DLR in C5 prime focus: cannot get fully in-focus images

When attaching our new Canon 350D SLR to the prime focus
of our little C5, we cannot obtain the sharpness that the
camera viewfinder (SLR) shows us.
Even though using lock-mirror, 2-10 sec. (remote) self-timer
and exposure times of 50th of a second and faster,
images taken are way to blurry (daytime).
Goddammit. What are we doing wrong ?
Reg. Pete J.