Calibration Group Phone Telecon Meeting Minutes - 2002Oct11 Attending: Becmann, Butler, D'Addario, Guilloteau, Hasegawa, Hills, Holdaway, Isaak, Koda, Lucas, Mangum, Mundy, Pety, Radford, Richer, Saito, Wilson, Woody, Wootten, Wright (apologies to anybody I missed!) Agenda: - results from ASAC meeting - action items left over from last telecon: . gain compression - is it a problem? . photonic device - discussion of memo review process and reviews already received - new memos/proposals . memo 434 (mangum) . larry's new proposal - discussion of level 2 milestones, specs documents, timeline... - next phone meeting? - next face-to-face meeting? Minutes: - Richer reported on the results of calibration discussions at the ASAC meeting in Socorro a month ago. He summarized Guilloteau's presentation thereat. The report has not yet been finalized, but when it is (and has been presented to the ACC), it will be distributed to the calibration group. The most important points were that the ASAC was keen to see a more detailed timeline (this should be answered to some degree by the Level 2 milestones), and to get another update in 6 months or so (since the calibration group is so new). Butler brought up the restructuring of the ASAC, and Richer & Wilson (with affirmation from Guilloteau & Wootten) answered that it was still uncertain what the new structure will be, or how that will affect communication with our group (i.e., whether there will be a specific 'calibration liaison' still). We will just have to wait and see. - Old action items were then discussed: . Guilloteau pointed out that gain compression was dealt with in memo 423 - unfortunately no reviews of that memo have been received yet (Myers & Blake are on the spot here). Further discussion was deferred until those (or other) reviews are received. . Butler pointed out existence of article relevant to photonic calibration device in Electronics Letters (Hirata et al., Elect. Lett., v.38, p.798-800 - online at: http://ioj.iee.org.uk/journals/el/2002/15/20020553.html for those that have access) - which he had found out about from Wootten, who had found out about it from Payne. Wootten pointed out that NRAO (through Payne) has a contract with NTO which has now gotten through the business office, and will involve testing of some photonic devices (testing at SRON - including work with Moseley's group and with the Herschel group). We will need to keep up with this work. - We then turned to the memo review process. Butler summarized the current situation - of 23 original requests for reviews, we had 4 declines; 4 who have never responded; 6 who agreed to review but have not done so yet; 9 completed reviews; and 2 completed unsolicited reviews. Unfortunately, we have 2 memos (including memo 423 as pointed out above - 402 is the other one) which have had no review to this point. Butler thought the exercise was still useful, despite mixed response, since it has encouraged discussion and will assist greatly in coming up with the 2 documents necessary to meet the upcoming Level 2 milestone deadlines. Guilloteau pointed out that he intends to formally retract memo 422, since it has errors, and he would like it not to be mistakenly taken as valid work (Butler notes posthumously that it's not clear how this can be done). Hills commented that the information in reviews received on memo 352 is being incorporated into a new document describing the WVR systems. It is unclear what form this report will take and how it can be distributed, however, since it is really a proposal to the European side of ALMA to continue the work there. Hills then reported briefly on some work done at JPL on MMIC-izing a 183 GHz WVR (this was work that had been reported to Hills & Butler by P. Napier at the URSI GA). There are some problems with the system, notably 1/f noise, and it is early on in the development, but the Cambridge group will keep abreast of further developments (if JPL/NASA pumps money into it, it could get interesting). - We next discussed 2 new memos/proposals: . Mangum summarized memo 434 (see it at: http://alma.aoc.nrao.edu/memos/html-memos/abstracts/abs434.html ). At low frequencies, either a single-load or dual-load system seem to be capable of reaching the required 1% accuracy. At frequencies > 350 GHz or so, however, they each have problems. For the single-load (semi-transparent vane) system, the difficulty is in measuring the mean atmospheric temperature. For the dual-load system, the difficulty is in determining the atmospheric opacity. Also, for both, the sideband gain ratio must be known to ~1%. This prompted a lively discussion on opacity variation across the ALMA site. How much does it vary? How densely will it have to be measured (in space & time) - and how accurate (expensive) will the devices need to be? Will the WVRs help in this respect? All of the amplitude calibration strategies assume accurate knowledge of various atmospheric quantities, so this is a serious issue. Holdaway volunteered to have a look at current site data to try to determine spatial variations from temporal sampling. . D'Addario summarized his new proposed scheme for calibration ( http://listmgr.cv.nrao.edu/mailman/private/almacal/2002-October/000067.html ). The main point here is that the calibration sources should have relatively well-known (and simple) structure, and can be monitored for absolute flux density, so use the power of the interferometer when observing them to derive the interferometric gain. This gain then must be transferred over to the single dish measurements. Also, the point that the cross correlation quantity is not simply a correlation coefficient, but can rather be turned into a real 'cross-power' measurement, is important (Butler - this is just what we normally call the Van-Vleck correction - or the quantization correction, along with a known 'gain' - isn't it?). This prompted lively discussion. Guilloteau pointed out that this scheme was not really so different than what was proposed in memo 372. He also pointed out that decorrelation is an important issue. D'Addario stated that he assumed that it was measured and corrected for. Woody brought up self-cal and power non-linearity issues. Guilloteau reminded that self-cal leaves the absolute power (or flux density, if you will) level unknown. This brought us to the monitoring of the secondary sources. How often? How does it interact with scheduling? This is a tricky issue, and is not only applicable to the D'Addario scheme. Several folks brought up the problem with not measuring the sideband gain ratio. This is only a problem with the single dish observations, of course. D'Addario admitted that this required more thought (Butler - and, in his defense, the ink is so fresh on his proposed scheme that issues like this were bound to arise). Another issue is the measurement of the bandpass, which D'Addario assumes does not vary except electronically - this ignores the atmospheric contribution. Butler asked if D'Addario would be providing a more formal description of the proposed scheme (e.g., in an ALMA memo) - D'Addario was noncommittal, stating he needed to think more about some of these issues. Wootten pointed out that we will be having a calibration sub-group meeting in Tucson on October 21 (at least Wootten, Guilloteau, and Butler will be there, and hopefully Mangum, D'Addario, and Radford can sit in on at least parts), and that it might be nice to have an update then (or we can at least discuss any progress or changes to it then). Radford pointed out that we should really finalize a schedule for this meeting, so that he can plan accordingly. - Time was pressing, so detailed discussion of the Level 2 milestones was deferred. Butler promised to send them out to the group after Wootten, Guilloteau, and van Dishoek had finalized the list along with descriptions. Discussion can follow by email, and it will be an agenda item for the next telecon. - Brief discussion of date for next telecon produced November 15 as a possibility. Butler will send out an email asking folks if this date is OK for the next one. dutifully scribed by bjb. 2002Oct11