Thanks for that excellent link David and regarding the issue of getting proper spacing would it be worth trying something like the Orion variable T spacer that can be set from 12mm to 17mm?
Re the back focus value I found it at
http://www.takahashi-europe.com/en/F...ifications.php . Normally I would take back focus to mean the maximum extent that can be taken up by gear before running out of inward travel on the focuser. However, I assumed that when a company selling a flattener quotes a back focus value but not a spacer value they intend the back focus value to represent the spacer value.
This could be wrong. Takahashi also provide a diagram with recommendations on items to be used in various image trains and they may be relying on this instead of a specification. I couldn’t find dimensions for any of the items.
Be glad of your opinion on my interpretation and on the use of a variable spacer.
All the best, Peter