leond1 I cannot focus correctly
Since you are using the EOS adapter, I am assuming that you are using a Canon EF lens, which has a flange focal distance of 44mm.
The EOS-II adapter from ZWO is 31mm thick, with a detachable 5 mm spacer (i.e., 26mm without the spacer).
Your camera, assuming that you do not remove that 11mm ring, has a flange to sensor focal distance of 17.5mm.
17.5mm + 26mm + 1mm (if your filter is 3mm thick -- cheaper filters are thinner, so add a little less) = 44.5mm, which closely matches the EF lens flange focal distance.
So, (A) unscrew and remove the 5mm extension part of the EOS-T2-II adapter to reduce the thickness of the EOS adapter from 31mm to 26mm, (B) keep the 11mm ring of your camera in place, do not remove it. Connect lens to EOS adapter, connect adapter to the 11mm ring on the ASI294, and you should be ready for fine adjustments.
(That additional 5mm piece on the EOS adapter is for use with ZWO's cameras that have 12.5mm flange-to-sensor distance.)
That should get you close enough (with a glass filter installed) and if not, use M42 shims -- do not use the ones that are included in the EOS-T2-II box; they are junk Delrin (plastic) washers. Go buy stainless steel M42 washers instead, if you need to shim the lens correctly. Places like Agena Astro sell M42, M48 and M54 stainless steel washers of various thickness.
ZWO cameras' flange to sensor diatances are not accurate (many of their spec sheets show +/- 0.5mm tolerance). That poor tolerance needs to be shimmed out.
However, with the small sensor of your ASI294, precise backfocus is probably not needed, especially with tele lenses. The errors at the corners of the frame might be acceptable even without careful shimming -- it will depend on the lens (and how much you personally tolerate distortion of star shapes), and you just have to check corner stars with some test shots.
Chen
