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

Thu Oct 27 13:37:19 EDT 2005

Hi Bryan

VLB isn't out only postponed--it is still in the science requirements. 
Like the last fourteen antennas, it is 'unfunded scope'.  But it is much
more likely to be funded quickly than the last 14 antennas. It is a good
point but I don't know how compelling Stefano will find it--since VLB is
unfunded scope at the present second.  I'll bring it up just to invoke a
reaction however.  All details are appreciated!

Clear skies,
Al
On Thu, October 27, 2005 1:06 pm, Bryan Butler wrote:
>
> 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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