Lack of orthogonality

[MarkS]

I'm trying to debug a problem that has recently appeared in my imaging train 
C11 - Visual Back - Optional Focal Reducer - Connecting Tube - DSLR. 

The symptoms are as follows:
Stars in the bottom right of the image are in focus but become progessively out of focus towards the top left.

If I rotate the camera 180 degrees then the stars that were in the bottom right are now in the top left and are still in focus - the stars then become progressively out of focus towards the bottom right.  This indicates that the CCD is parallel to the front mating flange of the DSLR.

Removing/replacing the focal reducer makes no difference so that has also been ruled out.

The DSLR and the Focal Reducer are therefore both ruled out - so what else could it be?
1) Maybe the connecting tube does not have parallel mating flanges at each end (or there is a foreign body on its surface)
2) Maybe the visual back is not orthogonal to the main axis of the scope (maybe it was knocked?)
3) Could it be play in the primary mirror? I'm not aware of any, but if there is, would that cause this effect?
4) Maybe the dew heater (which I now have to use in earnest) is creating a wedge of warm air next to the corrector plate.  Could that cause the effect?

Or is there something else it could be?

In any case I need to design a series of tests to gradually isolate the problem.

Mark

[Mike]

Do you get the same results if you set up the kit with everything pointing vertically and the kit at the bottom?

[mickw]

I don't know anything about your kit but if there are any threads involded in the attachments, if you over tighten anything, it could cause things to flex in the opposite direction to the point where the thread actually ends.
You could check the chances of this happening by partially screwing things together and move them around to see if there is any play in the threads.
This can be cured/bodged with thread lock 

Did you mention something like this before being caused by the glass "filter" you fitted being out of whack ?

[RobertM]

Are you sure that the DSLR and connecting tube are seating correctly in the visual back ? I have a VB which uses a ring inside but it pushes the DSLR nose piece off center.

[MarkS]

Mike, Mick, Robert,

Thanks for your suggestions - the problem could easily be to do with one of them.  I just need a clear night to do some further tests ...

[RobertM]

You won't need a clear night to check the nosepiece.  Seat it the way you normally do then push firmly on the back of the camera whilst releasing the locking screws.  If you feel it reseating then that's a problem, in fact in my case I could see it wasn't seated properly.