The 1.5-meter McMath-Pierce Telescope, with its 13.5 meter spectrometer, is employed. A 1 mm (2.4x2.4 arcsec) Bowen-type image slicer serves as the entrance slit. In front of this slicer is a 50 khz Morvue quartz modulator. A few angstroms of the spectrum are rapidly scanned with the spectrometer photomultiplier output, being phase detected by a lockin amplifier to yield Stokes V. Scans are averaged by the photometry system until the desired S/N is obtained. Stokes I results by placing a circular polarizer before the Morvue. An interesting aspect of the above scheme is that the Stokes V signal is, to first order, seeing independent (since non-umbral scattered light is not polarized).
© European Southern Observatory (ESO) 2000
Online publication: January 29, 2001