K and the 300 K loads (except for the Band 9 who are using the 373 K load) and even with this smaller range get close to the relative compression specified.
• In order to synchronize the specs we either need to relax the FE specs on compression @ 373 K to 8% or change the definition to a "5% large signal compression caused by the change between the 77 K and 373 K loads".

Pavel would like to converge on a preferred solution to the gain compression issue. Discussion on this topic revolved around several points:

• An unpleasant surprise in that the reported level of gain compression is much larger than we had been led to believe it would be. Also unclear how the spec was allowed to be set to what amounts to the full allowance of the amplitude calibration specification.
• RH suggested that someone investigate what effect the measured level of gain compression has on the amplitude calibration requirement. RH and JGM commented that Stuartt Corder would be a good choice to take on this task.
• JGM has posted Pavel's note to RH and JGM detailing this issue to the amplitude calibration wiki (see the "Frontend Compression and the ACD" item).

Ancilliary Measurement Devices

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

Plans for installation of first production weather station at AOS underway.

Atmospheric Temperature Profiler

From our June 11, 2009 telecon:

New contract to procure, install and test the profiler signed between ESO and the Cavendish Lab (MRAO).

• Profiler uses observations of atmospheric 60GHz O2 lines to calculate vertical temperature profile
• Current plan is to procure commercially-available unit and install at ALMA site late 2009/early 2010 for testing
• In procurement phase now: 1st draft of specifications written (too late for agenda but I can email it to anyone who is interested)
• LT will circulate draft requirements document for comment. Please provide comments to LT and EmilyCurtis within the next week.

-- EmilyCurtis - 2009-06-11

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Calibrator Database Development

See the ASA Catalogues wiki for suggested calibrator database content (developed in collaboration with EVLA).

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Date of Next Phone Meeting

Tentative date for next ALMA Calibration Group telecon will be 2009, August 13th 14:00 UT.

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-- JeffMangum - 2009-06-25