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

[Peter Napier]

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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