Europa Loc: West Hills, CA

Too much to get done...mount is already torn down and packed. I'm taking 3 telescopes, RASA, new AT and 12" Sky Watcher Newt. Need to pack things up to see if I can even fit it all in the truck. I figure if the AT makes the trip and it works, it's just a bonus.

Albuqshutterbug Loc: Albuquerque NM

Ok I am about to channel Sonny from a year ago.
Where do you get back focus numbers and how do they apply? Maybe that's part of my problem.
Jim or maybe Sonny

SonnyE Loc: Communist California, USA

Well, if the AT won't fit, I can baby sit it for you and try it out here on my mount....

SonnyE Loc: Communist California, USA

I can tell you how I dootit. I took the camera in question, like the DSLR or the Eye-piece camera, hooked them up to my computer, then moved them back and forth aimed at the place I wanted them.
When I got a rough picture, the distance I needed to bridge was what I'd use.
Currently, I have a 2" X 2"L extention tube, ~ 3/4" of filter wheel housing, a spool, and the camera (G3). The total back from the focuser tube = ~ 4". Or 101.6 mm of mechanical set back.

Interestingly enough, for the G3 the focus is achieved at ~ 400 mm of focus tube extension. When my DSLR is mounted, 100 mm of focus tube extension.

So real scientific and higher calculus used to get my back focus.

I did buy a Field Flattener once. It called for 55 mm of back focus spacing. There wasn't any spacers available in the USA for it. I was so pissed that they would even sell the damn thing knowing full well it was unusable without the proper assembly of spacers. As it were, I returned it (on their dime) and I don't need the damn thing anyway.
Back focus, smack focus, I got by my way.