[mmaimcal] Polarization specs

[Steven T. Myers]

    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].

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

    This non-orthogonality of -20 db to -12 db 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 that 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 the somewhat empirical of a "goal"
    of -20db and spec of -12db for both allowed non-orthogonality and
    mismatch:

        [2.1 Maximum non-orthogonality       Spec: -12db   Goal: -20db

        [2.2 Maximum polarization mismatch   Spec: -12db   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%?]

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.

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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