News | February 27, 2013

Low Stray Light Ellipsoidal Mirror Helps Push The Boundaries Of Solar Research


Optical Surfaces Ltd. has supplied a 350mm diameter on-axis ellipsoidal mirror to the Kiepenheuer-Institut für Sonnenphysik (KIS) in Freiberg, Germany.

The Kiepenheuer-Institut für Sonnenphysik (KIS) is Germany's leading institute for solar research with ground based telescopes. It runs two telescopes on the island of Tenerife: the Vacuum Tower Telescope (VTT) which has an aperture of 0.7 m and the GREGOR telescope that has an aperture of 1.5 m.

Dr. Dirk Soltau, an Experimental Solar Physics Scientist with KIS commented "The VTT telescope is equipped with optics supplied by Optical Surfaces in the 1980's. When it came to the replacement of the GREGOR tertiary mirror in 2012 - Optical Surfaces Ltd. was again the successful bidder. The new GREGOR M3 is an elliptical mirror with two foci that was specified with particular emphasis on low stray light levels.  Drawing upon their depth of expertise and experience in producing demanding telescope optics - the on-axis ellipsoidal mirror supplied by Optical Surfaces surpassed our specifications and was successfully implemented on the GREGOR telescope in February 2013.”

Dr Aris Kouris of Optical Surfaces Ltd commented "To achieve the demanding low stray light levels we chose to produce the GREGOR tertiary mirror in ClearCeram-z (HS). Using proprietary polishing techniques our skilled 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. In addition, the “structure function” was measured at the request of KIS. The structure function is a logarithmic plot of the wavefront error as a function of the spatial frequency components. In effect it describes the presence of individual spatial frequencies and their contribution to mirror’s performance. The resulting function measured, although it followed the predicted theoretical trend, was significantly better than expected".

About Optical Surfaces Ltd
Optical Surfaces Ltd has been producing optical components and systems for more than 50 years and is now accepted as one of the world's leading manufacturers of high-precision optical systems for telescope and satellite based space observation systems. The company's ISO 9001-2008 approved manufacturing workshops and test facilities are deep underground in a series of tunnels excavated in solid chalk where temperature remains constant and vibration is practically non-existent. With such stable conditions testing, particularly with long path lengths, becomes quantifiable and reliable. Working with these natural advantages is a highly skilled team of craftsmen with a commitment to excellence in both product quality and customer service. For more information, visit

SOURCE: Optical Surfaces Ltd