[mmaimcal] Re: [Almasci] Two antennas

[Bryan Butler]

you all may be interested in a paper in the most recent Radio Science:

Ng, T., T.L. Landecker, F. Cazzolato, D. Routledge, A.D. Gray,
R.I. Reid, & B.G. Veidt, Polarization Properties of reflector antennas
used as radio telescopes, Radio Science, v40, RS5014, 2005
(DOI 10.1029/2004RS003209)

ADS link: 
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2005RaSc...40S5014N&db_key=PHY&data_type=HTML&format=&high=412b55317309610

it would be nice to do some proper GRASP8 modelling of this for the two ALMA 
designs...

	-bryan


On 11/7/05 12:40, Mark Holdaway wrote:
> Bryan Butler wrote:
> 
>>
>> if it really *is* only 0.3%, then the nearly obvious conclusion would 
>> be that the antenna-to-antenna variation will be almost that large 
>> anyway, so nothing need be done.  i don't think we can even measure 
>> the beam (polarized or not) to 0.3% in any case.
>>
>> i'd be surprised if it really *were* only 0.3%, though, theoretically 
>> or in practice.  how much do we trust the beam simulation?  has 
>> anybody put it through GRASP yet?  if it's only 0.3% difference 
>> between the two designs, then this means that the entire effect of the 
>> feed legs on the polarization beam is only 0.3% (peak), and that the 
>> differences between the two designs have little to no area where they 
>> give opposite effects.  seems a bit small to me - if you simulate the 
>> polarization beam with and without the feed legs, you only get 0.3% 
>> difference?  and is that absolute or relative difference?
>>
> That 0.3% is an RMS difference for some PB level (ie, all pixels with PB 
> value ranging from 0.05 to 0.10). The peak difference (given the 
> geometrical shadowing profiles I was given)
> will be.....   huh?   3.3%    OK, I was expecting a bigger number too.
> 
> Keep in mind that there will likely be other beam effects which are 
> different between the two
> antennas -- for example, the dishes will deform differently from 
> gravity, so at high freqs, the
> beams will behave differently with elevation.   While things look good 
> at the moment for
> two different ant designs, there is a whole mountain of work we will 
> need to do, and at each
> step we will need to determine if we can treat the antennas as the same 
> or as different.
> 
>    -Mark
> 
>>     -bryan
>>
>>
>> On 11/7/05 11:35, Richard Hills wrote:
>>
>>> Dear All,
>>>
>>> It would certainly be good to get a definite statement from the 
>>> software IPT as to what is presently planned and budgeted in this 
>>> area.  Meanwhile, could we go back to the original issue please, 
>>> which was whether anyone thinks that the 0.3% (peak) difference 
>>> between the patterns caused by the different blockage patterns 
>>> requires that we implement special data processing and/or makes a 
>>> case for having the legs rotated.
>>>
>>> Best Richard
>>
>>
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> 
>