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Liquid Crystal Polarimeter Provides Fast Zeeman...(1Sep92)
Imaging at NSO/SP
(from NSO, NOAO Newsletter No. 31, 1 September 1992)

The liquid crystal polarimeter was assembled and tested at the NSO/SP
Vacuum Tower Telescope, during an observing run 9-16 March as part of
the Air Force-funded Liquid Crystal Filter (LCF) project. The system
provides a complete set of polarization spectral images and can be
operated in many spectral lines using the tuning properties of the
Universal Birefringent Filter (UBF) (4000 A to 7000 A). The system was
successfully demonstrated for use in vector magnetometry of active
regions. The complete polarization and spectral sampling from multiple
lines will be used to derive related physical information: magnetic
filling factor and electric current distribution, as a function of
atmospheric height.

The system consists of a polarimeter section, which is placed before
the UBF, and a spectral analyzer section which is placed immediately
following the UBF in the light beam. The polarimeter modulates the
light at 1-20 kHz to give nearly simultaneous polarization spectral
images by temporal integration in two exit CCD channels. A sequence of
16 image pairs taken with combinations of settings in the polarimeter
and spectral analyzer sections provides a complete polarization and
spectral sample, giving automatic gain correction for the CCD field
effects. The modulator control system provides multiple frequency
modulation of fast ferro-electric and slower tunable nematic liquid
crystals, for use with normal CCDs or for use with video recording. One
complete polarization spectral sequence requires 30 seconds in each
wavelength, due to the limitation of the CCD readout time.  With the
video upgrade, a complete sample will be obtained in less than one
second. The electronics section was designed and built by Larry
Wilkins.

The system provided initial vector magnetograms in a strong-field
complex region in 4 lines: Mg B 5173 A, Ca I 6103 A, Fe I 5247.8 A, and Fe
I 5250.2 A.  Initial tests showed the feasibility for studying electric
fields using the linear Stark effect by operating the system in the
Balmer lines Ha, Hb, Hg, Hd, as well as in other hydrogenic species
available in the visible. The system may have adequate sensitivity and
speed to study fast transient electric-field phenomena, using the
quadratic Stark effect or electric-beam impact polarization.

Laurence J. November

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