[alma-config] (no subject)

[Bryan Butler]

dan briggs' thesis (and the resultant "robust" weighting) does exactly 
this - try to get a "better" beam by reweighting the data.  the power of 
that method is that it reduced the specification to a single parameter, 
the "robustness", making it easy to implement, test, and understand (by 
the astronomers).  in practice, when CLEANing extended structure with 
VLA observations (which i do alot! :), it is almost always better to pay 
a ~5% penalty in noise to get the subsequent improvement in dirty beam 
(you can get significant reduction in the max sidelobe) - better maps 
result.

there is an old VLA memo which probably not very many folks have seen: 
Sault, R., The Weighting of Visibility Data, VLA Scientific Memo 154, 
1984, which is an early description of these things.  i'm sure there 
were earlier discussions (bob's memo has a few references which i didn't 
track down).  i can send or scan a copy of that memo to anyone that is 
interested.

we're always re-inventing things, which is usually fine because it often 
leads to new breakthroughs, or at least increased understanding on the 
part of those doing the re-inventing.

	-bryan


On 10/7/05 05:46, Frederic Boone wrote:
> Hi John,
> 
> Following Al's email and as I promised I will not
> discuss the design anymore. 
> I also swear this is my last email ever to this list.
> 
>> The
>> only (slightly, for radio astromomy) novel aspect of
>> what I did was
>> explicity to choose weights for each uv point in a
>> snapshot to
>> force explicity the beam to a  gaussian shape.
> 
> This is precisly the problem: this is not novel, this
> was done in memo 400 ("compensate the density excess
> with respect to an ideal distribution"). 
> 
> I don't pretend I invented "weights".
> My only contribution at this level was to propose
> weights could be used to force the beam to have a
> given shape. You can say your formulation (i.e. the
> words used and how this is achieved) is different than
> mine but the idea is exactly the same.
> 
>> Most implementations of beam forcing in radio
> astronomy,
>> generally force
>> uniform uv density on a grid, then gaussian taper
> the result (this is
>> I think what is described in memo 400)
> 
> First I don't know any reference mentioning this (I
> would be interested to know about all these very
> numerous references you imply with "Most
> implementations"). As far as I know radioastronomers
> use uniform weighting to gain in resolution and
> tapering to increase the brightness sensitivity but I
> don't know any example where both are used together.
> 
> Second, this cannot work efficiently because to force
> the shape of the beam it is required to get a sense of
> the actual distribution of weights in the data and
> this is very difficult with the gridding used in the
> method you mention (it requires super-Nyquist sampling
> everywhere to be really efficient).
> 
> I have to acknowledge the appendix of the memo was not
> complete enough, and I always thought I should write a
> memo to describe the method used in more detail (I am
> still planing to do this and you give me more
> motivation now). I gave only few more hints (but still
> not enough details) in my thesis manuscript (online on
> my web page, p39, sorry this is in french).
> 
> Regards,
> Frederic
> 
> 
> 
> 
> 	
> 
> 	
> 		
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