UC: ObserveFullPolarization ----------------------- S.T.Myers version 2000-10-17 Observations with ALMA in full-polarization mode (a single band and/or sideband with orthogonally polarized IFs at same sky frequency, with correlation of all four cross-products). Role(s)/Actor(s): Primary: AOS Secondary: Operator, Observer Priority: Major Performance: real-time; overhead should be minimal and dominated by hardware. Frequency: this observing mode is not likely to be the most-often used (single or dual polarization may be sufficient for most programs), but will be used some non-vanishing (~10-20%?) fraction of the time and thus must be supported Preconditions: 1. the observation requested requires full-polarization with dual polarization IFs from a single coincident sideband and four polarization cross-products at correlation in interferometer mode - single and dual polarization interferometry (ObserveSinglePolarization, ObserveDualPolarization) and total power observations are not supported by this Use Case. See the Note(3) below 2. all the user parameters have been properly entered and checked (SetupSingleField, SetupMultipleField) 3. the observer has selected a Polarization Calibration Mode for this program/project/session: 3.1 Session requires pre-existing valid calibration object. Some mechanism must be provided for to ensure that a calibration is available before exection of the observation - see IssuesTBD(1) below. 3.2 This observation is independently calibratable if no valid calibration available. For example, it may contain a single scan on an unpolarized source or source of known calibration and this is deemed sufficient for the purposes of calibration of this session for the science goals. 3.3 This observation is intended as a primary polarization calibration itself. It should then be flagged as such and the pipeline results (DetermineAntennaPolarization) fed into the Calibration Archive. Basic Course: 1. The preamble block is executed (only once in the session). This will set up system parameters (eg. Doppler correction, receiver setup, correlator setup) and is mostly identical to that found in the single or dual polarization case (UC-ObserveSingleField, UC-ObserveMultipleField, etc.) The choice of dual polarization for IFs from the receiver and full cross-products (x4) in the correlation MUST be chosen. 2. For each scheduled OB, the standard single or multiple field procedure is carried out (eg. UC-ObserveSingleField, UC-ObserveMultipleField). Polarization specific tasks might include, but are not exclusive to, observation of a scan on an unpolarized source or source of known polarization, use of an injected test tone signal (if available), hardware interchange of pathways for the polarizations (unlikely to be available). The default procedure is the same as per the non-polarization mode. 3. The system executes the associated postamble OB: the phase calibrator is reobserved one last time if necessary. The pipeline is instructed to make the final image (if requested) and is closed. Exception Course: preconditions are not met, or the observation must be aborted 1. The observer has requested calibration using pre-determined antenna polarization solutions, and there is no existing valid (ie. same band, sufficient number of antennas) solution in the Calibration Archive. In this case, the observation should be deferred until calibration is available, and the calibration observations themselves should be scheduled at high priority. 2. Abortion of the observing should occur if: weather degrades below the requested quality threshold; array failure occurs that exceeds a tolerance threshold; observations at interrupt priority are inserted into the array queue Postconditions: 1. The pipeline will reduce the data taken in this mode using the standard procedures (ReduceSingleField, ReduceMultipleFields) with the appropriate application of the polarization calibration (ApplyPolarizationCalibration) Issues to be Determined or Resolved: 1. How is the requirement that a pre-existing antenna polarization calibration at an observing band implemented? One possibility is that when scheduling for the first session for this observation is attempted, the precondition causes failure for this schedule, but a high-priority calibration schedule is inserted in the queue, with the target schedule slated for subsequent observation at high priority. Another possibility would be to have a "global schedule model" that is constructed on a weekly to monthly timescale that would include the observation of calibrations that are necessary for the observation of sessions that are forecasted for this period. My guess is that this is the most promising way to work at high efficiency. 2. Can polarization sensitive observations (eg. calibrations on a source for 1 minute every 15 minutes during transit) be "virally" inserted into another session in order to carry out system calibration? Or should dedicated calibration runs be carried out at somewhat lower efficiency but with the advantage of having a more standard composition and less of an impact on the "host" session? 3. What is the break-point in quality where a secondary polarization calibration within the session (ie. a single scan on an unpolarized source or calibrator of known polarization) will be sufficient? 4. What are the tolerances on the validity of previous polarization calibrations for applicability to observations? In particular, is it required only that a continuum calibration at band center be valid for some number of antennas, with the assumption that corrections can be made to line mode and/or the actual observing frequency and bootstrapped to those small number of antennas modified since the last full calibration? Notes: 1. Precision polarization observations at millimeter wavelengths, especially at submillimeter wavelengths, are largely uncharted territory. See the discussion by Crutcher et al. in the report of the September 2000 ASAC meeting http://www.cv.nrao.edu/~awootten/mmaimcal/asac/asacsep00/ for details of the goals for ALMA polarization observations. 2. I am writing this Use Case as an "addendum" to the standard SingleField and MultipleField Use Cases rather than splitting it out as a stand-alone UC (eg. ObserveSingleFieldFullPolarization). I think this is the best approach given that there are only a small number of important differences between the polarization modes. This also preserves the commonality of these modes. Are people happy with this? 3. See related UC: ObserveSinglePolarization, ObserveDualPolarization, ObserveSinglePolarizationTotalPower, ObserveDualPolarizationTotalPower. Should these be absorbed into this UC as alternate courses? 4. Note also that depending upon the polarization optics (eg. circular, parallel linear, or crossed linear) there may be some similar calibration issues for Single and Dual polarization modes. These will be dealt with in those specific Use Cases. 5. The papers by Sault, Hamaker and others are good references for the subtleties of polarization observations and calibration. In particular: Sault, Hamaker & Bregman (1996) A&ASup 117, 149-159; Hamaker (2000) A&ASup 143, 515-534. There seem to be three main optical geometries: a. circular - two orthogonal circualr poln. R and L each antenna b. parallel linear - two orthogonal linear poln. V and H oriented the same on all antennas c. crossed linear - two orthogonal linear poln. V and H oriented at 45 deg w.r.t. half of the antennas The differences between these cases are mostly in implementation, and we dont really know what ALMA will have yet! Created: 2000/10/17 smyers