Responses to Dick Crucher's points on the ALMA receiver design
LRD 2001-01-03


(Numbered sections correspond to similarly numbered points in Dick's
notes.) 

1.  The suggestion that half the antennas should be deliberately
cross-polarized by 3 dB with respect to the others seems very strange.
This creates an asymmetry in the array that will produce a wide range
of problems, not just for polarization measurements.  Dick claims that
it's an "established method" but he fails to cite any references.  We
need a detailed explanation of how it is supposed to work.  I
certainly question whether there is any advantage at all with respect
to alternatives.

   OTOH, if it is somehow established that it's desirable to orient
half the antennas at 45d polarization angle with respect to the
others, then it is probably feasible to do this for *some* bands.
Bands with more complicated optics within the cartridge cannot be
different among antennas, but simpler ones might be.  We can keep this
possibility in mind during detailed design, but I would ask that the
justification for doing so be worked out first.

2.  It is not true that the recommendation for polarization
optimization was ignored.  On the contrary, it was carefully
considered and every effort was made to minimize the beam squint.
However, it was found that putting one band on-axis would not have
much effect on this.  That's because refocusing optics are needed for
other reasons -- in particular, to keep the horn size reasonable --
and it is those optics that dominate the beam squint, not the offset
of the whole assembly from the axis of the main reflector.  The
refocusing is done with a pair of offset mirrors for most bands.  It
might conceivably be done with an on-axis lens, thereby avoiding beam
squint, but above about 200 GHz this would introduce excessive loss
and would limit the bandwidth.

3.  As you say, there will be significant variation of the antenna's
polarization state across the beam no matter what we do with the
optics, and this variation will require calibration.  This fact makes
it much less important that the design strive to make the variation as
small as possible.  Your point regarding the time stability of the
polarization pattern is well taken.  This is only one of several
reasons that the optics must be stable and reproducible, and achieving
this is a major goal of the design.  We believe that a very high
degree of polarization stability will be achieved for a given receiver
assembly at any fixed frequency, in the sense that changes will be
undetectable by any astronomical calibration.  However, we are
uncertain how reproducible the polarization will be when an entire
front end assembly or a band-specific cartridge is swapped out.

    The biggest problem with wide-field polarization imaging is
something that you did not address: even if the polarization of each
antenna's beam is perfectly known (calibrated), it is not
straightforward to apply this information to the raw visibilities so
as to produce accurate images of the polarized radiation.  Only under
the assumptions that the polarization state is constant across the
beams and identical among the antennas does this become tractable.

4.  The lack of understanding of the calibration process by the
astronomers makes it difficult for them to provide useful design
feedback to the engineers.  Most of the literature concerns methods
usable on existing instruments like the VLA.  But ALMA is a new
instrument, and it can have new features.  Darrel's explanation of the
subreflector-mounted cal source is an example of this.  Yet its
significance should not be overstated.  It provides a signal that is
strongly phase-coherent between the two polarization channels of the
same antenna, unlike any natural source, and this is its value for
polarization work.  But it does this at a single frequency (albeit
sweepable) rather than across the broad bandwidth of a continuum
observation, so it is not the answer to all problems.

5.  The TI will certainly be operated in polarization modes.  That is,
the correlator will be able to compute cross-correlations of opposite
polarizations in single-dish observations as well as cross-polarized
baselines in interferometry (although not all at the same time).  This
should enable us to try out various calibration techniques.  However,
it should be recognized that the front end optics of the evaluation
receivers will be very different from those of the array receivers.
Eventually, when the prototype array receivers are available, it may
be possible to test them in the TI, but this is far enough in the
future that the details are rather sketchy at present.

   It is almost impossible for any polarization experiments done with
the TI to have any effect on the contract for the production antennas,
but it is also hard to see how anything in that contract can have a
significant effect on the polarization performance of the array.


--Larry D'Addario