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Message: <blockquote data-quote="Binastro" data-source="post: 3160539" data-attributes="member: 111403">. There are very large telescopes with very fast primary mirrors. Such as 8 m or 300 inch F/1. Others of F/1.25.And designs for a 30 m telescope with an F/0.5 primary. Correctors are used.The primaries are usually spherical.The common low-priced Schmidt Cassegrains use F/2 mirrors.there is nothing very difficult about making fast mirrors if the design requires it.I also had a very strange Philips mirror system which seems to have perhaps an F/1 primary and you looked at it from the side. I never did find out what it was.I have a Sigma 500 mm F/4 mirror lens, which is complex. It only works at one focusing distance. If you focus away from this point the image becomes progressively worse. I do not know how fast the primary mirror is on this.There is also the Nikon 500 mm F/5 and I think the Zeiss 500 mm F/4.5. Again I don't know how fast the primaries are.As has been said above the secondary obstructions are large and the contrast low.The Minolta 250 mm F/5 .6 lens is very small and quite useful, but again the contrast is not very good.</blockquote>