The White Light Channel polarimeter is housed inside the spectrometer assembly. The polarimeter is designed to measure the broadband (4500-6000 Å ) linearly polarized radiance of the solar corona between 1.5 and 5 R, and consists of three main subassemblies: the entrance pinhole, the polarimeter group, and the detector. The three channels are aligned along the optical axis which is defined by the entrance pinhole and the center of the telescope mirror. Figure 12 shows the optical layout of the white light polarimeter.
The image of the solar corona is formed by the telescope mirror on the plane of the entrance pinhole, a square , which selects a spatial element of 14 x 14 arcseconds. Behind the entrance pinhole, a neutral density filter can be inserted to be used for solar disk observations. The radiation from the pinhole goes through the polarimeter group, and is collected by the detector. Table V summarizes the characteristics of the polarimeter optical components.
The optical components of the polarimeter are the half wave retarder plate, linear polarizer, lens, and passband filter (4500-6000 Å). The four components share the same optical axis and are fixed with the exception of the half wave retarder plate, which rotates around the optical axis.
The half wave retarder plate is the active component for the measurement of the linearly polarized radiance and consists of a stack of three polimeric birefringent plates following the achromatic configuration of Pancharatnam (1955), whose fast axis forms alternatively or with the final fast axis of the achromatic plate. In order to measure the linearly polarized radiance the retarder plate is rotated into 3 positions with angular separations of . The central position has been optimized to maximize the S/N ratio of the measured polarized radiance. The linearly polarized radiance, pI, is then given by the formula:
where , and are the radiance measurements for each position of the retarder plate.
The linear polarizer is a dichroic Polaroid HN38S with the transmission axis parallel to the UV slit length direction. In this way, the polarizer always selects radiation that has the same plane of polarization, thus reducing the errors due to the possible sensitivity to polarized radiation of the optical components that follow the polarizer in the optical path.
The lens focuses the light from the pinhole on a plane in the proximity of the detector. The passband filter selects the wavelength range 4500-6000 Å to meet the requirements of achromaticity of the retarder plate and the bandpass of both the polarizer and the detector.
The detector for the WLC is a photomultiplier tube, model EMI9130B, with S-20 photocathode, and low dark count rate, designed to work in the photon counting mode (the detector characteristics are listed in Table V.
The photocathode is positioned out of the image plane of the entrance pinhole in order to increase the maximum measurable signal, and to decrease the instabilities due to spatial inhomogeneities of the photocathode efficiency. The beam size is chosen to be not larger than 2 mm in order to meet the alignment requirements.