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

[Jeff Mangum]

Hi Mark,

Comment below...

Mark Holdaway wrote:

> On Beam issues:
>
> I'm not sure that the different quadripod positions will be a big deal 
> for
> polarization.  Or more exactly stated, I don't know how to do that 
> calculation
> to get the polarized beams.
>
> The total intensity beams are simple to calculate, and the feed legs 
> will make
> sidelobes of order 1-3%.   However, we never fixed the incorrect 
> specification
> that we know the primary beam to 6% accuracy, so it won't be too
> convincing a problem.  Doing the calculations will at least tell us 
> what level this
> is, and using real feed leg data (the two times I did this in the 
> past, I just made
> stuff up and passed  it by people like Tim Cornwell or Darrel Emerson
> or JingQuan, who said "Yup, a bit more like *this* and it should be 
> sort of
> realistic").

But doesn't this issue fit into what Stephane called "common mode 
errors"?  If all antennas have the same feed leg induced beam error 
pattern, then the "signature" of this error term in a polarimetry 
measurement would allow for its calibration/cancellation.  Won't 
dissimilar designs make this cancellation process much more difficult, 
or perhaps not doable at all?

>
> And a final security blanket statement: in principle, we will be able 
> to deal with
> either the differing total intensity or the polarization beams in 
> software;  this is an
> effort we knew we would have to address at some point -- these 
> algorithms may have
> been priority 3 in the SSR's list, having two antenna contracts 
> probably brings
> that priority up to 2.

Can the same be said of either pointing calibration or terms which are 
due to gravitational deflection?  The commonality of errors due to 
pointing, pathlength, and gravitational deflection seems to be a major 
advantage to being able to do all of these things which haven't really 
been done before.

Cheers,

Jeff

>
> I have no insight into the subreflector hole, except that at least I 
> have the right hole
> in my head.
>
>   -Mark
>
> 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
>> >
>>
>
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