ASTRO: NGC 2175 with (faulty?) new 6" achromat

On December 24 I got my new 152/900 mm achromatic refractor, which I bought
as a "light bucket" for narrow band images with short focal length (with the
option to get f/3.9 with a focal reducer).
Unfortunately the star test looks quite strange, see the image at
http://ccd-astronomy.de/temp2/FokusIRSperr.jpg
If anyone knows what is ailing this scope please let me know (looks like
undercorrection?)

Anyway, in Halpha the scope delivers sharp pictures, but brighter stars have
"dents" as if there was something in the path of light, which definately was
not the case.

I'll post two pictures, first is Halpha only, the second Ha:OIII:OIII

Taken from the middle of Berlin with a 152/900mm achromat, G11 mount, SXV-H9
camera, 7x10 minutes through Astronomik Halpha 13nm, 4x10 minutes through
Astronomik OIII.

Pictures can also be found at
http://ccd-astronomy.de/temp2/2175-600sHa7x10gut.jpg
http://ccd-astronomy.de/temp2/2175colourgut.jpg

Stefan