[mmaimcal] Widgets: further forwarded messages from Darrel Emerson

[Al Wootten]

From: Jeff Mangum <jmangum at tuc.nrao.edu>
To: Darrel Emerson <demerson at nrao.edu>
CC: awootten at pisco.tuc.nrao.edu, Andrey Baryshev <A.M.Baryshev at sron.rug.nl>,
        Matthew Carter <carter at iram.fr>, Brian Ellison <b.ellison at rl.ac.uk>,
        James Lamb <lamb at ovro.caltech.edu>,
        Yutaro Sekimoto <sekimoto at nro.nao.ac.jp>, John Payne <jpayne at nrao.edu>,
        Wolfgang Wild <wild at astro.rug.nl>
Subject: Re: [Fwd: Widgets]
Date: Wed, 18 Oct 2000 11:53:15 -0700

Hi Folks,

Just a minor comment on John's widget list...

The semi-transparent load could probably be incorporated into the
ambient load system, if we do it at all.  At this point, I don't see
any advantages to the semi-transparent load except for the fact that
it avoids possible compression in the receiver system.  Note too that
our intention is to test the two-load apex calibration (BIMA-style)
system.


---------- Forwarded message ----------
Date: Wed, 18 Oct 2000 12:00:15 -0700
From: Larry D'Addario <ldaddari at tuc.nrao.edu>
To: John Payne <jmpayne at nrao.edu>
Cc: Darrel Emerson <demerson at tuc.nrao.edu>,
    Wes Grammer <wgrammer at tuc.nrao.edu>
Subject: Re: Widgets

John Payne writes:
 >  -first the list of possible widgets:
 >              1) Ambient losd.
 >              2) Semi transparent load.
 >              3) Quarter Wave plate. ( maybe-for how many bands?)
 >              4) Solar Attenuator.

I see no point in 2) or 3).  Each causes more problems than it
solves.  So the list has only two required items, and I can't think of
any others that might be wanted.

--Larry

---------- Forwarded message ----------
Date: Wed, 18 Oct 2000 12:07:48 -0700 (MST)
From: Wes Grammer <wgrammer at nrao.edu>
To: Larry D'Addario <ldaddari at tuc.nrao.edu>
Cc: John Payne <jmpayne at nrao.edu>, Darrel Emerson <demerson at tuc.nrao.edu>,
    Wes Grammer <wgrammer at tuc.nrao.edu>
Subject: Re: Widgets


I'm not sure about 2), but don't we need 3) for doing circular
polarization?

---------- Forwarded message ----------
Date: Wed, 18 Oct 2000 13:31:34 -0700
From: Larry D'Addario <ldaddari at tuc.nrao.edu>
To: Wes Grammer <wgrammer at nrao.edu>
Cc: Larry D'Addario <ldaddari at tuc.nrao.edu>, John Payne <jmpayne at nrao.edu>,
    Darrel Emerson <demerson at tuc.nrao.edu>,
    Wes Grammer <wgrammer at tuc.nrao.edu>
Subject: Re: Widgets

Wes Grammer writes:
 > ... don't we need 3) for doing circular
 > polarization?

No, we don't.  A QWP enables us to produce a beam that has *roughly*
circular polarization, as opposed to one that has *accurate* linear
polarization.  For a point source, the issue is how well the actual
polarization of the instrument can be known.  For an extended source,
the main issue is how much the polarization varies over the beam.
Both issues can only be made worse by inserting the QWP.

Here I am assuming that we always have a dual-polarization receiver
and that we measure the full 2x2 polarization matrix.

---------------end forwarded messages----------------

Steve and Crystal sent along some comments on QWPs this am (SM in what follows).

If I can summarize the above comments and theirs:

Advantages:
1) 'roughly' circular polarization
2) SM: Q and U are derived homogenously from the RL and LR cross-products and
thus leakage is more easily calibrated.
3) SM: for the bulk of
observations that want high-quality I measurements, a single
polarization product RR or LL will suffice as a proxy in the absence 
of V signal.

Disadvantages:
1) lossy
2) narrow bandwidth (compared to receiver band; but we have no definition
of 'narrow'.
3) it is a widget, and inconvenient to implement.