Hi Dimitris,
I use the Pentax 165mm f2.8 (6x7) lens.
I've found it very sharp but it sufferes from a lot of fringing if not stopped down. So much that it's unusable at f2.8
These two were taken at f4 and still required a bit of work in post-processing to remove the magenta fringing:
http://www.astrobin.com/full/184790/0/
http://www.astrobin.com/full/187395/0/ (this one is a mosaic)
This one was taken at f5.6 and the fringing is almost non-existant (a significant crop of the full field)
http://www.astrobin.com/full/211010/0/
These are taken using an unmodified APS-C DSLR.
Hope that helps.
Pete