2007-10-11 ALMA Calibration Group Telecon Agenda and Minutes
Table of Contents:
ALMA Calibration Group Telecon |
- Date: October 11, 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
Action Items from Last Telecon
| Assigned To || Due Date || Description || State || Notify || |
Agenda and Minutes
NOTE: Discussion from the telecon is shown in italic.
- Richard Hills
- Lars-Ake Nyman
Calibration Examples Status
All complete. Please read and comment. See CalExamples
wiki for further information. Note that Calibration Sequence information is now included in each CalExamples
Amplitude Calibration Subsystem Development Reports
Multi/Dual-Load (M/DL) Calibration System (Ferdinand Patt)
To keep this issue moving forward, JeffMangum will draft a CRE to request changes to the total move time and a relaxation of the requirements on the hot load (no need to be used at Bands 1 and 2).
- Ferdinand Patt sent final version of ALMA Calibration Device: Prototype Calibration Load Test Report (FEND-40.06.04.00-005-A-REP); Murk and Duric (2007) (or on EDM ).
- As noted during our last telecon, 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. See CalAmp wiki page for further details.
- As noted during our last telecon, 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 hot load temperature stability requirement. On 2007/09/07 JeffMangum made some calculations of the accuracy of the single- and dual-load calibration systems at Bands 1 (40 GHz) and 2 (80 GHz). Following the same formalism described in Load Calibration at Millimeter and Submillimeter Wavelengths: Mangum; October 18, 2002; ALMA Memo 434 the situation is directly analogous to the calculation done at 230 GHz. Assuming 50th-percentile tau(40)=0.02+-0.002 and tau(80)=0.03+-0.003, and noting that Bands 1 and 2 will be HEMT detectors (intrinsically DSB), the single-load (traditional) chopper uncertainty is ~1% at both frequencies under "best" conditions (as laid out in Memo 434) assuming double sideband operation. The total uncertainty is significantly dominated by the uncertainty in the rear scattering and spillover plus radiative efficiencies (etal). Note that this analysis assumes that the frontends do not suffer affects due to saturation.
Berkeley Absolute Flux Calibration System (Jack Welch)
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 received first set of weather measurement devices at ESO in late September. Will assemble unit at ESO for testing.
for AOS site locations discussed during August meeting. A few kinks under discussion with computing and site include power distribution (440V needs transformer) and fiber lighting (only done when antenna occupies station).
This past week JeffMangum exchanged some emails with Eduardo Donoso regarding site installation logistics for weather stations. Eduardo will investigate proposed weather station siting (see CalAncillary).
We have done some further research on the oxygen sounders. There
appear to be two main approaches that are used to invert the set of
brightness temperatures that the sounder measures to
1) A neural network inversion approach that uses a large database of
temperature profiles from radiosonde launches at the site. This
approach appears to be in the widest use.
2) A maximum likelihood method that relies on meso-scale
meteorological forecast data, and then uses the radiometer data to
essentially make a small adjustment to the forecast model.
I would therefore like to know about:
1) Feasibility/cost of a significant number (1000 or more) radiosonde
launches at the ALMA site
2) Is ALMA already purchasing meteorological forecasting for the site?
What data products?
All of the above is assuming that actually want to know the
temperature profile. For the WVR we are only really interested in the
conversion factors between 183 GHz measurements and paths, so when
using inference techniques it may be simply possible to use the
outputs of the O2 sounder as inputs to the inference process and avoid
making un-informed guesses about the profile. But others may have
other uses for temperature profile data. So another question is :
3) Is the oxygen sounder data going to be used by any subsystem other
then WVR-based atmospheric phase correction ?
In terms of timescales, if we are going to be launching radiosondes,
then clearly it would be much preferable to have the O2 sounder at the
site when these launches are being made.
Finally, given uncertainty in the way the data are going to be used, I
would suggest that our procurement specifications concentrated on the
accuracy of the brightness temperature measurement by the sounder,
i.e., noise temperatures, bandwidths, absolute stability etc. This
would be in preference I think to relying on quoted accuracies of
temperature retrievals. BojanNikolic
- 11 Oct 2007
The requirement that the sounder needs ~1000 radiosonde launches will require some additional effort. Another option is to get an accurate site atmospheric model. Lars is working with one of the Chilean university groups to work on this modeling problem.
Solar Observations and Calibration
has written a document on Calibration of Solar Observations
. It was noted by Richard at our last telecon that Mark's scheme for doing solar amplitude calibration might be a bit pessimistic. If one could alternately observe with half of the antennas with solar filter in and the other half with load in, the loss in observing efficiency could be significantly reduced. Richard will bounce this off of Mark.
