Update: I mentioned in my original post that I hand contacted a Telescope and Eyepiece manufacture and got two different answers to the following scenario.
Assume there was correcting element that moves with the focuser that requires 50mm of fixed back focus and the imaging chain had a camera and filter wheel (with no filters) with a physical length of 25mm. In this configuration a 25mm spacer would be required to achieve back focus. If a 3m thick filter that adds 1mm of back focus is installed in the filter wheel, what would be the length of the spacer needed to achieve the correct back focus?
One person told me 24mm, another said 26mm.
This discrepancy is what led me to post the question here. I am happy to report that the person who told me 24mm tracked me down today and changed their answer to 26mm. They explained that adding the filter to the optical path increased the total back focus of the system. Introducing the filter increases the total back focus of the correcting element from 50mm to 51mm. Therefore the length of the spacer required to achieve the correct back focus for this scenario would be 26mm (50mm native BF + 1mm BF from filter – 25mm physical length of other components = 26mm spacer). I have to say, I was really impressed that the company would take the effort to contact me to correct their mistake. This is a rare thing.
This correction helped validate my initial thinking that adding a filter that adds 1mm of back focus changes the total back focus of the system as the starting point for the spacer calculation, before subtracting the physical distance of other components in the imaging chain. It is easy to get this backwards and add the 1mm to the components that are subtracted from the native back focus of the correcting element. This is the mistake the person made who said I would need a 24mm spacer.
I found this explanation helpful so thought I would pass it along. It also demonstrates how easy it is to make a mistake that would lead to a 2mm error in the calculation.
If adapters are ordered 1 – 2mm shorter than required and shims are used to achieve the correct back focus as described in Dale’s post then this mistake is easy to correct. If adaptors are ordered to the exact calculated length as I use to do, this can be a cost mistake to correct. After talking to the person yesterday who told me 24mm, I thought I was going to have to order a bunch of new adapters because I had ordered them to exact length and if what he told me was correct, it meant that all my adapters were 2mm to long. Thankfully, it appears I might be OK if I ignore the tolerances. Ordering them shorter than needed is definitely the way I plan to go in the future.
I hope this was useful.