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

[Mark Holdaway]

Mark Holdaway wrote:

Al,  I sent a version of this email with POSTSCRIPT files, it is
waiting for your approval for the lists -- KILL IT, as this
version with JPG figures will go through,

     -M

> Richard Hills wrote:
>
>> 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?.
>>
> I have included the beam images I made.  These are actually 4096 x 
> 4096 images filled with
> mostly empty space, derived from the Fourier Transforms of 
> illumination patterns with
> as 12dB edge taper and realistic feed leg shadows (yes, they are still 
> different even if rotated)
> with 1cm pixels  ---  which explains why these beam images look so 
> coarse.
>
> Anyway, as you can see from these figures
>   a) Sources way out there will come in through the sidelobes of any 
> of these PB's.
>   b) The range in the display is from 0.0 to 0.02 (actually, the 
> crossed PB goes
>        negative and is slightly complex)  ---  so we are worrying 
> about <2% effects
>        when the specification on the beam is 6%.
>   Note that surface/phase errors will result in the far out sidelobes 
> not being so circular.
>   As these beams appear now, as the beam rotates on the sky it seems 
> that a far out
>   source would just sit in a sidelobe and groove on it -- in 
> actuality, the sidelobes will
>  be going up and down as it rotates over that far out source.
>
> The differences in the EIE and Vertex beams is greatest at bout the 
> 5-10% point in the main lobe of
> the beam.   I don't know what the most important case is.
>
>
>
>> 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?
>
>
>
> I think the feed leg problems are fairly minor,
> and would rather not interfere with the integrity of the design.
>
>>
>> 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?
>>
> Well, I am imagining that for total intensity we will treat the beams 
> with
> a common model something like 80-90% of the time anyway.
>
>   -M
>
>> Best Richard
>> _______________________________________________
>> Almasci mailing list
>> Almasci at listmgr.cv.nrao.edu
>> http://listmgr.cv.nrao.edu/mailman/listinfo/almasci
>
>

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