Contact Information Agenda/Minutes for meeting Tuesday, 22 November at 4:00 pm EDT.

Date: 22 November 2005

Time: 4:00 pm EDT (6:00 pm Santiago; 2:00 pm Socorro, 2:00 pm Tucson)

Phone: Phone: (434)296-7082 (CV SoundStation Premier Conference phone 3rd floor). No Video planned.

Past minutes, etc on MMA Imaging and Calibration Division Page

News

  • ACA - News from reviews
  • SRR - Significant science IPT items.
  • SSR - Meeting just concluded; link to presentations, notes.
  • Characterization -- RIP. Travel cuts.
  • NAASC news -- Hibbard--Special session Calgary, AAS meeting plans.
  • Homework from today's IPT meeting

Topics

  • Beam squint requirements. B. Lazareff asks: "should the squint be negligible as-built, or will it be measured? Will the inidividual beam shapes (or, at least main parameters such as Az-El width and offsets) be actually measured for each of the two polarization channels? Or should there be a specification for their co-alignment? At least not a spec so tight that co-alignment errors could be neglected." Proposed Spec

JeffMangum: Bernard's suggested "revised proposal" sounds reasonable and mechanically feasible. It may in fact be difficult to mechanically adjust-out any measured beam squint, so I think that this should be part of our "optics calibration" measurements. Note that this is a simple pointing measurement but using each of the individual polarization channels separately. If this is a calibration measurement, it should be carried through to the archive. I'll check with TelCal.

  • Location of the ACA
    • Several possible locations for the ACA exist. "W' was approved but the new array design necessitates taking another look at this location. MH reports that the compact array exceeds the 50m which had been reserved for it, especially when the location of the 12m antennas is taken into account. While this is a favored location (less earth needs to be moved than for some other locations) it needs to be scrutinized. MH will do this, and AW will discuss with Morita and others at ACA review.
    • Holdaway reports that it is difficult to reconcile the position which is in a document from ED with our DEMs of the terrain. Solution--Master00 is not at (0,0).
    • Some sheilding from AOS TB RFI would be good. The move of the AOS TB from an area where it would be hidden from the array to the present location where it is the line of sight was perhaps not good from this perspective. As we discussed though, we aren't sure where it actually is.
Angel Otarola email:

On the location of the ACA, I think our japanese friends are right when they bring the issue that having the ACA westwards from the compact configuration might introduce turbulence advected by the main wind (which is regulary from west). This scenary is possible because the roughness length will increase from near 3 cm -typical of bare soils at Chajnantor- to about 100 cm. (rule of thumb is 10% of the height of the obstacles -here I assume 10m tall antennas-), and this implies larger transfer of energy from the surface into the inmediate atmosphere. The larger eddies advected to the east (towards the ALMA compact configuration) will add more power in the spectrum of the wind at all frequencies. Whether the wind spectra by the ALMA compact configuration will have significantly more power than that specified for the construction of the antennas is a matter of further analysis. At least placing the ACA somewhere north will help to reduce the risk of mechanical turbulence in the core of the ALMA antennas.

Discussion at Meeting:

Myers noted the desirability of some EW component in the baselines, perhaps NE or NW or SE or SW would be best. At the ACA review, we said that we would give some emphasis to imaging quality in the decision on location owing to the liklihood that we will have a 'Combined Array'.

  • Nutator Technical Specs: Here is the latest version of the Nutator TechSpec. Please review it soon and feedback your comments directly to Al. We need to have review comments back by November 28th so the document can be revised and then submitted into the formal DAR approval process.

JeffMangum: Several issues here:

Nutator Rotation Reference Point:

See the AlmaNutator wiki for our discussion of this issue. Minimal science impact puts it at the prime focus, minimal mechanical design impact puts it at the secondary mirror COM. The current techspec puts the nutator/subreflector COM (X,Y,Z)(COM) and nutator mirror pivot point (Zp) at the following position relative to the center of the focus translation stage:
  • X(COM) = 0+-25 mm
  • Y(COM) = 0+-25 mm
  • Z(COM) = -114+-50 mm
  • Zp = -160+-10 mm
(The coordinate system is defined as (right,down,out)=(X,Y,Z) when standing at the prime focus of the antenna looking in the direction that the antenna is pointed.) The question that I have is what calculation lead to the -114+-50mm and -160+-10mm specs?

