Peter I’ve had three over the years
- a cheap compact and lightweight 500mm f/8 picked up for a song at auction,
- a Sigma 600mm f/8 and
- a Russian MTO 1000mm f/10 likewise found at auction.
None were sharp enough to be considered as a telescope - at high power they all showed a quite messy star image nowhere near as good as an equivalent scope.
I did use the little 500 f/8 as a finderscope though for many years with a 25mm erfle. For finderscope duty most cheap telephoto lenses usually work quite well too.
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