I'm currently building a 10inch (254mm) F4 truss tube dobsonian telescope and have come to my first (and hopefully last) design problem.
The mirror is a SkyWatcher 10inch F4 mirror but I measured the focal length as being 1002.5mm making it aproximately F3.947, this means that with my current 46.5mm diagonal the focal plane is 11mm out from the edge of the 340mm upper end.
Here's where the problem lies though, (you've probably spotted it) where do I put my crayford style homemade focuser? The focuser is loosely base on Jerry Oltion's excellent design (see the link) used on his 12.5inch track ball scope, but I haven't actually made a final version of it yet.
http://www.jerryoltion.com/12inchtrackball.htm
The upper end of the scope is 340mm wide so I have 40mm on the inside on either side of the light path to potentially put some of the focuser but is that enough? My bearings (from bunnings) are 19mm wide so should I put two on the inside and two on the outside? Or should I get a bigger secondary mirror; a 50mm mirror would be 25mm and a 52mm mirror would be 33mm out from the tube.
I haven't fixed my secondary to its support or fixed the focuser to anything yet, so I have time to change things. Also in case it matters, my spider is a single vane going across the diameter of the upper end.
Thanks to those who read this long post and what are your thoughts on it?