Minutes for meeting Mon, 9 August 1999 at 4pm EDT.
Date: 9 August 1999
Time: 4:00 pm EDT (2:00 pm Socorro, 1:00 pm Tucson)
Phone: (804)296-7082 (CV SoundStation Premier Conference phone).
Past minutes, etc on MMA Imaging and Calibration Division Page
Minutes
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See Min's notes . The question of whether it is operationally desirable to have a crew committed to moving antennas always will be discussed at the 9 Aug DH meeting.
We should discuss Woody's MMA Memo No. 270. Oops ALMA.
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I proposed the following recommendation:
A review of the state of water vapor radiometry was held in Tucson on 7 June 1999. The following facts arose from this meeting:
1) Continuum water vapor radiometry has been employed at IRAM for some years to correct data for the excess path length caused by differing amounts of water vapor along the line of sight to astronomical objects. Simultaneous 1.3mm and 3mm observations are used for this, resulting in improved images about 70% of the time during which no liquid water vapor is present. A similar system is under development at Nobeyama and the technique has been explored at BIMA.
2) Water vapor radiometry in the 22 GHz line has been employed with good results using a system developed at OVRO, primarily by Dave Woody. Path length variations are corrected down to about 200 microns in virtually all weather, except for rain or snow. Similar systems are being constructed for use at the VLA, the AT, IRAM, and BIMA. They have proven stable, the radiometry shows good correlation with phase, coherence correction is straighforward and previously unusable data has been recovered. Residual delay errors are at the 100-200 micron level at OVRO. The system under development is expected to improve this to below 50 microns.
3) Water vapor radiometry in the 183 GHz line has been demonstrated to provide good correction at Mauna Kea by Wiedner, Hills, Yun and others (see ALMA Memo No. 252 and Wiedner's thesis). Path length corrections better than 60 microns were demonstrated under conditions which apply at Chajnantor 80% of the time. This is within a factor of a few of the ALMA goal of achieving 0.15 radian phase calibration at 230 GHz. Substantial improvement should be achievable on the performance of the receiver which would allow ALMA realization of the goal.
We conclude that water vapor radiometry in the 183 GHz water line will provide a most promising technique for correcting interferometric data for phase noise due to water vapor path fluctuations in the troposplhere. We recommend that construction of a water vapor radiometry system operating at 183 GHz for this purpose become a part of the ALMA project. The systems currently stationed at Chajnantor should be further improved to develop into the final more sensitive system.
Division heads discussed this on 9 August and adopted it as a baseline. Further elements of the baseline plan are that the receivers will be separate inserts and will be cooled.
Just after this, Thompson told me of a JPL plan to put up 9-12 low earth orbit 183 GHz cw transmitters to monitor the atmosphere through grazing incidence. ImCal should send something to Gergely.
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Update on URSI: There will be a 17 Aug meeting with the European DHs at URSI. Kawabe has circulated a tentative agenda for the 14 August workshop. Mangum will speak on amplitude calibration of the MMA. Yun will speak on the strawperson configuration. Radford will give a short presentation on future site testing. Wootten suggested other topics to be covered might include:
Dave Woody has just issued a configuration memo of considerable interest, which I commend to your attention.
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I will update everyone on the status of this event.
The 2nd announcement went out last week to the SOC, LOC and science99 list.
Phil Solomon will give an invited talk.
Posters will be ready for distribution in a day or two. 200 to ESO, one to all on the Jansky advertisement list. Others?
Kate has posted these to the WWW.
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The data rate figure I've been carrying in the project book is 1MB/s average, 10MB/s max sustained. This comes from the Scott et. al. memo (#164) which in turn apparently came from a Tucson science meeting (40 antennas). Mark Holdaway should know about this number. Rupen in a later memo comes up with a science case for 100's of MB/s (OTF synthesis surveys). Note that the NRAO correlator design is capable of dumping data from the LTA at a rate of 1GB/s.
What is the science-driven rate?
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Progress is slow owing to a lack of suitable simulation software. We discussed a) breaking the memo up into parts describing 1) Noise in e.g. EKH chopped and restored maps and 2) Noise in OTF maps. A qualitative description with numbers can be prepared for the project book, which aleady has 1/f noise constraints in a table.
Pending resolution of the problem of a lack of suitable simulation software, I propose that a table of target total power sensitivities be made similar to that which we have for interferometric sensitivities. For specificity, I propose that these be made for a 5' x 5' region at all bands under median atmospheric conditions. ------------
Major issues are definition of the configuration, radiometry as a calibration tool, and imaging.
Kogan and Helfer have specified a strawperson configuration set, discussed at the review on 9 June in Tucson. They are in the process of mating the three smaller configurations to the site. This process in larger configurations is limited somewhat by lack of accurate topographic maps near the Bolivian border; Radford is addressing this problem. The strawperson configuration is then confronted with astronomical images to test its qualities.
Problems include the software packages for the latter test (Helfer uses miriad, Kogan AIPS, Viallefond uses Gypsy, Holdaway used SDE. AIPS++ has agreed to provide a standard package in a several month timescale. C&I has discussed qualities a 'good' imaging configuration should have, with some input from the MAC.
European array designs have supported spiral designs, which we do not feel offer enough short spacings; we have emphasized dual ring designs to provide these.
European ideas have included the continually reconfigured array, in which antennas are moved continuously. This poses an operations question which will be considered at the 9 Aug DH meeting: it requires a crew to be present at the high site devoted to this each working day.
Simulations so far suggest that even the optimized 64x12m array recovers (interferometrically) only about 20% of the flux from a mosaiced image.
Helfer will leave the ALMA/US project next week.
A review of water vapor radiometry on 7 June resulted in a recommendation that the array employ 183 GHz water vapor line radiometry to correct phases for the varying vapor content along the path to the source.
Problems include:
The state of the art at 183 GHz is in its infancy. Twin receivers at the ends of the phase monitoring interferometer are in place at Chajnantor,
and are being improved.
Wilson is building a 183 GHz radiometer to work at the SMA in conjunction with the CSO and JCMT.
The character and location of the receiver and the definition of its backend are still under investigation.
Imaging techniques were reviewed at a workshop at the AOC a few weeks ago.
Problems include:
Total flux recovery into an accurate image still appears difficult with the 64x12m array alone.
Total power models lack the robustness of interferometric models of array performance, in part owing to the lack of simulation software.
Measurements of 1/f noise on prototype receivers, or on any other receiver, are difficult and lacking for most relevant cases.
Frequency coverage:
Problems include:
US Scientists favor a 7mm band, reiterating this view at the MAC meeting last week, while European scientists are unconvinced.
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DECISION: Configurations--where are we?
DECISION: 183 GHz or 22 GHz phase correction?
DECISION: Is a nutating secondary necessary?
DECISION: What is the effect of 1/f noise in the HEMT amplifiers of SIS receivers upon our ability to combine total power and interferometric images into a faithful representation of the sky?
MEETINGS: MAC meeting 25 August 1999 at noon. ------
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If your travel isn't on here you have not sent me a travel authorization.
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T. Helfer:
A. Wootten: 13-19 Aug URSI.
J. Mangum:
M. Yun:
B. Butler:
S. Radford: