Minutes of 05 March 2003 Telecon of USSKA Consortium Written 05-07 March by JMC In attendance: T. Beasley, D. Bock, J. Cordes, G. Cortes, D. DeBoer, J. Dickey, N. Duric, S. Ellingson, P. Goldsmith, K. Kellerman, J. Lazio, C. Lonsdale, J. Tarter, Y. Terzian, S. Weinreb, J. Welch At the very least, please see the section labeled IMPORTANT. 1. Introductory Comments (Cordes et al.) We need to respond by 31 May 2003 to EMT and ISAC comments on the Large-N/Small-D (LNSD) Whitepaper of 2002 June. The format of the response is as follows (from the ISSC as conveyed by Russ Taylor 11 Feb): "The updates are to be stand-alone documents containing three elements. 1) Up to three pages with a succinct summary of any substantive changes to the proposed design concept. 2) Up to 17 pages with specific replies to questions posed by EMT to the proposed design concept. This should include a description of any linkages between the technology concept and an SKA site. 3) Up to 10 pages containing a reclama to ISAC assessment of the proposed design concept as represented in the Compliance Matrix and associated notes. The Compliance matrix can be found at http://www-astro.physics.ox.ac.uk/%7Esr/ska/ska_matrix.html. The updated white papers will be published in the SKA Memo Series. Please send the updated white papers by May 31 to Richard Schilizzi. Contact information is provided below." There are 8 explicit questions on page 9 of the EMT Document, "SKA Concept Designs (Whitepapers) --- EMT Comments" by Peter Hall, 1 October 2002. Two additional issues are mentioned on pp 19-20. Tarter pointed out two additional issues, one on page 1 of the EMT document referring to "the ISSC's firm cost target of USD 1 billion." Ensuing discussion contested whether this was truly an ISSC dictate and the sentiment expressed was that we should concern ourselves with a $1 to 2B range and concentrate more on scientific specifications. The other issue, on p. 15 of the EMT report, is a recommendation from the Interference Mitigation Working Group that there be elaborations on the pros and cons of each design with regard to RFI vulnerability and any systemic advantages in RFI mitigation. We agreed to go through these points during the telecon. Cordes also pointed out that a second task is to update our Whitepaper on a self-imposed deadline of 1 July 2003 in advance of the Geraldton meeting and with the following time table: 1 April Drafts of new chapters 25 April Meeting in Washington, DC (primary purpose: discuss responses to EMT, ISAC comments; we also discuss our updated chapters; and discuss the Siting and Hosting White Paper ) 1 June Updated drafts of sections 1 July All sections placed on web publicly in advance of the Geraldton meeting. 2. Immediate comments on frequency coverage: Welch argued that the scientific needs for high-frequencies (e.g. > 22 GHz) were slim and suggested that for that reason, there may be a large cost benefit in considering 22 GHz (or so) as the upper frequency. Weinreb counterargued that the cost differential might not be significant. There was general discussion about overlap with ALMA and whether it would subsume SKA science above 30 GHz. It was felt that the much greater area of the SKA would be very beneficial at this frequency. 3. Tarter pointed out a memo by Carole Jackson, "SKA Science: A Parameter Space Analysis" as a useful discussion for our deliberations on frequency coverage, multibeaming, and other specifications for the SKA. This memo may be found at http://www.skatelescope.org/memos/skamemo29.html 4. Dickey summarized discussions by the ISAC and a talk by Frank Briggs about the implications of recent CMB results from DASI and WMAP for EoR studies. Polarization results suggest two phases of ionization, one beginning at about z=20 by a population of early AGNs or giant stars; the second phase finishing the job by z=6. The implied range of frequencies is thus 70 to 200 MHz in which frequency structure might be seen from redshifted HI. The new finding doesn't necessarily alter the nominal low frequency cutoff for the SKA (150 MHz) but it underscores the role LOFAR will play in scouting out potential signatures. There was some discussion about whether the EoR band should be analyzed with dishes of the LNSD concept or whether we should contemplate augmentations to LOFAR itself as a means for further study of the EoR. JMC comment: as a Consortium we should address this issue in both our 10 page response to the ISAC about the compliance matrix and also in the science section of our updated WP. My opinion is that we anticipate possible hybridization of the SKA that would include LNSD combined with some kind of low-frequency array such as the tile design or a LOFAR design. 