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WUPPE and Orion

WUPPE Spectropolarimeter


The WUPPE spectropolarimeter is a modified Monk-Gilleson spectrometer: a plane grating is placed between a spherical relay mirror and the detector. Following the light path, at the focal plane of the telescope a 16-position aperture wheel serves to define the portion of the sky to be observed. The apertures (constructed of etched Corning Photoceram) include a 40 arcsec diameter acquisition aperture and a 4.2 arcsec diameter stellar aperture, in addition to a number of aperture shapes for diffuse sources, for instance 6x12 and 3x50 arcsecond rectangles. The 3 arcsec slit provides a spectral purity for a diffuse source of 0.6 nm. Next one encounters a polarimetric analyzer wheel which provides for modulation of the polarimetric state of the beam. A conventional linear polarization analyzer is provided by switching among a set of 3 pairs of matched 1/2 lambda MgF2 retarders at angles of 0, 45, 15, 60, 30 and 75 degrees. An all-Stokes analyzer is provided by a MgF2 Lyot plate which modulates the polarimetric state of the beam in wavelength, with different modulation patterns for the different Stokes parameters. Next a 1 cm MgF2 Wollaston polarizing beam-splitter placed between the filter and the relay mirror splits the beam (perpendicular to the screen in the figure above) into the two orthogonally-polarized beams. All the MgF2 polarizing optics were fabricated by Karl Lambrecht Corp. After encountering a spherical relay mirror which provides a final f/15 beam, the light is dispersed by a 600 l/mm plane grating blazed at 190 nm, fabricated by Hyperfine, Inc. The grating is coated with Aluminum and overcoated with 500A MgF2.

At the spectrometer focal plane, the first order of the grating from 135 to 330 nm is seen as two orthogonally polarized 25mm long spectra, split by 3.5 mm. The image size parallel to the dispersion is less than 25 microns (0.2 nm in wavelength); the image size perpendicular to the dispersion is dominated by residual astigmatism: the angles of the relay mirror and grating in a standard Monk-Gilleson design are optimized to cancel astigmatism at the central wavelength; but the addition of the astigmatism from the Wollaston prism results in this point falling near the lowest wavelength for one split beam and near the highest wavelength for the other. The resulting astigmatic images are up to 0.5 mm in length.

-taken from Nordsieck et al. 1994, SPIE, Vol. 2010, p.2.