Mirror lenses in general do not perform as well as refractive lenses, but are cheaper to make and more compact. 500mm ones are far more common than 1000mm. I have a 500mm which I was thinking of selling, but the prices they go for on Ebay are so low that I decided to keep it. It does not stop some sellers asking silly prices though.
Originally posted by zzaletel 
No idea on the flange, but would assume you could swap it to K mount
Don't be so sure. My 500 was made by Centon (but sold under other names too), and it uses a T-2 mount which takes an adaptor to the actual camera mount. So if you bought a used Centon you could fit the appropriate T-2 adaptor (if it does not already have it) which you can find on Ebay for about $2. Using a T-2 mount was cheap for the maker because they did not need to make their lenses in a dozen different mounts. However, Sigma were generally a cut above this practice, so the mount on that 1000mm might not be changeable. However, it does look like a K mount, but perhaps someone more expert than me can tell. Better still, ask the seller what it is.
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