I just received a 70mm X 500mm rokinon achromat ota that I was hoping would be a grab and go or super finder. Its quite inexpensive. I decided to put my collimation chesire EP in it to see how it looked. I racked out the focuser (plastic) and I see 2 airy discs, side by side, but each have very good concentric rings! The 2 airy discs are touching / overlapping each other just a small amount. I checked the chesire on my 102mm achro and it looks just fine.
So how can this be? What can I do? I loosened the objective's retaining ring, shook it a bit and retightened it to just touching. No change. It has 3 spacers which look OK.
I loosened the 3 focuser bolts and moved them around and it did change the discs closeness to each other, but I really had to put a lot of pressure on the focuser to the side to do this.