To my dismay I could not reach focus with my ATK16IC camera with Scope Teknix Filter Wheel on my William Optics Zenithstar 80 ED II APO: I did not have enough back focus so I needed an extender.
I experimented with a plastic 10cm extender which worked nicely and I found out that a 5cm extender would do the job.

2" Extender.
I bought this 2-inch extender [with an optical length of 5cm] so that I can mount my cameras in a more secure way and still reach focus when using my filterwheel and ATK16IC camera. Supplier: Teleskop-Service - Muenchen (Germany)
The whole set up:
William Optics Zenithstar 80 ED II APO + the 2-inch 5 cm extender + Scope Teknix Filter Wheel + Astronomik LRGB Filter set + ATK16IC camera