Maximum Chop Throw and Resolution:

The current version of the nutator specification document has a TBD in a few places for these specifications. Section 5.1.2 (Range of Motion) of the spec states: "The maximum chopping throw shall be continually adjustable with a resolution of TBD arcsecs up to ±20.0arcminTBD considering the neutral position to be when the axis of the subreflector is parallel to the local axis ZN." Two questions for Science:
  • What should the resolution (I think that they really mean "granularity") of the nutator throw be? Perhaps naively, I would think that we want to set the subreflector throw to some fraction of the smallest primary beam. At 950 GHz, the primary beam is something like 6 arcsec. How about 1 arcsec for the resolution? Note that I believe this is not a design driver.
  • Is +-20 arcmin a large enough maximum throw?

Angular Accuracy:

The spec currently states: "The following angular accuracies measured at the SubreflectorÂ’s boresight shall be achieved:
  • 0Hz < f < 6Hz TBD arcsec (RMS)
  • 6Hz ≤ f ≤ 10Hz TBD arcsec (RMS)
The angular accuracies apply to the period of time between the end of transition and the initiation of the following change of position." What is a reasonable number for the angular positioning accuracy of the subreflector? Probably some fraction of the 950 GHz primary beam? Shouldn't it be a small contributor to the offset pointing performance? This would make it something like 0.1 arcsec. Note too that these should be angles on the sky. Not clear if "at the Subreflector's boresight" means also "on the sky".


  • Scattering Cone design. Proto-Memo from R. Hills: Pascal Martinez: "a conceputal drawing of the new Central cone including the comments that have been discussed in the last weeks. The only point that could be open is the play we have between the "cone" itself and the subreflector once assembled. A dia60mm H7/g6 (typical feasible machining that could be assembled at 5000m by people on a cherry-picker), gives a play, therefore a discontinuity in the surface of the subreflector, of +10 to +59 microns (not to be misunderstood with the step of +/-10microns in the axis of the subreflector)."

Clarification

  • How many correlator subarrays does the baseband correlator support? A: John Webber writes: "Let me put it this way: the correlator supports up to 16 configurations, where a configuration defines a static, unchanging state. Each defined subarray can switch among these various configurations at will. You could, for example, have 16 different subarrays each pointing in a different direction and with its own bandwidth and frequency resolution. However, if you want to switch any subarray among several different configurations rapidly, as would be the case with fast switching or with 90-degree phase switching used to measure sideband ratio, then the number of subarrays decreases. The practical number is probably 4 or 6, but there are constraints to all these statements."

Science Corner:

Calendar

Official JAO Calendar

Events of Interest

(see also Al's ALMA Biweekly Calendar)

2005
 
 
 
 
 
Day Date Time (EST) Event location details
Weds
Dec 7
2100 UT
ASAC Telecon
 
 
Weds
Dec 7
 
 
 
Thurs
Dec 22
1500UT
ALMA Board telecon
Telephone
 
Thurs
Jan 26 2006
1500UT
ALMA Board telecon
Telephone
 
Weds-Fri
22-24 March
all day
ALMA Board Meeting
Japan
TBD
Tues-Weds
13-14 June 2006
all day
ALMA Board Meeting
Santiago
TBD
Thurs-Fri
9-10 November 2006
all day
ALMA Board Meeting
Madrid
TBD

Upcoming Meetings

"IR Diagnostics of Galaxy Evolution". 14-16 November 2005, Pasadena, CA

Physique stellaire avec ALMA November 14/15, 2005, GRAAL - Montpellier.

Star Formation Workshop 2005 Dec 11-14 in Taipei, Taiwan,

Latin American IAU Regional Meeting 12-16 December, Pucon, Chile

National Radio Science Meeting 4-7 January, Boulder, Colorado

From ZMachines to ALMA 13-14 January, Charlottesville, VA

Complex Molecules in Space: present status and prospects with ALMA May 7 to 11, 2006, Fuglsoecentret, near Aarhus, Denmark

50th anniversary of the founding of NRAO October 15-19, 2006; Charlottesville, Virginia

ALMA November 13-16; Madrid, Spain

See also NAASC listing

-- JeffMangum - 22 Nov 2005
Topic revision: r7 - 2005-11-22, AlWootten
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