[mmaimcal] oops - a corrected version Pol Specs...

[Steven T. Myers]

It was pointed out to me quite rightly that my versions of the specs were
for VOLTAGE, not POWER, so they are even worse by a factor of 2.  I should
have known that.  Thus, I now agree with Larry's numbers...

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    POLARIZATION SPECS FOR ALMA

    Version 27 March 2000

    S.T. MYERS (NRAO, Socorro)

    To get the ball rolling, here is my first go at suggested 
    polarization specs for ALMA.    

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1. Background

In his memo with suggested receiver specs 

       http://www.tuc.nrao.edu/~ldaddari/rcvrSpecs.txt

Larry writes

    Polarization: Simultaneous reception of two orthogonal
    polarizations is required, with each converted to (one or more)
    separate IF output(s).  The nominal polarization states may be
    selected separately for each band so as to minimize the receiver
    noise temperature; that is, either linear or circular is
    acceptable as the nominal polarization.  [At any frequency within
    the receiver's tuning range, the polarization states of the two
    channels should conform to:]

    [2.1 Maximum non-orthogonality              TBD, ~-25 dB]

    [2.2 Maximum polarization mismatch between any pair
        of antennas in the array                TBD, ~-20 dB]

    [Detailed specifications on polarization performance are under
    study.]

    In addition, over a limited portion of the tuning range of any
    receiver (typically 5% of center frequency), it shall be possible
    to make the nominal polarizations circular within 1.0 dB.  This
    may be accomplished by an insertable optical device, which may
    cause an increase in noise temperature not to exceed [TBD].

Note that these spec (dB) are in POWER, not voltage, so the voltage
(eg. interferometer amplitudes, phases, and polarization angles) are
TWICE this!

2. Current VLA performance

In the set of VLA/VLBA polarization calibration data that I have
been compiling the past 6 months

     http://www.aoc.nrao.edu/~smyers/calibration/

I find typical cross-polarization terms ("D-terms") of 1% at C
and X band to as much as 5% -- 6% at K and Q band.  For an
example solution, see 

     http://www.aoc.nrao.edu/~smyers/calibration/antpol.html

(The non-orthogonality is technically the difference between the d_12
and d_21 terms, but these seem to be on the order of the magnitude of
the D-terms themselves) A non-orthogonality of -40 db to -24 db (
-20 db to -12 db in voltage) limits the believable fractional
polarization (even in bright sources) to around 0.5% for standard
observation and analysis schemes due to limitations in the solutions
for the D-terms using the usual software.  The VLA solutions also do
not seem to be time-stable.  I am currently looking into this problem
as part of the calibration program.

3. However, it does seem that the polarization performance of the VLA
is acceptable to most observers and does not seem to seriously
limit the scientific applications of polarization observations.
Therefore, I would adopt Larry's specs of -25db and -20db 
respectively for allowed non-orthogonality and
mismatch:

    [2.1 Maximum non-orthogonality       Spec: -25db   Goal: -20db

    [2.2 Maximum polarization mismatch   Spec: -20db   Goal: -20db
         between any pair of antennas

These specs should yield polarization vector accuracies of a few
degrees which seems sufficient.

4. With the large tuning bandwidths (as high as 30% fractional
bandwidth) it will be difficult to make OMTs, quarter-wave plates
or quasi-optical devices that will perform to the above specs over
the entire band.  For example, for a band-edge 1.15 times the band
center, a waveguide quarter-wave retarder (with a dielectric vane
for example) could have as much as a $13^\circ$ phase error at
band edge, which would roughly give a cross-polarization of 23%
[DID I ESTIMATE THIS CORRECTLY?  I AM JUST TRYING TO GET ROUGH
NUMBERS HERE.]

Therefore I propose the above specs and goals to apply only to the
central 5% of bandwidth (eg. 5 GHz at 100 GHz, 32 GHz at 650
GHz), and allow the performance to deviate at band edges.
[IS THIS TOO LAX A STANDARD?  SHOULD THIS BE OVER 10%?]

As for circular versus linear, should we specify anything beyond
these specs (eg. Larry's 1db circularity)?

5. If we want tighter specs, I think we need pretty strong scientific
drivers (such as expected fractional polarizations for dust
emission at sub-mm bands) to argue for this.  AGN polarizations
seem to be high enough (5% - 10%) that the VLA-level specs are
sufficient.

>From what I hear from the engineers, getting even the VLA-level
performance will be very difficult!

6.  COMMENTS ANYONE?

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|:| Steven T. Myers                      |:|  Associate Scientist      |:|
|:| National Radio Astronomy Observatory |:|                           |:|
|:| P.O. Box O                           |:|  1003 Lopezville Rd.      |:|
|:| Socorro, NM 87801                    |:|  Ph:  (505) 835-7294      |:|
|:| smyers at nrao.edu                      |:|  FAX: (505) 835-7027      |:|
|:| http://www.aoc.nrao.edu/~smyers      |:|                           |:|
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