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

[Bryan Butler]

i don't believe it's a show-stopper for polarimetry.  painful, yes. 
costly, yes.  but not a show-stopper.

it may be a show-stopper for differential astrometry.  some of us here 
at the AOC have been talking a bit about that.  VLB is also potentially 
impacted (harder to phase up the array), but VLB is out anyway with no 
maser...

	-bryan


On 10/27/05 06:07, Jeff Mangum wrote:
> Hi Mark,
> 
> Understood.  I fear, though, that we are slowly sliding into a situation 
> where we *can* correct for many of the problems two antenna designs 
> present, but the effort involved prevents many of these corrections from 
> being made, independent of the good intentions of those involved.  I 
> think that this was Stephane's main point.  Taking the pointing case as 
> an example, most telescope operations spend a great deal of time during 
> their operational lifetime tweaking pointing performance.  For many of 
> these telescopes, these efforts consume a significant fraction of the 
> operational resources, when integrated over the lifetime of the 
> observatory.  The fraction of the total resources devoted to pointing 
> issues was about 20% for the 12m (cryogenic/receiver 
> development/maintenance/diagnosis/repair was the big "winner" for the 
> 12m).  Most of this time is personnel resources.
> 
> So, echoing what I believe Stephane was suggesting in his 2003 comments, 
> two antenna designs adds significantly to the operations cost of the 
> observatory.  Note too that this is a recurring cost, so the integral 
> over the lifetime of the observatory makes this a big number.
> 
> I still worry about polarimetry, though, as being a possible 
> "showstopper" for the two antenna problem.  As Steve is our expert, I 
> hope he chimes-in on this issue.
> 
> Cheers,
> 
> Jeff
> 
> 
> Mark Holdaway wrote:
> 
>> Jeff Mangum wrote:
>>
>>> 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?
>>>
>>
>> If we have one design, we've got one primary beam to deal with, and one
>> polarization beam to deal with.  That is even overstating it, as each 
>> antenna's
>> VP will be a bit different, so we'll have an average beam with small 
>> increments.
>> To perform wide field imaging or wide field polarization imaging, we 
>> will need to
>> take this beam into consideration  --   thats what mosaicing is.
>>
>> Now, if we have two antenna designs, we actually have three primary 
>> beams and
>> three polarization beams.  We'll have one average beam, and the 
>> increments are
>> from that average are larger, but clustered into three camps:  Vert x 
>> Vert, EIE x EIE, and
>> EIE x VERT.
>>
>> Low to moderate fidelity imaging can be done using the average beam.
>> Higher fidelity imaging will require that we use the three different beam
>> models.
>>
>> In some things, "common mode" errors really just disappear -- if you 
>> suddenly
>> added 1 mm to the cables to each antenna, the phase error is zero, and 
>> you would
>> never know about the 1mm excess.    Just beause we have the same 
>> primary beam among
>> all antennas doesn't mean we don't have to treat that primary beam -- 
>> and if we can
>> treat the effects of one primary beam, we can treat the effects of 
>> three primary beams.
>> The cost?   Manpower to characterize those three primary beams, 
>> implement them in
>> software, and make judgements about when we actually need to go to 
>> that trouble....
>> and when we DO go to that trouble (??? 10% of the time ???)  we will 
>> need to do
>> 3 FFT's for every FFT we used to have to do --- if these FFT's 
>> dominate the
>> computing budget, and IF we need to deal with the 3 beams X% of the time,
>> our computer budget needs to increase by  a factor of 1 + 2*X/100  ---
>> 10% ==> 1.20  --- a modest factor.
>>
>>>>
>>>> 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.
>>
>>
>>
>>
>>
>> OK -- if ALL antennas are mispointed the same way and we know it, we 
>> just say
>> "the antennas were actually pointing HERE", change the header in the 
>> data, and pretend
>> that is what we intended to do and proceed with mosaicing (this is 
>> similar to OTF mapping
>> when the wind blows you off-- as long as you know where you are 
>> pointing).  Generally,
>> it will be more complicated than that, and you will have different 
>> pointing errors for each
>> antenna.
>>
>> BUT, lets take the analog of ALL antennas mispointed the same way for 
>> each type of antenna.
>> SO -- Vertex ants are mispointed by (x1, y1),   and EIE are all 
>> mispointed by (x2, y2).
>> SO, just as we have three different primary beams and need to do 3 
>> different FFTs.
>> we treat all Vert x Vert baselines as pointing to (x0+x1, y0+y1) -- 
>> then we treat all
>> EIE x EIE baselines as pointing to (x0+x2, y0+y2)  ---   and EIE x 
>> Vert would be a bit more
>> complicated ---- the Primary Beam then depends on the two 
>> mispointings, but all
>> EIE x Vert baselines will have the same effective primary beam and 
>> effective pointing center.
>> And we are just back to using three different beams, pointing centers, 
>> and FFTs.
>>
>> SO:  its a pain in the BUTT -- but it isn't the end of the world.
>> I think the maintenance and operational issues are a much bigger 
>> argument in the end.
>>
>>   -Mark
>>
>>>
>>> Cheers,
>>>
>>> Jeff
>>>
>>
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