[mmaimcal] Polarization Teleconference

[Al Wootten]

ALMA/US Imaging and Calibration Group

Agenda for meeting Thursday, 4 Jan 2001 at 12:30pm EST.

Date: 4 Jan 2001

Time: 12:30 pm EST (1730 UT, 10:30 am Socorro, 10:30 am Tucson)

Phone: TBA

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Special Polarization Meeting Thursday

Crutcher has proposed a special meeting on polarization on Thursday. 
Dick's discussion guide:

Polarization observations have been defined by the ALMA Science Advisory
Committee as being one of the important science drivers for the ALMA
telescope(ref: March Report). The polarization science given the highest
importance was the mapping of linear polarization, both over ALMA telescope
primary beams and over multi-pointing mosaic fields. Hence, both
interferometer and single-dish polarization must be addressed. The March
2000 ASAC report proposed the requirement for 0.1% polarization mapping
fidelity after calibration. This requirement of 0.1% polarization fidelity
after calibration does not lead directly to specifications on antennas,
receivers, and software, since the combination of instrumental polarization
induced by receivers and antennas both play a role, along with the
procedures for calibration. In order to insure that the polarization
requirements will be met, it is essential that coordinated planning across
these areas take place.

The present ALMA Project Book does not address how the science polarization
requirements will be met. Indeed, things seem to be going the way they
usually do with radio telescope design. One discovers that to make
polarization work well will compromise other things, so polarization is
given low priority and polarization science is compromised. ALMA examples
include adopting linear rather than circular polarization feeds, having all
feeds oriented identically rather than half rotated at 45 degrees (which
would allow use of software to derive optimally all 4 Stokes parameters
simultaneously), and having all feeds off-axis (which will introduce
significant primary beam instrumental polarization). All of these design
considerations have sound justifications in order to optimize performance
for Stokes I observations, but at the expense of polarization observations.
There does not appear to be any plan for insuring that the potential of
ALMA for polarization observations will be met.

The following design features should be reviewed to see whether it might be
possible to give some weight to polarization in the ALMA design.

1. Having the linear polarization response of half of the antennas at 45
degrees to the rest would provide an established method for measurement of
all four Stokes parameters. This arrangement would require measuring 4
correlations, which would reduce the allowable maximum bandwidth to be
correlated if only Stokes I is desired. Is this loss of proven polarization
capability worth the tradeoff of higher bandwidth? If so, could it be
recovered by having half of the receiver units rotatable through 45
degrees?

2. Having all receivers off-axis ignores the recommendation that one (at
least) receiver be optimized for polarization observations. Off-axis
receivers will produce instrumental polarization that is dependant on
position within the primary beam, making it more difficult to map
polarization over large fields of view. Different antennas, particularly
those of different sizes, will have different instrumental polarization
characteristics. Is it possible to have a prime polarization receiver
channel (345 GHz) on axis?

3. Even if the ALMA design were most carefully optimized for polarization,
there will a variable instrumental polarization response over the primary
beams of the antennas. To have any realistic hope of being able to do
polarization mapping, it is essential that the polarized beam patterns be
stable over time, temperature, antenna pointing, etc. In order to calibrate
polarization science maps, the polarized beam of the antennas will have to
be measured by mapping the polarized beam pattern at the 0.1% level on the
spatial scale of the synthesized beam. This will be a time consuming
operation - to be practical, it will have to be done once (or infrequently)
and applied over extended periods of time. This will require that the beam
pattern be stable and repeatable. Is this a formal design consideration for
the antennas? How will this be tested before an antenna design is accepted?

4. How will the polarization calibrations be done? Darrel Emerson presented
a design for a bandpass calibration system at the Berkeley ASAC meeting
that could also be used for polarization calibration of the receiver. This
would consist of an amplitude and phase stable signal that would be
broadcast into the receiver, so one could essentially continuously
calibrate the gain and phase difference between the two nominally
orthogonal receptors on each antenna. If both receiver gain and phase can
be very accurately calibrated, it should be possible to derive all four
Stokes parameters from orthogonal linear feeds with common orientation,
with acceptable loss of fidelity in the Stokes parameter derived from $X -
Y$. Is this a firm component of the ALMA plan?

5. Provision for testing the polarization characteristics of the
two-antenna test array should be made. This will give the opportunity to
test the system hardware and software for interferometer and single-dish
polarimetry and identify problems before additional contracts are signed.
What is the plan for a full testing of the ALMA polarization system to
insure that the science polarization goals will be met?

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Richard M. Crutcher

Professor & Chair, Astronomy Department

Chief Application Scientist, NCSA

University of Illinois

1002 W. Green Street

Urbana, IL 61801

Voice: 217/333-9581

Fax: 217/244-7638

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