[evlatests] 3-bit polarization/stability test

[Michael Rupen]

Note that we will be taking 8-bit data, at the same frequency, at the
same time; we are really looking for differences in the time dependences
of the pol'n leakage between 3- and 8-bit data, which should help to
isolate the origin of any variability we see.

          Michael

> Rick-
>
> On Wed, Nov 9, 2011 at 9:27 AM, Rick Perley <rperley at aoc.nrao.edu> wrote:
>>   I think we don't have to worry excessively about this.  The tests will be
>> done at Ku-band, and we have abundant evidence that the gain variability (in
>> outstanding weather, which we know we'll get this evening) at this band is
>> very, very low, following correction by the switched power.  (And of course,
>> the ionosphere is of no concern at 14 GHz and D configuration).    In any
>> event, we have to try.   The polarizers are extremely good at this band and
>> frequency (should be better than 2%, and superbly stable), and the source
>> polarization is less than .5%.  The devil may well be in the details, but we
>> have to look nonetheless ...
>
> You miss my point.  Indeed, the ionosphere is of no concern at Ku-band.
> And the tropo weather is unpolarized (so commutes with everything).  The problem
> is that applying the net gain calibration (switched power, plus
> whatever a standard
> 'CALIB' yields) as if it were all downstream of the samplers, when, in
> fact, some of
> the pol-dep gain(t) originates upstream of them,
> will substantially complicate your analysis.  What one might conclude
> is variability in the
> cross-talk in the samplers might just be the upstream gain(pol,t) not
> commuting with it.
> (I noted ionosphere merely to point out another context where this
> sort of thing
> causes problems.)   Basically, pol-dep gain(t) (incl phase) upstream
> of the polarizing
> element _heuristically_ prohibits the assumption of stable leakage
> that we like to
> make, or equivalently, frustrate attempts to test this assumption.
> This is so even
> if the leakage is _physically_ stable.  It is not insoluble, and
> knowing something
> about the upstram gain (sw power) as an isolated factor helps.   That
> there is gain
> variability occurring _between_  two (or more?) imperfect polarizing
> elements--even if they
> are physically stable--only further complicates the description of
> 'net D-terms'.
> It is just not a scalar problem, and so not simply factorable in the
> traditional manner.
>
> I don't intend to discourage; by all means, take the data!  (Just
> don't jump to conclusions.)
>
> -George
>
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