I have a Celestron Starhopper 8" dob.  I got the t-ring and t-adapter for my Canon 30D.  I can focus just fine with the eyepiece, but when I remove the eyepiece and put in the t-ring/t-adapter/30D stack, the camera won't focus no matter which way I turn the focus knobs.  The scope focuses perfectly with the eyepiece.  I know I won't be able to do "true" astrophotography with this set up, but at this point, I am just trying to get a good shot of the moon, maybe a planet or two.  It's just a big blur looking through the camera's viewfinder.

 BTW I really tried to find an answer using the search function, but every search I tried ended up with no results or 2,000 results, none of which seemed to be applicable.  Sorry....

 Thank you for your help!

 Few newtonians will allow you to get to focus with a camera body attached, even with a low profile focuser. Moving the primary mirror closer to the secondary to move the light cone further out of the tube is usaually the only answer.

Or, you can stick to afocal photography. 

Thank you for the reply John.  Bummer.  What kind of distance would I need to move the primary mirror?  At the risk of asking the (second?) stupid question of the day, would adding something like a barlow lens allow me to get a focus without moving mirrors (which sounds kind of tricky)?

I got my Canon XTi to focus on a Meade SN-8 by using the 1.25" adapter at http://www.stellar-international.com/index1.html and a T-ring.  This is about as close as you can get and I have a JMI EV-1 focuser that is fairly low profile.

CFB

 You can place a barlow in the focuser, add a tube that will clamp over it to attach your camera to and then you are doing negative projection photography.

Basically the primary needs to be moved a distance equal to the distance of the adapter rings etc to the film or chip plane of the camera measured from the top of the focuser draw tube. 

Hi

If you have a 42mm thread on your camera you could try adding some M42 camera extension rings. They are cheap on ebay and I use them to solve all sorts of focussing problems.

There are some examples on my website in the equipment and project sections

Cheers

Danny

Thank you for all of your help everyone.  I'm going to make this thing work! I'll let you know how it goes.