Date: JAN-04-2001 (Thursday) 4 January 2001.
Time: 12:30 EST (10:30 MST, 1730UT)
USA Toll Free Number: 1-888-456-0327
International Number: 1-312-470-0065
Passcode: ALMA
Conference leader: Al Wootten
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Agenda
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Crutcher has proposed a special meeting on polarization on Thursday. We will make a proto agenda. 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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