[mmaimcal] Re: Q wave plate and VLBI

[Larry D'Addario]

Peter Napier writes:
 > 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.

There are various ways to handle the VLBI case.  If many ALMA antennas
are to be "phased together" for VLBI, as is now done with the VLA,
then a linear-to-circular conversion can be done in analog hardware at
baseband, before recording.  It will be a long time before VLBI
recording technology is expanded to ALMA bandwidths.  We can also
consider doing it digitally, by programming some of ALMA's FIR filters
to do Hilbert transforms and then adding/subtracting the results.
Finally, the simplest option of all is to allow the ALMA data to
remain in linear polarization even though other stations are circular,
but to require that the full 4-element polarization matrix be produced
by the correlator; then no sensitivity is lost, but accurate
polarimetry requires calibration that is arguably more difficult.

Thus, I do not believe that the need to accomodate VLBI should affect
the design of the initial optics ("widgets") at all, nor do I think it
should be allowed to have a large impact on the signal processing at
IF.

--Larry