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Optical Surfaces has supplied a 350mm-diameter on-axis ellipsoidal mirror to the Kiepenheuer-Institut für Sonnenphysik (KIS) — an institute in Freiberg, Germany, that carries out solar research with ground-based telescopes.

The institute runs two telescopes on the island of Tenerife: the Vacuum Tower Telescope (VTT), which has an aperture of 0.7m; and the Gregor telescope, which has an aperture of 1.5m.

The VTT telescope is equipped with optics supplied by Optical Surfaces in the 1980s. When it came to replacing the Gregor tertiary mirror last year, Optical Surfaces was chosen for the job.

The new Gregor M3 is an elliptical mirror with two foci that was specified with particular emphasis on low-stray light levels. The mirror was successfully implemented on the Gregor telescope in February 2013.

Key benefits of product application

  • According to Dr Aris Kouris of Optical Surfaces, in order to achieve the demanding low-stray light levels, the tertiary mirror was produced in Clearceram-z (HS).
  • The company’s craftsmen were able to achieve a surface accuracy of nearly Lambda/17 P-V at 633nm, a surface quality of better than 20/10 and an ultra-smooth finish measured at less than 0.60nm RMS.
  • The ‘structure function’ — measured at the request of KIS — is a logarithmic plot of the wavefront error as a function of the spatial frequency components; the resulting function measured, although it followed the predicted theoretical trend, was significantly better than expected, said Kouris.

Optical Surfaces

Optical Surfaces Ltd has been producing large and demanding high-precision optical components and systems for more than 40 years and is today internationally acknowledged for its technical expertise, attention to quality and customer service.

With the main workshops and test facilities deep underground in an area excavated from solid chalk, temperatures remain constant year-round and vibration is practically non-existent. These are important considerations when making, mounting and testing high-precision optics. Working with these natural advantages Optical Surfaces Ltd is able to supply high-precision off-axis parabolas, spherical and flat mirrors, etalons, aspherics, reference flats, windows, collimators and beam expanders for the most demanding applications. With such stable conditions, testing, particularly with long path lengths, becomes quantifiable and reliable. Investment in large-aperture Fizeau, Zygo GPI, Flashphase, Twyman Green, Scatterplate, and Laser Unequal Path interferometers allows detailed testing of the largest-diameter optics. Topographic and fringe analysis provides precise testing of surface roughness and confirms the wavefront of various surface forms. A rolling programme for calibration of test optics where possible to national standards and production approval to ISO 9001-2008 ensure that the quality of optics from Optical Surfaces is second to none.

Today the company is a trusted supplier to leading scientific, astronomical and defence establishments including many space projects worldwide.

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