2007-06-21 ALMA Calibration Group Telecon Agenda and Minutes


ALMA Calibration Group Telecon
  • Date: June 21, 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. Richard Hills
  2. RobertLucas
  3. JeffMangum
  4. Morita
  5. Matsushita
  6. BojanNikolic
  7. Tomohiko Sekiguchi
  8. BaltasarVilaVilaro
  9. AlWootten


Calibration Examples Status

Still waiting on final versions of bandpass (RobertLucas), delay (RobertLucas), polarization (EdFomalont), optics (BaltasarVilaVilaro), and ACA calibration issues (EdFomalont) calibration examples. See CalExamples wiki for further information.


Calibration Sequence Development

Will begin process of turning CalExamples into calibration "sequences" during 2007Q2. Need to first meet with Computing IPT to figure out which Computing IPT personnel will be available to participate in sequence development.

Noted by RobertLucas: During Computing IPT Leads meeting in Socorro in June RobertLucas planned on discussing temperature scale calibration issues with Computing IPT. RobertLucas made the case that TelCal should handle amplitude calibration exclusively, rather than current design which involves both TelCal and Offline. See RobertLucas presentation to the Computing IPT for further information.


Amplitude Calibration Subsystem Development Reports

  • Berkeley Absolute Flux Calibration System (Jack Welch)
    • No report.
  • Semi-Transparent Vane (STV) Calibration System
    • Need only to get final report from Jesus Martin-Pintado. This is still pending.
  • Multi/Dual-Load (M/DL) Calibration System (Ferdinand Patt)
    • Ferdinand will give us a report next meeting.


Amplitude Calibration Device Specification

Richard Hills has reviewed amplitude calibration device hardware specifications. The specifications are summarized below. As Richard suggests, there specifications are described in engineering terms - i.e. the physical temperatures of the loads and the RF mismatch. What is needed are the requirements from one level above from which these requirements can flow down.

Richard suggests that the main issue is how well we need to know the real radiometric temperatures of the loads as a function of frequency, taking into account both effects like the front surface of the absorber being colder than the sensor, and the fact that the load is not a true black body. This has to be achieved when the load is in use with a real ALMA receiver.

Richard believes that the effective temperature cannot be confidently defined just by setting an RF mismatch level of -40dB. The point is that the reflections do not necessarily go directly back to the feed - we have things at very different temperatures in the system and you may have multi-path effects that produces coherent signals. In the worst case (fully coherent) -40dB means +/2% ripple and we have something like 350K temperature difference between the mixers and the hot load.

The main amplitude calibration relies on the difference between the sky and the cold load. This might be ~200K at the lower frequencies but could be only 100 or 50K at the shortest wavelengths. Since we can only allow the load temperatures a small fraction of our calibration error budget, this suggests that we can't allow an error in the effective temperature of greater than 1K and I would be happier with a target of 0.5K.

An obvious problem is how to test this. Essentially I think this has to be done with an even better load. Fortunately there are some cone absorbers available from Keatings which Axel Murk has shown are very good. So at least for the ambient load that should not be too hard.

One other point that may not be covered in the existing requirements is what wind speed this has to operate in. I ask because there is rather a lot of air blowing around in the cabin it is obviously much harder to keep the absorber warm if it is in gale. (I guess "air flow velocity" would be a more proper term.)

Performance Requirements for the Calibration Loads

Numbers are from the actual specification document. See http://edm.alma.cl/forums/alma/dispatch.cgi/iptfedocs/folderFrame/100028/0/def/ccf7 for full documentation.