5. Discussion about the questions/comments in the EMT report then took place. Addressing this constitutes up to 17 pages of our report due on 31 May 2003. These questions and comments are from pp. 9, 15, 19 and 20 of the EMT report: (1) Compared with filled aperture, large-D, proposals, the instrument outlined has somewhat reduced surface brightness sensitivity for low spatial frequencies. Can the authors quantify the brightness sensitivity at various array scales and mention how the reduced sensitivity might affect the science done with the instrument? (2) With 15 m minimum spacing, the 12 m antennas will be closely packed. Would the authors clarify the low-elevation shadowing situation? What is the minimum un-shadowed elevation? (3) The 12 m dishes are shaped for efficiency. Have the authors considered the effects on the off-axis performance and the implications this might have for any future retro-fit with focal plane arrays? (4) Can the authors give any more details of the dish mount and its likely mechanical performance (including reliability)? (5) Have the authors had any further thoughts on the form of the "swing away" arrangment for the prime focus receiver ? (6) Could the authors outline further the operation of the new-generation cryo-coolers and the commercial drivers for the assumed cost reductions? (7) In the ATA, cooling to ~ 80 K is considered to be adequate and it appears that the path to cheap, reliable, pulse tube coolers is clear. Could the authors comment on the technologies and tradeoffs involved in cooling to 15 K in their proposed design? (8) Could the authors clarify the feed proposals for the highest frequencies? Are ATA-style feeds a possibility, both in terms of electrical performance and mechanical mounting arrangements for e.g. internal cryostats? Given the possible applicability of the TRW feed to many SKA concepts, are the authors able to supply any further details at this stage? If the TRW feed proves unsuitable, can conventional feeds be used in the available space near the Gregorian focus? Could the authors outline the cost and operational consequences of using conventional feedhorns above 10 GHz? (9a) Question of confidence in being able to scale-up from SETI 6m design to a 12m using current hydro-forming techniques; (9b) Transportation of 12m diameter antennas to remote sites is not a trivial problem. (10) The USA and India should be encouraged to collaborate to see if the Indian low-cost design concepts can be extended to the USA reflector design. (11) Future comparisons of SKA concepts would benefit from elaboration of the pros and cons of the designs with regard to their RFI vulnerability or their systemic advantages in RFI mitigation. Brief summaries of our discussion follow and names of those responsible for drafting text and relevant analysis are included. Note that some of the discussion referred to an email sent by Weinreb to usska@seti.org on 15 Feb 2003, where he included his responses (as of 05 Aug 2002) to questions (3)-(8). Weinreb also sent a 17 Feb 2003 update of sections 5-6 of the WP to usska@seti.org and others. This update will contribute to both our response document and our updated WP. ******IMPORTANT******************************************************** Below, I've designated LEAD AUTHORS by an asterisk. They should draft their section in consulation with the others. JMC would like to receive these sections two weeks from today, Noon, 21 March. Please send them to Cordes and Kellerman with cc to all. We will incorporate them into a draft reponse and circulate them for discussion in a succeeding telecon. *********************************************************************** (1) [Bock, Cordes, Dickey, *Lonsdale] Our current WP already contains information needed to address this question but it is tersely presented. We can address this by posing two or more alternative configurations, including those with more area within 1 to 2 km, say, and perhaps within 1 to 10 km as well. Adjusting the configuration of the Large-N/Small-D (LNSD) concept is a very positive feature. The issue of surface brightness sensitivity on large scales is important for all designs. Answering the question requires interplay between optimization of beam shapes, etc. and science goals. The collecting area in a core array is not only an issue for imaging but also for blind surveys of pulsars and fast transients. Dickey said that, for imaging, having significant area within 1 to 10 km is crucial. Phasing of a core array into all-necessary beams to cover the FOV for blind surveys (transients, pulsars, ETI) pushes the configuration toward the smallest possible size. Lonsdale pointed out that LOFAR will have an all-sky monitor mode, with the implictation that transient science with the SKA will be influenced by LOFAR results. (2) [*Bock, Cortes, Lonsdale] The minimum unshadowed elevation is 60 deg for 15m spacing and 45 deg for 24m spacing in a central core array. For reference, shadowing begins at 33 deg elevation for the ATA which has minimum 1.8 x diameter spacing (10.8 m). The onset of shadowing is not a hard limit on usage! One can always trade sensitivity against shadowing and we might present configurations optimized for observing the Galactic Center from a SW North-American site. Details about appropriate spacings and implications are TBD with regard to science goals. We can sharpen the discussion. Should refer to an EVLA memo by L. Kogan ("An EVLA Compact Configuration with Optimized Side Lobes" = EVLA Memo 30, http://www.aoc.nrao.edu/evla/memolist.shtml), which contains a section on shadowing. DeBoer pointed out that a groundscreen can reduce Tsys increases from shadowing. Cortes has detailed drawings of the 12m offset design that can aid considerations of shadowing and physical interaction and will send them to everyone. (3) [*Cortes, Weinreb, Welch] There is a tradeoff between shaped and unshaped reflectors with regard to focal plane array capabilities. Our stance is essentially that the LNSD concept meets FOV specifications already, so any future need for focal plane arrays goes beyond overall current specifications for