I don't know if I've mentioned this before - a piece of software on a French website for performing Roddier testing:
http://www.astrosurf.com/tests/roddier/projet.html (http://www.astrosurf.com/tests/roddier/projet.html)
By taking extremely intra and extra defocused images and processing them in this software it will show inaccuracies in the optical system and other distortions such as pinching, astigmatism etc.
I used it very successfully on the C11 to prove just how good the optics were. I intend to use it on the Tak for finely adjusting the "mirror pressure screws".
If you don't understand the French then I'm sure Google translator will come to your aid.