[alma-sw-ssr] Re: ALMA-SSR review

[Richard Crutcher <crutcher@astro.uiuc.edu>]

Comment on polarization calibration (page 10 of report: "Polarization
calibration This can probably be derived from phase calibration
observations."

The science goal will be to obtain maps in all 4 Stokes parameters
routinely, at the 0.1% level, without special time-consuming
polarization calibrations. (See report of the ALMA Science Advisory
Committee, especially the appendix on polarization observations.)
Polarization mapping introduces many problems for calibration. The
telescope primary polarized beam is not a simple (near) gaussian, but
has a much more complex structure in Q, U, and V than in I. Moreover,
this structure is time variable with physical changes such as sag of the
feed and reflector with elevation, focusing, etc. The polarized beam
pattern will rotate with telescope tracking, and for feeds not on the
optical axis even the polarized response at beam center will change.
Moreover, single-dish data for zero and short spacings will be required.
Single dish polarimetry is often done by subtracting total power
measurements made with changes in the polarimeter (right - left, or
linear at different position angles). This technique severely limits
polarization accuracy. For ALMA, investigation of cross-correlation of
the 2 receivers on each single dish to derive the 4 Stokes parameters is
essential. Mosaic mapping will require accurate knowledge of the time
variable polarized beam pattern down to very low levels. For example,
the proposed single-dish technique for ALMA is on the fly mapping, which
will emphasize the effects of strong sources in the polarized primary
beam sidelobes. Thus, the software requirements for polarization will be
much more severe than can be satisfied by deriving polarization
calibration from standard phase calibration observations.
-- 
Richard M. Crutcher
Chair, Department of Astronomy
University of Illinois
1002 W. Green St.
Urbana, IL  61801
Voice: 217/333-9581
Fax: 217/244-7638