2009-06-11 ALMA Calibration Group Telecon Agenda and Minutes
Table of Contents:
ALMA Calibration Group Telecon |
- Date: June 11, 2009 (Thursday)
- Time: 14:30 UT (NOTE: Special 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
Agenda and Minutes
NOTE: Discussion from the telecon is shown in italic.
- Darrel Emerson
- Richard Hills
- Harvey Liszt
- Bojan Nikolic
- Ferdinand Patt
- Dick Sramek
- Kamaljeet Saini
- Pavel Yagoubov
- Jack Welch
- Tom Wilson
- Mel Wright
Amplitude Calibration Device Development Status
Pavel will lead a discussion on the status of the amplitude calibration device (ACD). During our February 12, 2009 meeting
we discussed the following:
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).
- Additional hot loads of the prototype design have been ordered from RAL so that the first 7 calibration devices will have these on them when they are delivered to Chile.
- A draft of the preliminary report from Bern on the shroud plus pyramid design was received. The shroud does improve the thermal but does not have much effect on the backscatter which remains higher than we require at frequencies of order 100 GHz and below.
- Pavel has provided a plan for the development of the production loads and an initial design proposal.
- RH: Note that use of this load below 85 GHz was removed as a requirement. Plan allows for use down to Band 1. Are there any compromises in the design with allowing for operation down to Band 1?
- PY: Perhaps. Will need to test/simulate this.
- RH: Concerned about air flow and use at low elevations. Are we concerned about this?
- PY: Simulations due by mid-April should shed some light on what can be accomplished.
- JM: Would rather not set a low-elevation limit at this time. Wait for simulation results.
- RH: Why can't an actual measurement be made, rather than relying on simulations?
- GHT: Can do IR camera measurements of a prototype.
- PY: Would like to have a meeting/workshop at the end of phase 1 so that we can decide on a design. Phase 1 should be done in May 2009. Is this a good thing to do?
- RH: Yes, as long as information on design/testing results are distributed before the meeting.
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).
ACD Development Update
To augment this discussion Pavel provided the following draft reports on 2009-06-10:
- 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.
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
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.
Calibrator Database Development
See the ASA Catalogues wiki
for suggested calibrator database content (developed in collaboration with EVLA).
Date of Next Phone Meeting
Tentative date for next ALMA Calibration Group telecon will be 2009, July 9th 14:00 UT