At present extremely large telescopes are planned with primary mirrors from 20 m up to 100 m. Such telescopes need not only huge mirror arrays but also downstream refractive optics like atmospheric dispersion correctors, cola: correctors for imaging and beam shapers for spectrometers. For classical boro-crown and lead-flint glass types blanks have been made in the past up to about 1 m. Now there is a increasing demand to be expected for optical glass blanks with diameters up to 1.5 m for use as lenses or prisms. Additionally optics designers ask for glass types like the low dispersion fluoro-phosphate glasses. The production of high quality blanks of such glasses has been a challenge even for diameters around 200 mm. This presentation shall give information about the feasibility of large glass blanks and recommendations how to specify the quality balancing the requirements of the application on one side and the possibilities and conditions of the production and the measurement for inspection on the other side.
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