2007-08-16 ALMA Calibration Group Telecon Agenda and Minutes


ALMA Calibration Group Telecon
  • Date: August 16, 2007 (Thursday)
  • Time: 14:00 UT
  • Duration: 1.0 hr
  • USA Toll Free Number: 1-877-919-7148
  • International: +1-203-566-1039
  • Passcode: 5 1 0 4 6 8
  • Leader: JeffMangum

Table of Contents:



Complete List of Action Items


Action Items from Last Telecon

Assigned To Due Date Description State Notify  


Agenda and Minutes

NOTE: Discussion from the telecon is shown in italic.


Attendees

  1. JeffMangum
  2. Ferdinand Patt
  3. Alison Peck
  4. Matsushita
  5. ToddHunter
  6. Axel Murk
  7. BaltasarVilaVilaro
  8. AlWootten


Calibration Examples Status

Still waiting on final versions of delay (RobertLucas) and optics (BaltasarVilaVilaro) calibration examples. See CalExamples wiki for further information.


Calibration Sequence Development

Complete. Added measurement sequence information to CalExamples documents. Have contacted Computing IPT to suggest that their staff begin reviewing CalExamples documentation. Integration and testing of these calibration examples will be incorporated into ATF test planning currently being developed.


Amplitude Calibration Subsystem Development Reports

Multi/Dual-Load (M/DL) Calibration System (Ferdinand Patt)

  • Ferdinand Patt sent load test report (from IAP) to group.
  • Several issues from load report need resolution. Basic problem is that it is not possible to optimize the backscatter, total scatter, and thermal performance of a load system with a single type of absorber. Since the amplitude calibration sequence will involve comparisons of spectral measurements of the loads, standing waves are a problem.
  • Based on the analysis of the "chopper wheel" amplitude calibration technique presented in ALMA Memo 434, there isn't much room for relaxation in any of the load temperature regulation requirements. It would seem that adopting a more relaxed +-0.5 K (0.15% at 300 K) load temperature regulation spec would result in a missed 1% amplitude calibration spec at frequencies less than 370 GHz. Need further discussion on this.
  • A revised version of the Calibration Device Technical Specs is available. Still several issues:
    1. Conflicts with amplitude calibration device review results identified in 2006/07/10 have not yet been addressed. See CalAmp for details.
    2. Seems that QWP specs have been eliminated from the FE requirements.
  • Richard Hills has produced a summary and recommendation document describing the current status of the amplitude calibration device testing and development. In summary, Richard points to four issues one must overcome when determining the effective temperature of the current hot and ambient loads:
    1. Determining the actual physical temperature, as determined by thermometers embedded in the structure of the load.
    2. Temperature gradients between the temperature sensors and the actual emitting surface. These will depend on IR cooling and convection (both that due to the gradients in the air temperature which in turn depend on the orientation, and that due to the forced air circulation produced by the HVAC system).
    3. The emissivity of the load.
    4. Most critically, the backscatter characteristics of the load.
  • Note that the specification for the required accuracy of the loads is +-0.2 K (+-0.07%) for the 20 C (ambient) load and +-0.6 K (+-0.2%) for the 60 C (hot) load. This specification refers to the effective black body temperature of the loads.
  • Richard recommends the following:
    1. Continue the procurement of two sets of loads to get us through the initial test phases. The Calibration Group concurs.
    2. Confirm the figures in the calibration design document as being the requirements for the knowledge of the effective black-body temperatures of the loads when used with the ALMA receivers at all frequencies of interest. The Calibration Group concurs.
    3. Suggests that it might be productive to slightly soften the load temperature stability requirement and set the specifications at +/-0.5K for the ambient load and +/-1K for the hot load, while maintaining the values of +/-0.3K and +/-0.6K as goals. The Calibration Group discussed this and opted not to follow this recommendation. Since the current 0.2/0.6 K spec only marginally allows one to meet the 1% amplitude calibration specification, there is no room for relaxation in the load stability specification.
    4. Start immediately on a program to develop an alternative design of load that does meet these requirements. The Calibration Group concurs. Richard is convinced that the cone design is sufficiently promising to make this worthwhile, but clearly we should not start on such a study without a clear understanding of what is to be done and on what timescale.
  • The Calibration Group further discussed the temperature stability limitations of the prototype systems with Patt and Murk. It appears that the need to provide a hot load which meets the temperature stability requirement from 30 to 900 GHz is the main limitation. According to Patt and Murk, if the hot calibration load were not needed for Bands 1 and 2, then it would be easier to meet the overall load temperature stability requirement. With a follow-up analysis JeffMangum will look into whether we can get away with using just an ambient load for relative amplitude calibration for Bands 1 and 2.
  • Richard noted in an additional follow-up email that there appear to be three options to resolve the load temperature stability issue:
    1. Insist that both the ambient and hot loads have good performance at all bands (i.e. ~30 to 950GHz).
    2. Revise the system so that the ambient load is for 30 to 900GHz, while the hot load is used for 85 to 950GHz only (i.e. not Bands 1 and 2). The advantage of this option is of course that the hot load can be made smaller (roughly a factor of two in the size of the aperture), which eases the design considerably. The argument is that the atmospheric opacity in Bands 1 and 2 is low enough that one does not need to use the two-load calibration method: one (ambient) load is good enough to meet ALMA's accuracy requirements. This will be investigated by JeffMangum. It seems that this approach was adopted in e.g. the Calibration Unit Design document, but it was not directly taken on board in the development work done so far.
    3. Make two (essentially identical) loads for 85 to 950GHz with only one of them heated, and then a third ambient-only load for 30 to 90GHz. This option is attractive in that the ambient load can be made with a smaller aperture as well and one could perhaps make saving by using the same design as the hot load. (Remember that the ambient load has the tighter requirements on the accuracy. Note also that even the ambient load is not really at thermal equilibrium - the air may be at one temperature but much of the radiation environment will be the cold sky or even the cold interior of the cryostat.) A point here is that, since there are no active plans to build Bands 1 and 2 at present, we can postpone actually making the third load until those bands are funded. We would of course need to be sure of the feasibility of making a low frequency ambient load that fits in the space available.
  • A point to appreciate about having only one calibration load for Bands 1 and 2 is that it does not directly give you a value for the receiver noise temperature - only the overall system temperature. Given that we will have other ways of estimating the atmospheric contribution (e.g. the WVR plus model), Richard believes that this is not a problem.


