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

[Alwyn Wootten]

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