[tempestwizz (Brian)]

Frustrated in my attempts to obtain fine collimation of my TOA, I disassembled the optical assembly to learn how it is constructed in the hope that with this understanding I will now be able to collimate.

Removal and disassembly is surprisingly simple.

Firstly I removed the lens hood. In my version it simply unscrews from the retaining ring. (Removal of the hood is not absolutely necessary, but it opens up the assembly to allow gripping while removing screws)

The TOA has two sets of adjusting screws visible from the front. One set is recessed into the face plate, and the other set sits on the front.
The adjusting screws themselves are in sets of three, with one larger screw in the centre, and two smaller screws either side. They operate in a push-pull arrangement. The two smaller screw each pushing, and the larger screw pulling.

Removal of the three larger screws that are inset into the assembly release the whole lens assembly which can be lifted out for inspection. I made note of the orientation prior to removal.

All three glass elements are contained in this assembly.

The innermost two lenses are fixed. There is no individual adjustment for them.
Hence, the adjusting screws that were just removed, allow only adjustment of the assembly as a whole.

The other set of adjusting screws affect the front element with respect to the other two fixed elements. If the larger screws are removed, the front lens element can be separated from the assembly holding the other two elements.


(In my case, after removal I noted that my rear element was loose in the housing. The lens retaining ring had loosened by about a turn and a half. After retightening, I put a drop of Loctite onto the thread.)

So now, I’m guessing that I should replace just the assembly with the two rear elements and align it via the laser reflection from focuser with the inner set of adjusting screws.
Then, replace the front element and align it to centralise laser reflections.
Then use an artificial Star to fine tune the airy patterns. This is indeed fine tuning, as 1/10 turn makes a huge change to the patterns.

What I am not sure about is if the spacing between the front element and the other two has any affect on CA. If so, I may have to step the spacing of the front element in and out by trial and error to minimise CA.

I hope this info can be of assistance to others. I’ve not been able to locate any information on the physical construction of the TOA assembly anywhere else.