  • 4.1 Temperature Requirements for Hot RF Load and Ambient RF Load
    • The calibration device shall have two calibration loads “Hot RF Load” and “Ambient RF Load” according to the following specifications.
    • Each Calibration Load shall be designed as line replaceable units [AD13].
  • 4.1.1 "Hot RF Load" Temperature
    • The hot RF load temperature shall a set in the range between 60 to 100 degrees Celsius.
    • 4.1.2 "Hot RF Load" Temperature Stability
      • The temperature stabilities specified hereafter shall be reached within 1 hour after the calibration device is going from its off-mode or transport-mode to operation-mode.
      • The temperature change of the hot RF load shall be less than 0.2 deg.
Celsius on timescales of 100 seconds [TBD].
    • 4.1.3 "Hot RF Load" Thermal Insulation
      • The hot RF load thermal insulation shall be sufficient to cause no increase in temperature of any external part of the hot RF load of 5 deg. Celsius above the ambient temperature.
  • 4.2 Temperature Sensors
    • The three temperature sensors, one each for the hot RF load and the ambient RF load mentioned in section 4.1. and an additional environmental temperature sensor shall have an accuracy of better than 0.1 deg. Celsius over a temperature range specified in section 4.1.1. and 9.1.2.
    • The temperature sensors for the hot RF load and the ambient RF load mentioned in section 4.1. and the environmental temperature sensor shall have a repeatability of better than 0.05 deg. Celsius.
  • 4.3 RF Requirements for Hot RF Load and Ambient RF Load
    • 4.3.1.1 Frequency Coverage
      • The loads shall be designed for a frequency range from 30 GHz to 960 GHz.
    • 4.3.1.2. RF mismatch
      • The reflected power from free space to any of the two calibration loads
shall be in average less than -40 dB across the frequency range specified in section 4.3.1.1. with the exception for Band 1 (30 GHz to 45 GHz) where in average less than – 30 dB is required.


Weather (Ancilliary) Measurement Devices

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 has started to push purchase through ESO purchasing system.

Before the meeting BaltasarVilaVilaro sent around a plot showing two proposed locations for a central weather instrumentation installation which could better serve the needs of the ACA. On this plot, black dot = 12m antenna pad, red dot = ACA antenna pad, blue circle = 30m radius from antenna pad, red circle = 40m radius from antenna pad. Location "A" would be preferred over location "B" as "A" is a bit less crowded by neighboring antenna pads. Will include these ACA requirements in detailed siting of weather instruments to be done in coming months.

Also before the meeting BaltasarVilaVilaro sent around a plot showing the error made in the refraction calculation as a function of elevation for the recently-respecified (by Systems Engineering) OSF weather station redesign. The relatively poor specification for the hygrometer (2% accuracy) drives the error in the refraction calculation. BaltasarVilaVilaro suggests that the specified OSF weather station instrumentation do not have the required accuracy to properly characterize radiometric pointing at the OSF site. Richard suggested that, since this process is well-along, that we consider installing an existing (CR4) or purchased AOS hygrometer in the OSF weather instrumentation package if the poorer performance of the purchased OSF devices is a problem.


Oxygen Sounder

As noted previously, JohnRicher has developed a contact with one of the companies that produces O2 sounders. With BojanNikolic will investigate issues related to operating such a device at 5000 m, among other things.

Our contact has written to us that he expect O2 sounders to work about as well at high altitude as at sea level. In particular, he had been doing a study for a site at 3300m asl and claims accuracy of about 0.5 K in the first 1000m of the atmosphere and about 1K thereafter. One thing which was pointed out by him that a long time series of radiosonde data at a site of the O2 sounder, or a very similar site, is required for retrieval of temperature profiles as the method employed is a machine learning approach. We should investigate availability of long time series of radiosonde data at locations close to the ALMA site -- BN


Solar Observations and Calibration

It has proven difficult to keep the nutator both light enough to nutate and cool enough in the face of the expected solar flux. We have been asked if the four nutators will be necessary for solar observations. Bastian thinks not and is thinking on it. We may need to ensure that these nutators never observe the Sun.

Issue raised by BaltasarVilaVilaro. Has heard rumor that there is a problem with nutators and solar observations and/or observations near the Sun. Perhaps further information available from recent PDR.

MarkHoldaway has written a draft document on Calibration of Solar Observations. We discussed this document at our last telecon. Few revisions are needed. MarkHoldaway will update and submit final version within the coming few weeks.


Date of Next Phone Meeting

The next ALMA Calibration Group telecon will be 2007, July 12th 14:00 UT.
-- JeffMangum - 19 Jun 2007

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Topic revision: 2007-06-21, JeffMangum
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