Attendees: Robert Brown (NRAO), Charles Cunningham (HIA), Darrel Emerson (NRAO), Horst Stüer for R. Güsten (MPIfR), Mark Harman (RAL), James Lamb (OVRO), Peter Napier (NRAO), John Payne (NRAO), Richard Plambeck (BIMA), Marc Rafal (NRAO), Richard Wade (RAL), and John Webber (NRAO)
John welcomed everyone. Wolfgang Wild and Carolyn White were not in attendance, both due to illness.
Asia A, CSIRO, JPL, etc., photonic work. 100 GHz distribution over fiber. All interested in 100 GHz photodetectors. Main thrust is just to get a simple 100 GHz system. RAL work on integrating available chip, being packaged in WR-10 waveguide. No plans for higher frequency detectors.
Receiver installation package drawings available for Vertex and EIE antennas. Napier says we need side view as well as plane view of receiver packaging. ICD1 has serious constraint on space above dewar. Show volume taken up by optics. Air conditioning uses space inside cabin, but Vertex is working on putting it outside. Plambeck asks if cold head can go in first. Harman has looked at PTR configuration, worried about temperature stability. Payne says it would be better if PT didn't poke out the side. Payne doesn't think shield temperature stability is important. Harman thinks 70 and 4K shield fluctuations CAN affect 4K stage. Wade says baseplate has not much space either; a lot of holes needed in baseplate. Lamb asks about temperature specifications for different stages. Webber says amps unaffected by 25-12 K change.
Webber preliminary report. Needs info from Pan on SIS mixers, and measurements of receiver stability with wide IF. Will be one month more for full report. Conclusion is that 1 in 10e-4 cannot be met with simple HFET receivers. More complicated switching scheme might do it.
Optics drawing without cold load: Carter has supplied a diagram. Mentions Stephane's arguments about cold load, already distributed. Plambeck doesn't like cold loads because of windows, etc. Plambeck/Carter mention importance of cold load at lower frequencies. Wade/Guilloteau: can't say for sure if cold load needed, more study needed, but cold load IS better. Plambeck/Lamb say there are many other issues, 1% is not only a matter of cold loads. Non-heterodyne effects, harmonics come into it. Gain compression is a problem. Payne would like to see Carter's scheme simplified, uneasy about rotating mirror. Guilloteau prefers rotating mirror in Rx system to rotating mirror in S/R. Plambeck says maybe you need a rotating mirror even with a warm load. Maybe can go over these details at Rx face-to-face meeting in Cambridge, rather than trying to solve by teleconference. Brown proposes Cambridge meeting for design review covering all this. Wade agrees, but says advice (from Guilloteau) is cold load below 300 GHz anyway. Payne doesn't like complexity of Carter's design. Carter says only 2 moving mirrors. Carter adds, WVR - does it need a cold load? Wade says 1 Cambridge design needs cold load, 1 doesn't, but should consider WVR independent design. Band 1. Carter says can't use WVR with band 1. Will ask ASAC, but Guilloteau and Emerson think it's not needed. (Kurz reminds us that ASAC says band 1/WVR NOT needed simultaneously.) Band 2 question: how much below 86 GHz?
Plambeck's memo could be modified after Guilloteau's comments. Brown suggests revising and issuing in time for Cambridge. Should Plambeck memo be combined with Mangum? Mangum does not consider gain compression. Plambeck should update memo and put out. Encourage Mangum ditto.
Napier: Offset ellipsoids. References: MMA 115 for VLA, MMA Memo 116 for MMA work. Problem is that offset reflector generates cross-polarization, so principle planes rotate, equivalent to generation phase shift of circular across aperture, so circular polarization beams point at different positions in sky. Large instrumental Stokes V. e.g. VLA 6% bw offset=10% instrumental polarization. Will be worse for ALMA than VLA because of most objects filling primary beam. Start from Murphy paper. Sign error in analysis. Can generate simple formula for beam squint. Napier concentrates on single reflector system (2 can cancel). 5% beam squint, about same as VLA. With more quantitative optics data, could predict more detail. New Carter design has dual reflectors, so all can be cancelled. So, Napier confident we can predict the effects well; can give to astronomers to ask if an issue. 5-6% serious, but 1-2% probably ok. Carter agrees to give details to Napier.
Face-to-face 7-8 Sept in Cambridge. Review meeting or working meeting? Brown says goal to make refinements to Rx design. Brown says "read documents & make decisions so Rx can be built." ASAC mtg Sep 9-10. Webber says premature for PDR. Wade says should be working mtg. in Cambridge, with later formal review, which could be smaller. Payne brings up "how many reflections" and what difference in Tsys does this cause? Lamb says mtg should be carefully planned to make best use of time. Payne asks everyone to send email on whether can attend the meeting in Cambridge and what issues interested in.