Is it possible to adjust the mainmirror first? I mean so that the distance to the secondary and camera increases
no, the mirror cell is bolted to the bottom plate.
the easiest spacer, mechanically, is a spacer under the focuser. but you'll have to see if the 2ndry is big enough... but your out that far already... but I'd still check.
but if you install spacers under the mirror box truss, you'll still need to look at how that will affect the 2ndry size also. but not a bad idea, keeps the weight away from the front of the scope.
so 1" will put your focusing plane somewhere close to 1/2 the focuser travel length?
That sounds about right.
Well, isn't it the case that the closer the focal plane is to the secondary, the smaller that it needs to be? And also I am a bit worried that the coma corrector is being vignetted by being so deeply recessed in a drawtube / focuser body. (haven't measured to see if that would be a problem)
Not wishing to blame the victim but this setup looks very wrong. Usually a focuser must be racked inward to achieve focus with a long imaging train. Is there any chance the coma corrector is reversed? If not, then if this scope is new I would return it. The mirror focal length is incorrect.
No, the coma corrector is not reversed.
My best guess for what the problem might be is that if the entire secondary assembly was somehow flipped over and reattached to the truss, everything would be an inch or two further out from the primary. You can see in the pictures that the focuser is closer to the truss side than the end.
Otherwise the primary mirror is too long of a focal length or the primary mirror is too far forward in the mirror cell. Sounds like a call to the manufacturer is needed.
I am not sure it matters much which end the spacer blocks go to. I can think of pros and cons for each location. Probably be pretty much a wash. My inclination would be to make them beefier and tighter tolerance than you would think are needed though.
No, it's not flipped over. The secondary is "centered under the focuser" according to a sight tube.
I think other people with this scope have the same issue. See here:
Now that I think more about it, a better location to put the spacer blocks is at the joint near the secondary. The reason is that this configuration actually has the center of mass lower. If I put the spacers near the primary, that shifts all the truss tubes, plus their mounting blocks, upwards.