[mmaimcal] Text of AI on antenna quardupod design from Beasley

Wed Oct 26 11:30:30 EDT 2005

Cancel my previous comment. I didn't read carefully enough and thought 
the issue was the primary vertex hole rather than the subreflector hole.
Peter

Peter Napier wrote:
> Al,
> Like Darrel I would assume Point 2 has to do with adequate clearance 
> between the feed beams and the edge of the vertex hole. If it has not 
> yet been done someone needs to use Gaussian beam propagation to 
> determine if the edge of the hole is far enough out on the edge of 
> whichever beam is closest to it. One issue that constrains the hole size 
> is finding a window material that is completely transparent at 1 THz but 
> sufficiently robust that you never get window damage on 50 antennas.
> Peter
> 
> Alwyn Wootten wrote:
> 
>> Folks,
>>
>> Here is the text of our action item and my preliminary response.  
>> Comments?
>>
>> Action Item to the Science IPT (Al and Tom) from Tony Beasley could 
>> you write me a few-paragraph response to each of these and return 
>> asap? Regarding the Quadrupod position - the text you sent yesterday 
>> [from the May 2003 Report to the Board] should cover it. ASAC opinions 
>> the only one we respond to.
>>
>> (second point - question to Jeff Z- what did we do for Vertex?)
>>
>> Thanks... Tony
>>
>> Text from SS:
>>  > Science related:
>>  >
>>  > 1.     Quadripod position.
>>  > Members of ESAC/ASAC have different views on the impact of the feed 
>> leg
>>  > design on polarization. Instead of throwing numbers and statement 
>> on how this
>>  > is going to affect the science of the antenna, it would be better 
>> to use a program
>>  > (Grasp?) to quantify the effects.  It must be remembered  that the 
>> beams of the antennas
>>  > will be different anyway. The japanese also have different feed 
>> legs.   This may justify
>>  > or not  a request for a change the feed leg of AEM. Please pass an 
>> action to Science IPT ( I will discuss it with AEM anyway)
>>  >
>>
>>  > 2.      Subreflector hole diameter
>>  > We have specified 48 mm. In the past  I do believe I was told that 
>> this should be 60 mm
>>  > (probably by Richard Hills?) This is something that I would like to 
>> be checked by science.
>>  > If implemented now (on both antennas it is likely to be a minor 
>> issue). If not needed
>>  > better so. Please pass an action to Science/SE
>>  >
>>
>> Draft Response:
>> Question Number One:  The view of the ASAC is well-expressed in their 
>> April 2003 report:
>>
>>  From the Executive Summary of the ASAC report of the April 2003 
>> meeting, presented to the ALMA Board at their 26-7 May meeting and 
>> noted in those minutes:
>>
>> ...
>> 6. Inhomoegeneous Array:  The ASAC strongly recommends that a single 
>> antenna design be adopted for ALMA.  Having two different antennas 
>> designs seems certain to impact the science capabilities of ALMA for 
>> wide field mosaics and polarization observations, while in a 
>> worst-case scenario, imaging of any significantly extended source 
>> could be affected.  The ASAC recommends that the project consider 
>> whether additional specifications  will [be] required to enforce 
>> consistency between two different designs.  If two different antenna 
>> designs must be adopted, the ASAC recommends that an identical 
>> quadrupod design be used for both antennas, which should reduce any 
>> adverse effects on the science.
>>
>> Later in the report...
>>
>> The ASAC reviewed two written documents on the impact of an 
>> inhomogeneous array that had been prepared by A. Wootten and by the 
>> ANATAC.  We also heard a presentation by S. Guilloteau.  The science 
>> implications of having two different antenna designs arise primarily 
>> from "common mode errors", which would cancel if the antennas were 
>> identical. Common mode errors identified include pointing errors, 
>> phase/pathlength/focus errors, phase effects due to changes in the 
>> fiber length, and polarization matching and primary beam shape.
>>
>> For common mode pointing errors, errors due to wind are likely to be 
>> common in the compact configuration, while solar heating in this 
>> configuration may vary from one antenna to the next due to shadowing. 
>> In contrast, in more extended configurations, common pointing errors 
>> are likely to arise from solar heating, while the wind and its 
>> associated pointing error may vary across the (large) site.  For 
>> errors in phase
>> due to pathlength and focus changes, all mechanical deformations 
>> except that due to the non-intersection of the axes (likely the 
>> dominant effect) would benefit from having identical antennas.  Phase 
>> effects due to changes in the fiber length are dominated by the run to 
>> the antenna; this normally common mode error could probably be 
>> monitored and compensated for in software.  Polarization and primary 
>> beam shape are determined by the quadrupod leg design; having two 
>> different antennas with very similar quadrupod designs could mitigate 
>> the problems here. However, it is worth noting that the Vertex and 
