Date: (Friday) 18 May 2001
Time: 10:00 EDT (08:00 MDT, 1400UT)
Phone: (804)296-7082 (CV SoundStation Premier Conference phone).
Past minutes, etc on ALMA Configuration Group Page
Attendees
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Wootten, Woody, Conway, Heddle, Sakamoto, Butler, Kogan, Guilloteau, Holdaway
Agenda
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Leonia brought us up to date on his studies for the compact configuration. These included designs with 20 and 30m road widths, the latter with the ability to access any antenna at any time and the former requiring movement of some antennas to access others. Leonia consulted the antenna group and upon a suggestion investigated center out transport via a single road. Kogan's work on a one road (30m) plan was discussed, along with other designs and factors. Minimum spacing is 15m. This design features good brightness temperature sensitivity, a good beam (1".1 at 345 GHz) and small (<6.4%) sidelobes. Leonia's document details one sequence of moving antennas; several possibilities are allowed. Stephane noted that a systematic approach might lead to systematic uv spacing holes where ideally one might want to pluck antennas randomly. The extent to which this was possible was briefly discussed; there seem some possibilities which might be better explored.
Currently, foundations are designed for the EU and US prototype antennas for the VLA site (no bedrock). For the US antenna, the pad is 11m, which suggests that the transporter will traverse pads as it moves antennas. The pads are level with the ground and can withstand this, though some discussion ensued on this. The EU antenna has a pad of 4m radius. When soil tests are obtained at the site of the compact array and the pad is designed, care should be taken that the site will support antenna foundations anchored in bedrock, not a difficult task from what we know now about the site. A pad designed for Chajnantor will be somewhat different from those designed for the VLA site and it seems likely that they may be somewhat smaller, and/or more capable of supporting transporter passage. Note that in ALMA Memo No. 369 Sakamoto reported on soil resistivity in parts of the science reserve.
There was general agreement that this was a good start for the next step, definition of the exact scale-free configuration by which the array would expand.
Conway reported on his recent exploration of the Chajnantor site. In general, he found that the main 3km area was not as rough as he had worried that it might be. Sites noted by Butler, Min and others all looked good, but the site chosen at PDR has no problems and will be the site for which the design is tailored.
Conway also reported a discussion with ESO engineers about the transporters. These could handle gradients as large as 7 or 10 degrees over short distances; a road could be bulldozed with minimal effort if necessary. Larger gradients incur larger maintenance problems, in general, and are to be avoided if possible. We will maintain close contact with the transporter and antenna engineers as we design the array in detail.
As for the larger configuration, Conway and others explored a number of routes pioneered by Sakamoto, Yun, Butler Radford and others on previous expeditions. Several routes are possible; Max Kraus has built roads on Paranal and has experience. Conway believes the area south and east will be good for the design of hybrids between 3 and 14 km in extent. Various routes were measured via GPS and will be put onto maps for general interest and use. Eventually we will describe the large array as a ring around Chascon, consult with the road engineer.
Conway can begin optimizing a self-similar intermediate configuration soon, coordinating with Kogan and Boone.
Please read Woody's Memo.
David Woody described his consideration of the question of what sidelobe distribution might be expected mathematically from an configuration independent array of N antennas. This would provied an independent measure against which any specific array design might be compared. In a companion memo he will show that a pseudo-random array meets the mathematical expectation exactly. The two memos will appear in the memo series together.
ALMA Memo No. 364 describes Bryan's work to produce a new mask based on the configurations PDR committee recommendations. The mask may be obtained from Bryan's WWW Site.
Last month, Angel Otarola gave me the data for a new digital elevation model (DEM) of the Chajnantor region based on the published IGM topographic maps. The new DEM covers about 51 x 51 km with either 50 m or 250 m pixels. The model's irregular northern edge reflects missing data. The coordinates are UTM zone 19 N and E [m] on the SAm56 datum and altitudes are in m.
The new model complements two existing DEMs, a high resolution map prepared for NRAO and NRO from aerial photographs and a synthetic aperture radar (SAR) map prepared at Cornell. The high resolution map covers about 18.5 x 19.5 km with 10 m pixels. Alignment between the new DEM and this existing model is good, as should be expected since the map maker used the published IGM maps in lieu of ground control.
The SAR map covers 40 x 40 km with 20.3 m pixels. The SSE slopes of the mountains are, however, shadowed. As was previously noted, the SAR map is offset in both position and altitude from other, published maps. Comparing the new DEM to the SAR map, I determined these equivalencies:
| SAR (Andes TM) | UTM (SAm56) | |
| Longitude: | 2185 km W | 630 km E |
| Latitude: | 7449 km N | 7450 km N |
| Altitude:* | 5250 m | 5150 m |
* at Cerro Agua Amarga
The new DEM is now available in several formats here or from the main map page here. Contact R. Giovanelli for the SAR map.
Heddle had noted the availability of his simulator and Conway suggested some interesting simulations. One interesting experiment would be to simulate imaging with missing antennas or groups of antennas. It was thought this would be particularly problematic near the center of the array. Conway suggested simulations incorporating a range of scales and offered to send some test arrays to Heddle. Investigations of the importance of an offset center could also be of interest.
Next Meeting?
We agreed to meet again sometime in perhaps 6-8 weeks during July.
Respectfully submitted, Al Wootten