Ultra precise prisms produced for atmospheric corrections

Optical Surfaces has announced the delivery of a pair of ultra high precision counter-rotating doublet prisms for incorporation into a network of ground-based astronomical observatories.

The SONG project is a collaboration between scientists at the Department of Physics and Astronomy, University of Aarhus, and the Niels Bohr Institute, University of Copenhagen, both of which are in Denmark. SONG stands for Stellar Observations Network Group and the purpose of the project is to design an observatory that consists of a network of telescopes distributed around the planet. The design of the SONG telescope will target two scientific purposes: it must measure stellar oscillations that can probe the internal structure of stars, and be able to find Earth-like planets in our Milky Way.

SONG will be a network of 1-metre class telescopes distributed worldwide for measurement of stellar oscillations and planet detection via microlensing events. Currently, the prototype telescope and instrument suite is being designed. Two Lucky Imaging cameras, observing in the VIS and RED bands, are situated at the Nasmyth focal station. Via a Coudé train, a f/37 beam can be fed to a high-resolution spectrograph. In order to deliver diffraction-limited performance, important especially for the observations in the VIS and RED bands with zenith distances of as much as 70 degrees, an atmospheric dispersion corrector (ADC) was required. The ADC consists of a pair of counter-rotating doublet prisms to be placed approximately 75cm before the focal plane, in the combined VIS/RED band.

Reflecting on their choice of supplier, Per Kjærgaard, SONG project manager, commented: 'Optical Surfaces was selected to produce the critical ADC prism doublet because of their impressive track record in preparing high performance optical systems for telescope and space projects.'

Using proprietary production techniques, Optical Surfaces' skilled craftsmen produced two identical glass prisms of 45mm diameter with a surface accuracy of lambda/20p-v and microroughness of better than 1nm rms to provide the diffraction-limited performance required by the SONG project team.

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