Lyot Polarimeter Instrument Description

The spectropolarimetric observations were acquired with the University of Wisconsin's Pine Bluff Observatory 0.91 m Cassegrain reflector and Boller and Chivens Cassegrain spectrograph with a Lyot analyzer placed just after the entrance slits. This system is simultaneously a moderate resolution spectrometer and broadband polarimeter. The detector was an electrostatically focussed Varo microchannel plate intensifier fiberoptically coupled to a Reticon dual photodiode array. The Reticon consists of two parallel linear self-scanned arrays, each composed of 1872 diodes (or pixels). Each pixel is 15x750 microns, and the arrays (referred to as arrays A and B) are separated by 2.5 mm.

The spectrograph entrance aperture consisted of a double slit and decker arrangement. The two slits were 168 arcsec apart, and the beam from each slit was dispersed onto one of the Reticon arrays. This allowed simultaneous measurement of the source and sky signals. The observing setup usually utilized a slit width and length of 6.5 and 11.3 arcsec, respectively. The dispersing element was a 400 lines/mm grating used in the first order and blazed at 5000 Å. This provided coverage over a spectral range from 4300 to 7600 Å and a measured spectral resolution of 13 Å. During the reduction process, the polarimetric range was limited to 4600 -7200 Å to eliminate pecularities near the edges of the detector. (Prior to March 1982, the dispersion element in the spectrograph was a 300 line grating used in first order, blazed for maximum efficiency at 7000 Å. The 2.5 Å/pixel dispersion provided spectral coverage from 4500 to 8200 Å.)

The Lyot analyzer, located just after the entrance slit, is a polarimetric filter assembly (see Figure below) designed to modulate the spectrum in wavelength with a pattern whose amplitude is proportional to the polarization. The optic elements in the filter assembly are two quartz phase-retardation plates of different thickness, followed by a polaroid. This are mounted on an order-separation filter slide. The wave plates are of thickness 1.6 and 3.2 mm, mounted with their optic axes 45 degrees apart.

Lyot Polarimeter