[mmaimcal] Q wave plate and VLBI

[Peter Napier]

Sorry if this is a repeat of a point already made but I think we need
the quarter wave plate to be flipped in to generate CP for any bands
where we plan to support VLBI with ALMA. Or else be prepared to generate
CP in the IF.
Peter

Al Wootten wrote:
> 
> 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.

-- 
Peter Napier
NRAO, PO Box 0, Socorro NM 87801
Ph 505 8357218, Fax 505 8357027