[alma-config] (no subject)

[Frederic Boone]

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




	

	
		
___________________________________________________________________________ 
Appel audio GRATUIT partout dans le monde avec le nouveau Yahoo! Messenger 
Téléchargez cette version sur http://fr.messenger.yahoo.com