Richard believes that Mark said that this is correct.
Frontend Requirements and Pointing
Richard Hills notes that, while participating in the Optics CDR he noted quite a few items that seemed to be missing or incorrect in the current version of the frontend specifications
. One of those issues is the frontend contribution to pointing errors. There is apparently no reference to this in the specifications but it is obviously an important item, given the emphasis on pointing errors in the antenna requirements. It may well be that this has all been covered by detailed design and analysis in documents already (awaiting Frontend IPT comment on this).
The key point regarding frontend positional stability and pointing is that, at the focal plane, a shift in the position of the receiver beam relative to the telescope axis gives rise to pointing error on the sky. By contrast a tilt of the beam at this point causes an error in the illumination of the telescope and phase gradient across the far-field beam but no pointing error. We are at present putting a great deal of stress on the beam tilts but apparently ignoring the possibility of shifts, which are arguably more important.
In the top-level specifications the pointing errors are in fact described as the "antenna" pointing, but it is clear that this has to be a seen as a system requirement. In the antenna requirements the pointing errors are given at three levels:
- "Repeatable": 90 arcsec,
- "Absolute": 2 arcsec,
- "Offset": 0.6 arc sec.
The first of these is a peak error. The others refer to the rms radial error - i.e. both axes combined. Unfortunately the antenna designs have essentially no margins on these figures, so we don't really have any "spare" budget to assign to the frontend. To get an idea of the sort of level we should be looking at, let us suppose that we can allow errors due to the front end (i.e. from the mounting ring down) to be one quarter of those for the system. IF the errors can be added to those of the antenna in quadrature this produces only a 6% penalty overall. In fact, errors arising in the FE are likely to be highly correlated with those in the antenna, so this is not a safe assumption, but taking a quarter of the antenna numbers seems a reasonable starting point.
The relationship between shift and pointing is set by the focal length, which is 96m at the Cass focus. This gives the secondary "plate scale" of 0.465mm per arc second. The allowable shifts are therefore: Repeatable ~10mm, absolute, 0.23mm, and offset 0.07mm. If both axes are to be given the same error budget, the absolute and offset numbers come down to 0.16mm and 0.05mm in each direction.
In the case of the antenna, the repeatable term is mainly gravity and the intention is that this is taken out by the sin and cos of elevation terms in the pointing model. Obviously there will be some flexure of the FE as the system is tilted, but so long as this is elastic it will be taken out in the same way. The figure must surely be a lot smaller than 10mm anyway. Of much greater concern would be the differential movement of one receiver with respect to another (see below).
In the case of the antenna the absolute pointing errors are dominated by thermal effects. These are much less likely to be a problem for the frontend given that it is in a temperature-controlled environment. In view of the "drift" problems seen in the beam direction, the immediate concern here is similar behaviour in the beam position - i.e. that it moves around in a way that depends on the history of the elevation moves. Here a tolerance of ~0.16mm does not seem to be extremely tight for a system like this, but we should certainly have a look at any analysis that has been done and see what the measurements that are being made at present say about this.
As with the antennas, by far the hardest requirement is however the "offset" pointing. This is intended to apply to the case where we make the pointing measurement at one frequency and the astronomical observation at another. This will surely be a common
circumstance since most of the best objects for doing pointing - quasars - are only strong at the lower frequencies. We are therefore concerned about the relative movements between the beam positions of the different cartridges. Although the offset requirements on the antennas are only for movements of up to 2 degrees on the sky and 15 minutes of time, we need to know the relatively positions of the beams of the different
cartridges "absolutely". That is to say, we would expect to measure the pointing offsets for each receiver with respect to the reference one (presumably Band 3) once, presumably using a "broadband" object such as an asteroid, and then use these offsets at all elevations and for a long period of time. It seems to me that a tolerance of 50 microns on the stability of these beam separations might be quite hard to guarantee.
If we are absolutely stuck, then having offsets that are a function of just one variable (elevation) might just be tolerable. Essentially it would mean that one or two parameters in the pointing model would have to be changed depending on which receiver was being used. Offsets that depend on the pointing history or offsets which drift on time scales of hours or even weeks, would certainly not be acceptable.
Richard Hills - 11 Oct 2007
Richard will continue to iterate with the Frontend IPT on this issue.
Date of Next Phone Meeting
The next ALMA Calibration Group telecon will be 2007, November 8th 14:00 UT
- 09 Oct 2007