[alma-config] Re: Near-in sidelobes

[Leonia Kogan]

May I remember that in the case of the natural weighting:

THERE IS NO NEGATIVE THAT IS MORE NEGATIVE THAN 1/(N-1).

Where N is numberof elements in the array.

If N=64, then the most negaive is -1.6%


Leonia

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>From root at cv3.cv.nrao.edu Fri Jun  9 07:27 MDT 2000
From: "Stephane Guilloteau" <guillote at iram.fr>
To: <alma-config at zia.aoc.NRAO.EDU>
Subject: Re: [alma-config] Re: Near-in sidelobes
Date: Fri, 9 Jun 2000 09:57:20 +0200
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>From Min Yun
>> The VLA example I mentioned
>> was just an illustration of the problem I was trying to describe.
>> As Leonia will discuss further in his own e-mail, the dirty beam
>> from zoom spiral has a large wing around it, most of which also
>> has to be negative (to have zero net power under the beam, the
>> true dirty beam should have a negative DC offset).  I suspect the
>> ring of near-in sidelobes in the donut arrays will ultimately
>> limit its imaging capability at some point, but the Gaussian
>> wing, which is negative, is also of some concern.
>
>From Mark Holdaway
>Rather than speculate theorize, lets go further and figure out what sorts
>of imaging problems will be affected by the broad wing.  It seems
>that imaging very large sources would be.  So lets find the cases
>and figure out how big (or small) the effects are.
>
> -M

    A particularly interesting case is that of proto-stellar envelopes and
circumstellar disks. They do have a centrally condensed emission, with broad
low surface brightness feature around.

    At some point also one should try to check how the conclusions are
affected
when short spacings are included. ALMA will be designed for that (as far as
we
can), so if the problem of the negative DC offset becomes so critical, we
should
use that possibility to fill it in...

    I suspect (oops speculate again) that only the level of positive
sidelobes become
important then ...

        Stephane


>




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