the SKA (in the LNSD concept). Moreover, wideband feeds are not particularly compatible, spatially, with focal plane arrays. Concerns about wide-field polarization capabilities were raised but were assuaged. Nonetheless, it is felt that the jury is still out about the pros and cons of shaped vs. unshaped reflectors and attendant tradeoffs between sidelobes, FOV and sensitivity. Welch argued that the 10% cost-savings from the greater efficiency of a shaped reflector (as discussed in Weinreb's initial response) might be negated by the need for greater surface accuracy on the outer edges. There was sentiment that we should outline concept alternatives that consider both shaped and unshaped reflectors. On this issue and more generally, we should make our presentation in an updated WP more sophisticated with regard to alternatives and flexibility of the overall concept. (4) [Bock, Goldsmith, Weinreb, *Welch] Welch reported that pointing of ATA antennas is much better than the specification (10 arcsec rms, but in unspecified wind conditions). Bock and Welch stated that observations in 40 MPH winds would be possible. Weinreb stated that reliability of the ATA mount will be much better known in the next few years. Larger (e.g. 12m) antennas that go to higher frequencies obviously require greater precision in the mounts. The first 12m antenna for the DSN prototype array (cost approx. $167k/antenna, fully outfitted) will be designed by the end of 2004 and will be operating in 2006, yielding much information on the mount issue and other performance issues. Bock and DeBoer point out that there is a continuum of possible dish sizes. There was disagreement as to whether electronics costs would push the optimal dish diameter to lower than 12m. Our stance on this should again be one of flexibility in the design. (5) [Cortes, *Weinreb] The swing-away prime-focus feed was felt to be a non-issue because the receiver is uncooled and thus lightweight and the longer wavelengths imply that precision of positioning is not difficult. A detailed design can emerge on the appropriate time scale, perhaps with a prototype in 2007. There was also discussion about TRW and ATA feeds which is included in item (8) below. (6+7) [Goldsmith, *Weinreb, Welch] Weinreb's initial response discusses cost issues between cooled and uncooled receivers, assuming design-to-specification. Cooling is therefore cost effective from an A/T point of view, but life-cycle, maintenance and power costs must be folded in as well. One of the prime issues is the break point frequency between uncooled and cooled receivers. Weinreb is contemplating moving the BPF to 2.7 GHz. There are anticipated improvements in transistors and cryocoolers, so the optimization point will evolve. However, the considerations of the BPF and whether 80K or 15K is needed depend on science issues. Welch argued that since HI is a showcase target for the SKA, that we should optimize for its detection, implying that at 1.4 GHz the system should be cooled. Weinreb and Dickey pointed out that redshifted HI is a prime target; Dickey said that the the BPF is between 1.1 and 1.2 GHz in considering detectability of galaxies. Cryogenics cost go up with increasing wavelength. To address the stated questions, we can allude to the ATA timeline and what we will learn from the ATA on these issues. (8) [Cortes, DeBoer, *Weinreb] Weinreb has an update on specifications for the TRW feed contained in his redraft of sections 5 and 6 of the WP (see above) and also in a stand-alone document that he has sent to some. More information will emerge about the TRW feed performance and improvements made by TRW on the VWSR. Per-Simon Kildal is looking into feed concepts as well. Information about the ATA efficiency will be available on a time scale about that of the due date (31 May 2003) of our response (Welch). DeBoer will provide a paragraph or two on ATA feed performance that parallels that of the TRW feed. Cortes has considered feed placement with respect to overall optics and physical constraints. (9) [Weinreb, *Welch] At present all we know about scaling the hydroforming process from 6m to 12m (or intermediate size) antennas is from a CAD model (Weinreb) that includes springback. Over the next few months, we may not be able to say much more than this. The CAD model needs more sophistication on other issues, such as the uniformity of the aluminum. Welch asked if the figure of the dish is usable as is, simply bolting into place. Weinreb answered that (for 12m and performance up to 34 GHz), some manufacturing adjustment may be needed in assembly. (10) [*Welch] There has been discussion with G. Swarup about the drive screw mount, so there is some cross-talk. Might we consider the Indian design for an array of low-frequency optimized antennas in a hybrid design? (11) [Bock, Cordes, Dickey, *Ellingson, Lonsdale] We did not discuss RFI issues at all in regard to this specific comment in the EMT report. However, reflector shaping and the wideband feeds enter in to this discussion. I suggest that we include a brief discussion about some of these issues and about development work on RFI mitigation underway. ------------------------------------------------------------------------- Abbreviations and Acronyms: ------------------------------------------------------------------------- FOV = Field of view WP = White Paper document containing the LNSD concept LNSD = Large N, Small D