re: Is focus really an issue?
The focal plane may be fixed by the optics, but the detector (film or CCD) may be anywhere, depending on how it's held in place, thermal expansion, etc. So there's always a focus knob. This was almost certainly shot with eyepiece projection, so there were additional optics
in the light path, too, so the distance between objective and eyepiece had to be tweaked, probably by doing some test shots with some object (like a star) at infinity.