[mmaimcal] Re: [Almasci] [Fwd: Antenna Differences]

[Richard Hills]

Dear Mark,

Thanks for yours. 

> V*A and A*V are the same, right (or complex conjugates of each other)?


Sure.  No great significant implied in writing it this way - just 
counting up the cases.

> AND, why do you say that V*A correlations are immune to sources 
> bubbling up through the
> sidelobes?
>
Point was that in the previous paragraph I had concluded that the 
sidelobes due to the legs would be in the form of a cross, with the 
appropriate orientation,  ~100 times larger than the main beam.  I was 
therefore thinking about bright sources that are many beamwidths away 
from the main beam.  In that case the source can only be in the sidelobe 
for one of the antenna types at a time - hence my conclusion.  If 
however we are talking about sources that are in the outer parts of the 
main beam or in the first sidelobe, then I agree that they will 
contribute in both cases.  I imagine that in practice this is the most 
important case because of the dealy and fringe-rate discrimination 
against far-out sources.  Is that right?.

> There are now pretty good techniques for imaging the confusing source 
> outside the main beam
> with "MX" type methods.  Those techniques will probably break down 
> when you get the 3
> primary beam situation ---  BUT we can then make 3 different images of 
> the offending source,
> one for all A*A baselines, one for all A*V baselines, one for all V*V 
> baselines (each image
> will be quite different, depending on how the sidelobes fall on the 
> source).
>
>> 5) It is not obvious to me that the configuration of the legs has any 
>> significant effect on the instrumental polarization within the main 
>> beam.  Can anyone see a mechanism that would do that?
>>
> In an email which you didn't see, I argue that it doesn't matter what 
> the feed legs do to the
> polarization leakage beam -- We have to implement an algorithm which 
> corrects for the
> pol-beam-leakage for a SINGLE antenna design -- and once that is in 
> place, it will be
> simple to extend that to 3 different polarization patterns.
>
>
So where do we stand regarding conclusions?  Do we think that it is 
worth asking AEM to turn the feed legs 45 degrees or not?

One relevant point is the question of whether the patterns would be 
similar enough that we could treat them as identical.  The point being 
that even after they have been rotated the details of the leg structure 
are still very different.  More generally has anyone actually done 
calculations of the sidelobe patterns predicted?

Best Richard