[The Mekon (John Briggs)]

I have long held that most modern refractors are manufactured with tubes that are way too short for visual use. For example my 106mm Sharpstar needs the focuser racked out all the way when using a Type 6 Nagler in a 2” prism – and even then I need to add a 3mm spacer to get focus.
This year I ordered a 132mm F6.9 CFF. Prior to ordering, the first question I asked Catalin Fus was about back focus and whether the scope would come to focus using a prism diagonal.He answered that the back focus was 180mm so any diagonal would need to use up a minimum 66mm of the light path with the Feathertouch 114mm travel focuser fitted.So focus seemed easily achievable with most prisms.
Last night I took some measurements of my 132mm refractor to find out just what the true backfocus value is.
Using an extension tube of 115mm (checked with verniers) I observed Achernar straight through with various eyepieces, noting in each case the reading on the focuser gauge.
7mm Nagler focuser racked out 92mm
17mm Plossl focuser racked out 85mm
17.3 Delos focuser racked out80mm
This would seem to indicate a back focus of between 195mm and 207mm, not 180mm
Question: am I missing something here?Perhaps I should check the back focus using the projection method as well?
I would like some comments before I go back to the manufacturer to say that the backfocus claim is all wrong.
The reason I am interested in backfocus figures is that I want to use a Baader T2 prism, which has a light path of around 70mm, this would leave me around 20mm-25mm short of focus.My 2” mirror diagonal has a measured light path of 108mm, my 2” prism diagonal one of 88mm.I much prefer prisms – last night the companion of Antares was easy with the prism, but quite hard to spot with the mirror.