Very good, Stefan: There is nothing wrong with neither your camera nor your skills as an astrophotographer. It is just your lenses that are to blame.
I can show you similar results with some of my lenses. Here are just a couple of examples:
On top, you have an off-centre crop (200%) of an image talken with a Tamron Adaptall-2 35-80mm lens and below you
have a similar 200% crop from an image in the same region (not the exact same spot) taken with a Super Takumar 55mm
lens. The difference is quite obvious, isn't it?
And here's another example:
An image taken with an SMC Pentax-M f/2 50mm lens. Looks fairly decent perhaps,
but click on image to get a closer look........
Mind you, there isn't really anything "wrong" with these lenses. They were just designed with terrestrial use in mind and not those point-like sources of light that we try to capture. Therefore, coma will often be a problem with even good wide field primes from about 50mm focal lenght and below.
Anyway, as long as we do not blow up our pictures enormously, they are still quite decent, I think.
Last edited by Stone G.; 09-02-2012 at 04:11 PM.