Tonk

I think the problem was that the instructions of camera manufacturers where misinterpreted as subtracting from the scope/reducer metal back distance when its actually "subtract from the camera back focus cost". Don Goldman (Astrodon) pointed this common trap out many years ago.

"Do filters add to back focus?"

They ADD to metal back distance (MBD) of your scope or reducer or they SUBTRACT from the camera back focus cost . You calculate the spacer requirements needed by either the following methods:

(First calculate the filter offset via t * (n - 1) / n  where  t is filter thickness and n is filter substrate refractive index - note that most filters have n close to 1.5 so the equation can be treated as t / 3)

1. Adding the filter offset to the metal back distance of your scope or reducer and using this adjusted MBD as the base for subtracting the back focus costs used up by filter wheel and camera

OR

2. Subtracting the filter offset from the camera back focus cost and using the original scope/reducer MDB as the base to subtract back focus cost due to filter wheel  and due to the filter-adjusted camera back focus cost. (Some camera manufacturers instruct you to use this  approach - particularly if the camera has an internal filter wheel where you add your own filters)

The two methods are equivalent - one instructs you to add the filter offset  to something - the other to subtract it from something else - somehow this subtle point has lead to 3 pages of tail chasing - lol

The equivalent equations in words

1. Spacer length = metal back distance + filter offset - (filter wheel back focus cost + camera back focus cost)

2. Spacer length = metal back distance - (filter wheel back focus cost + (camera back focus cost - filter offset))

Camera back focus cost is measured from front flange to sensor surface