That is true, Ken is right - it may be difficult to focus with commercial lens.
Sometimes it is possible to modify lens as well, to compensate for lack of filter and move the focus plane further (there will be stops inside the lens that need to be moved or removed).
Or, use different lens, fixed focus primes are usually better for AP than zooms.
see here:
http://www.iceinspace.com.au/forum/s...ht=canon+200mm
Another possible way is to remove the shims between sensor and mirror box.
And adding flat glass as suggested by others.
Another point - if you are using glass lens with full spectrum modified camera, you will definitely need UV-IR cut filter to avoid bloated stars. See here, post #25
http://www.iceinspace.com.au/forum/s...rocolax+filter
Better option is to use mirror lens (catadioptric) if you do not have filter.
Have a look
here for focus screen removal.