Berkeley Absolute Flux Calibration System (Jack Welch)

  • No report.


Ancilliary Measurement Devices

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

In December 2006 JeffMangum submitted weather station instrumentation (RH, P, T, and wind) RFQ documentation to NRAO procurement system. After having reviewed information received from bidders and consulted with Calibration Group (see CalAncillary for discussion), on 2007/03/21 JeffMangum submitted purchase information to RobertLaing for procurement.

Leonardo Testi continues to push purchase through ESO purchasing system. Will likely obtain a loaner set of weather devices from Vaisala to allow us to test the integrated system either at ESO or NRAO.

Location Proposal

Main.JeffMangum has assembled a suggested siting for the AOS/OSF weather stations to CalAncillary. AlWootten noted that the weather station siting description should note that the other "ancillary" measurement devices (tipper, O2 radiometer, phase monitor, etc.) should also be sited at or near the central weather station, which is located near antenna station number 83.

Oxygen Sounder

JohnRicher has developed a contact with one of the companies that produces O2 sounders. With BojanNikolic, they are investigating issues related to operating such a device at 5000 m, among other things.

One of the companies that produces O2 sounders has recently been doing a study for a site at 3300m asl and so they have a good idea of expected performance at high elevations. They have told us that they expect accuracy of about 0.5K in the first kilometre of the atmosphere and about 1K accuracy higher up. This is a bit better than previously expected and is likely to be good enough for atmospheric phase correction purposes.

There is, however, one obstacle. The algorithm to invert the radiometer measurements to a vertical temperature profile is based on a neural network and requires a large training set to function properly. We have been told that something like 10 years of daily radiosonde launches are required. We currently looking at what data are available in the vicinity of the ALMA site. -- BojanNikolic

Question regarding purchasing process. Since budget in on NRAO side of the ALMA budget, what are requirements for purchasing? Likely requires an RFQ with bid evaluation. JohnRicher and BojanNikolic will continue to investigate potential vendors and devices.


Solar Observations and Calibration

MarkHoldaway has written a draft document on Calibration of Solar Observations. We discussed this document at our last telecon. Few revisions are needed. Main.JeffMangum will pick-up the last draft produced by Mark and update.


Date of Next Phone Meeting

The next ALMA Calibration Group telecon will be 2007, September 13th 14:00 UT.

-- JeffMangum - 12 Jul 2007

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Topic revision: 2007-08-27, JeffMangum
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