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

[Mark Holdaway]

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
>