Sure, if you have a 400mm f2.8 telescope and expect it to work only
manually.  I suspect it will be a lot more convenient, better results
(optimized for a flat plan focus) and maybe even cheaper by buying a real
lens.

Hi,

Telescopes as Swarovski-Habicht and Kowa do have a telescope adapter but it
is at F8 or F11 : an 800mm adapter at f8 and a 1100mm adapter at f11.

Cheers,

Luc

You will have a hard time getting f/2.8 from a telescope.
Telescopes with that fast of an aperture are special designs
that often have curved film planes, like Schmidt telescopes,
or would be quite expensive.

But you could get f/4.5 to f/6 effectively.  I've tested
a 6-inch aperture f/6 homemade Newtonian, and it
delivers very sharp images, in fact sharper than
top end telephotos.  That equates to a 914mm f/6 telephoto,
that cost only a few hundred $ to build.
The problem is focusing.  If you intend to do astrophotos,
there are focusing aids (do a google search for DSLR focus),
but if you want to do wildlife photography,
where focus is constantly changing, it would not be
a good solution.  Focus is very difficult with
DSLRs unless you can change the focusing screen and
can install a better microprism.

I settled for a 500 mm f/4 L IS telephoto.

Roger
Photos at: http://www.clarkvision.com