Tube Inside Diameter

So I have never built a telescope before, but I have a mirror cell and a 12.5" blank sitting here. I am getting ready to grind the mirror and have an elaborate set of plans in my head for how to make this scope work. I am designing a 4 pole truss dob, and my main question before making any drawings, is how much larger should the inside diameter of my tube be than the mirror? I am straying away from the typical "mirror box" and am going to design a circular primary mirror assembly. Is there any rule on how large the scope tube ID should be? Or does this depend mainly on where the secondary puts the focal point at so one can know where to place the focuser. Any help or advice for a first timer would be appreciated. Thanks.

Taylor,As I too am new at building telescopes,the diameter of the tube should be 2" larger than your primary in most cases.I'm sure that some are less.But almost all the plans I've seen and even used one required the tube to be 2" larger.12"tube for a 10" mirror etc.Clear skies,Darrell Hensley P.S. Good luck and have fun with it.

A couple of comments:

1. Dale Kellner's Newt Program is good for designing Newtonians. It's free and it addresses the various issues like focuser height, tube diameter, illuminated field of view, secondary size etc.

http://home.att.net/~dale.keller/atm/newtonians/newtsoft/newtsoft.htm

2. Upper tube diameter has to do with cutting off the light cone, the lower has to do with air flow around the mirror.

3. From what I see, it is normally recommended to first do a small 4 or 6 inch mirror before taking on something like a 10 or 12.5 incher. Since the tolerances are independent of the mirror diameter and a 12.5inch mirror is will need to be faster, getting one's feet wet first with something easier seems to be wise.

Jon

Taylor,
A couple of things to suggest. First, the 4 poles, well, that is not a truss and there are reported problems with flexure unless you use some pretty large diameter tubes. However, when you are building the type of scope design you describe, it could be a good thing for mounting the Alt bearings.
Then the round mirror box, make it larger than the mirror by 2 inches or so on each side at least. With a 12.5" mirror, cooling is a real factor. You may want to add fans latter so leave yourself some room for that.
There are a few scopes around that are basically a combination of a upper tube assembly (UTA) and a lower tube assembly (LTA). The lower assembly being just a larger version of the upper with the truss tubes between and the Alt bearings mounted to the truss tubes. A simple and elegant solution to the structure. You might do some looking around for a picture of this type to give you ideas.