>> Alcatel prototypes do not have identical quadrupod designs.
>>
>> Inhomogeneous array designs also have cost implications during the 
>> construction, commissioning and operations phase.  In the construction 
>> phase, the cost effect could be either positive or negative, depending 
>> on the details of the antenna contracts.  For commissioning and 
>> operations, it is clear that having an inhomogeneous array implies
>> extra costs due to the extra work involved with commissioning and 
>> maintaining two different antennas, maintaining two software 
>> interfaces (for example, different pointing models), etc.  The bottom 
>> line is that anything that increases the cost ultimately affects the 
>> science return from ALMA in a negative way.
>>
>> The ASAC reached the following conclusions concerning the 
>> inhomogeneous array:
>> 1. The ASAC stronly recommends that a single antenna design be adopted 
>> for ALMA.  Having a single antenna design will factilitate several key 
>> observing modes with ALMA, in particular polarization observations and 
>> wide-field mosaics.  It will also reduce the effort and cost required 
>> to commission and operate ALMA.
>>
>> 2.  If two different antenna designs are adopted, the ASAC recommends 
>> that the identical quadrupod design be used for both antennas.  Having 
>> an identical quadrupod design should help to minimize science impact, 
>> again particularly for polarization and mosaic observations.  
>> Minimizing the problems introduced by having two different antenna 
>> designs implies that there should be additional specifications placed 
>> on the designs, for example on the lack of axis intersection, the 
>> thermal coefficient for the expansion of the quadrupod legs, the 
>> profile for the quadrupod legs, etc.  It might be possible to minimize 
>> common mode errors with appropriate specifications on the change of 
>> the antenna with tempreature and gravity and on the wind response.  
>> However, placing a number of additional specifications on the antenna 
>> designs could drive the costs up.
>>
>> 3. If ALMA consists of an inhomogeneous array without stringent 
>> specifications on the quadrupod and other aspects of the 12m antennas, 
>> the ASAC believes the biggest potential impact on the science 
>> capabilities of ALMA will be in the areas of polarization observations 
>> and wide field mosaics.  Polarization mosaics are probably the most 
>> demanding use of ALMA and would likely be extremely difficult with an 
>> inhomogeneous array.  In a worst case scenario, imaging of any sources 
>> larger than roughly 1/4 of the ALMA primary beam could be adversely 
>> affected.
>>
>> For any type of inhomogeneous array, the potential extra costs 
>> involved will take money and effort away from other ALMA tasks and the 
>> end result will be a less powerful instrument.  Having two types of 
>> antennas will have a negative impact on commissioning and operations, 
>> with extra training, software, spare parts, etc. required.  In this 
>> context the ASAC wishes to highlight the impact on the software 
>> effort, as many of
>> the corrections required to operate ALMA with different antennas will 
>> fall to software.
>>
>> Summary
>>
>> The ASAC strongly recommends that a single antenna design be adopted 
>> for ALMA.  The ASAC recommends that the project review the antenna 
>> specifications to see whether additional specifications would be 
>> required to enforce consistency between two different antenna designs. 
>> If two different designs are adopted, the ASAC recommends that the
>> identical quadrupod design by used for both antennas, which should 
>> help to minimize the impact of the different designs on science.  If 
>> two substantially different antenna designs are adopted, the biggest 
>> potential impact on the science capabilities of ALMA will be in the 
>> areas of polarization observations and wide field mosaics.  In a worst
>> case scenario, imaging of any sources larger than roughly 1/4 of the 
>> ALMA primary beam could be adversely affected.
>>
>> Part b. Quantify the results.  We are currently planning to run 
>> simulations to quantify the results.  The results cannot simulate well 
>> the effects of non-cancellation of common mode errors, expected to 
>> present some difficulty to getting the best science performance out of 
>> the array.  The results will focus on using beam patterns to simulate
>> observations of Level 1 Science Goals for ALMA.  The results are 
>> expected to be most severe for measurement of polarization in 
>> protoplanetary disks (Goal 2) and for high dynamic range imaging (Goal 
>> 3); the simulations will focus on those.  The differences in the feed 
>> leg design for the Japanese antennas will not cause a great effect as 
>> these antennas will be used independently of the Vertex and AEM 
>> antennas in a majority of situations.
>>
>> Question Number Two:
>> I'm not sure what the issue is.  Darrel guesses it has to do with a 
>> study of the scattering cone.  Any more guesses?
>>
>> Al
>>
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> 
> 