What length camera tube on a DSLR?

[Ovenpaa]

Is there a formula or guide to determining what length tube I need to enable a Canon 450D body to focus on a 14" Orion Dobsonian or is it a trial and error thing?

I ask because I have the Canon and tube off a 102 and would like to try it on the 14"

[Rallemikken]

37 minutes ago, Ovenpaa said:

Is there a formula or guide to determining what length tube I need to enable a Canon 450D body to focus on a 14" Orion Dobsonian or is it a trial and error thing?

Trial and error. Not sure how the 102 is pieced together, but I guess the dob has a 2" focus tube, so you will need a 2" barrel with threads that interact with the Canon adapter. A stock 2" T-ring adapter, with other words. If what you have fits the dob, then take it out on a cloudy day, and start fiddling. Or the moon at night. If you cant get a clear image, you most likely lack inward travel on the focuser. If so, try anything that gets the camera closer to the telescope tube. Is there a rim on the focus tube that can be removed? Can you raise the primary mirror by changing to longer screws (not unusual, pretty common if you want to image with a reflector that is mainly constructed for visual)?

[Ovenpaa]

Yes the dob is 2" and I have the T ring and a tube that screws in and fits the focuser. The issue is it is all about to be packed down ready to move to a new location/country end of the year and I suspect sourcing a different length tube will be time consuming

[Rallemikken]

32 minutes ago, Ovenpaa said:

The issue is it is all about to be packed down ready to move to a new location/country end of the year and I suspect sourcing a different length tube will be time consuming

Not sure if I understand. The tube (or barrel) connected to the camera should always be pulled as far down the focuser tube as possible, this is to avoid that the focuser tube sticks out in the inside of the telescope tube (avoid reflections and such).  The optimal setup is when the camera is in focus, and the focuser tube is flush with the inside of the scope wall.  As long as the focuser tube goes all the way in, so it touches the body of the focuser itself, nothing is gained in replacing it. If you lack inward travel, you could opt for a low-profile focuser, but that is risky as long as you don't know how much less distance you need. I'd raise the bottom mirror (maybe clean it as it is out) and add a 2" extension under my eyepiece when I use it for visual.

[Ovenpaa]

The issue is the tube has a step of a slightly reduced diameter so the tube does not side through the focuser. This is easily resolved with a skim on the lathe to reduce the diameter.

[Rallemikken]

I see. Think I have one of those, I belive it came with a SW ST80, the little brother of yours. If I'm not mistaking, this tube is slighly less than the standard 2", don't ask my why. But I think the threads on the part that connects to the camera is standard, so a full-width tube should be a drop-in replacement. Maybe simpler to order the two things together. Cheap as chips. Not sure about the f-ratio on your dob, and if you are in for serious AP, but if it is 5 or lower you will benefit greatly from a coma corrector. Both Baader and SkyWatcher have such, looks just like an ordinairy DSLR-adapter, backfocus taken care of. At the moment the Baader is sold out, but I recently got a SW 0.9 CC. I can confirm, it works great, so my straight-through adapters are put aside. Something to consider. I've already spent far to much money on things I later put aside or upgraded....

[Ovenpaa]

Turning the tube down slightly gives a snug range of adjustment and once it is in the middle I can machine a stop ring.