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Optical Elements Highlight the Skies

Photonics Spectra
Jul 1999
Daniel C. McCarthy, News Editor

As astronomy expands the known universe, it pushes the boundaries of optical engineering. The European Southern Observatory recently unveiled a fiber-fed extended-range optical spectrograph, called Feros, high atop the Chilean Andes. The instrument incorporates in its 1.52-m telescope a number of high-precision optics from Optical Surfaces Ltd. to help record spectra of stars 2500 light-years away. The instrument was designed and specified by researchers from Heidelberg State Observatory.

The European Southern Observatory atop La Silla mountain in Chile includes the Feros spectrograph. The telescope is comparatively small, but high-precision optics from Optical Surfaces Ltd. will help it record spectra of stars 2500 light-years away. Courtesy of European Southern Observatory.

An echelle grating provides Feros with spectral coverage from the near- ultraviolet to the infrared region (360 to 920 nm) in one exposure. Two off-axis paraboloidal mirrors beside the grating reimage its 136-mm pupil near a cross-dispersion prism. The mirrors and prism are made by Optical Surfaces. The reimaging of the pupil helps to reduce attenuation of illuminance at the edges of the optical field to less than 4 percent. The prism provides the cross-dispersion to separate the high echelle orders to deliver the high resolution.

The cross-dispersion prism is one of the largest ever made, its high optical quality producing superior efficiency over a grating cross-disperser, said Andreas Kaufer, a researcher at the observatory. Besides offering an attractive price for the two mirrors, Optical Surfaces was the only manufacturer that demonstrated the ability to provide the material and that had the techniques to manufacture the large prism, Kaufer added.

All three elements and a high-reflectivity flat were manufactured at Optical Surfaces' unique production facility, which is embedded deep within a chalk cliff with a year-round temperature variability of less than 1 °F and extremely low vibration.

Despite the large number of optical elements in the Feros instrument, the observatory reports that 46 percent of the light entering the spectrograph is recorded by the detector. Its full spectral range is divided into 100,000 pixels and linked by fiber to an optical bench in a stabilized environment. The stability and efficiency of this configuration enable the comparatively small telescope to deliver high-resolution images that will help astronomers search for planets orbiting distant stars, measure the chemical composition of their suns or determine the chemical abundances in Magellanic Clouds.

The scientific observation of celestial radiation that has reached the vicinity of Earth, and the interpretation of these observations to determine the characteristics of the extraterrestrial bodies and phenomena that have emitted the radiation.
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