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Published online by Cambridge University Press: 12 April 2016
Large aperture telescopes usually have an unavoidably long focal length so that stellar image diameters are often much larger than the resolution of the detector. The detection of extended sources can take advantage of a faster focal ratio and both stellar and extended monochromatic sources can be detected with great sensitivity by using interference filters and a focal reducer, without much loss in resolution. Extragalactic results obtained with the 6 meter telescope at focal ratio f/1 are given as examples.