Wisconsin Ultraviolet Photo-Polarimeter Experiment

Contents

Introduction

The Wisconsin Ultraviolet Photo-Polarimeter Experiment (WUPPE) was one of three ultraviolet telescopes on the ASTRO-1 mission flown on the space shuttle Columbia during 2-10 December, 1990. 98 observations of 75 targets were obtained. The same three instruments were later flown on the space shuttle Endeavour from 3-17 March, 1995, as part of the ASTRO-2 mission. During the longer ASTRO-2 mission, 369 observations of 254 targets were obtained.

Light Polarization

Most of the light we encounter every day is a chaotic mixture of light waves vibrating in all directions. Such a combination is known as unpolarized light. However, if the light – or other type of electromagnetic energy – passes through certain materials or is reflected, the waves will tend to vibrate more in one direction and the light is said to be polarized. You can observe polarized light by looking through a pair of polarizing sunglasses at the brightness of the blue sky about 90 degrees from the Sun. As you rotate the glasses, the brightness of the sky will vary because the light has been polarized by being reflected in the atmosphere.

By determining the amount and direction of polarization and how these change with wavelength, one can learn about what causes the energy to become polarized. By using polarimetry (the study of polarization), scientists can learn a great deal about the object being observed and the space between that object and Earth.

Instrument Description

WUPPE consists of a telescope with a 50-centimeter (20-inch) mirror to reflect ultraviolet light to a spectropolarimeter, which splits the beam of radiation entering the telescope into two beams with perpendicular planes of polarization; the beams are then passed through a spectrometer and focused on separate array detectors. This results in a measurement of the degree and direction of polarization at many different wavelengths. Before the development and flight of this instrument on the Astro-1 mission, virtually no such UV data existed because of the difficulty in obtaining these measurements above the Earth's atmosphere with the degree of accuracy required for astronomical observations.

During the ASTRO-1 mission (launch GMT 1990/336/06:49:01) and the ASTRO-2 mission (launch GMT 1995/061/06:38:13), WUPPE obtained spectropolarimetry of a variety of astronomical objects, including distant stars to study the interstellar medium, hot stars, stars with circumstellar material, interacting binaries, novae, solar system objects, and active galaxies. The data gathered have allowed astronomers to refine theories and develop a better understanding of the universe. WUPPE paves the way for future instrumentation and research using spectropolarimetry.

The instrument was designed and built at the University of Wisconsin Space Astronomy Laboratory in the 1980's.

• Developing Institution: University of Wisconsin
• Telescope Optics: Cassegrain system, f/10 focal ratio
• Instrument: Spectropolarimeter with dual electronic diode array detectors
• Primary Mirror: 50 centimeters (20 inches) diameter
• Field of View: 3.3 x 4.4 arc-minutes
• Spectral Resolution: 6 angstroms
• Wavelength Range: 1,400 to 3,200 angstroms
• Magnitude Limit: 16
• Weight: 446 kilograms (981 pounds)
• Diameter: 70 centimeters (28 inches)
• Length: 3.7 meters (12.14 feet) length

Detail: WUPPE was designed to obtain simultaneous spectra and polarization measurements from 1400 to 3300Å. WUPPE has a 0.5m f/10 classical Cassegrain telescope (area = 1,800 square centimeters, 279 square inches) and a spectropolarimeter, with a field of view of 3.3 by 4.4 arc-minutes and a resolution of 6 angstroms.

The telescope feeds light to a low resolution spectrometer equipped with various polarimetric analyzers. The spectrometer is a modified Monk-Gilleson spectrometer: two rotating wheels are used to select the focal plane aperture and the polarimetric analyzer; a magnesium fluoride Wollaston polarizing beam-splitter placed between the aperture and the relay mirror splits the beam into two orthogonally-polarized spectra which fall on the detector.

The detector consists of dual Reticon self-scanning linear arrays of 1024 pixel photodiodes (to detect both beams simultaneously) coupled by fiber optics to a microchannel plate intensifier with a cesium-telluride photocathode. A charge-coupled device camera is provided for target acquisition and for viewing the aperture during a measurement. The focal plane scale is 26 arcsec/mm and the dispersion is 78 Å/mm, blaze at 2000 Angstroms. A set of halfwave plates at 6 different angles provided spectropolarimetric modulation with 5 angstrom resolution on point sources through apertures from 6 to 40 arcsec. A "Lyot" analyzer was used to provide 50-100 angstrom spectropolarimetric resolution on faint point targets and diffuse nebulae.

External Links

• Wisconsin Ultraviolet Photo-Polarimeter Experiment - Space Astronomy Laboratory, University of Wisconsin.

Preview Image

"Astro 1 In Orbit" - Wisconsin Ultraviolet Photo-Polarimeter Experiment on-board the space shuttle Columbia.  (Source: Astronomy Picture of the Day, December 3, 2005. STS-35 Crew, NASA.)