Pentacon 500mm f/5.6 vs. ???

[dave_t]

I'm "reasonably" happy with the Pentacon 500mm, although I'm starting to question its sharpness a little bit, and am getting a bit tired of the weight and difficulty of focusing such a beast.

What kind of 500mm could I look into for a similar level of sharpness? I've been quite impressed with the results from some of the 500mm mirror lenses on these boards. Fortunately, OOF highlight rendition isn't too relevant for me as I try to keep everything within the plane of focus, so I wouldn't be too worried about the mirror lens donuts.

Is there a suitable mirror replacement for the Pentacon 500mm that would at least match it in contrast and sharpness in the M42 mount?

[CarbonR]

Takumar 500/4.5 ? ~300€, brighter than the Pentacon, and rather sharp even at full aperture (see my site with examples : http://kajiwara.weebly.com/s-m-c-takumar-50045.html )