The Geoptik is designed for 12" f5, my mirror is 10" f6, so based on newt calculation I don't really need to adjust the tube length. I had to modify the mirror cell to align with the plop calculation for my mirror. The 13.6" OD UTA is of course oversized but I still have 100% illumination with the 1.83" secondary if I use my 7mm and 5mm Pentax EPs. Great planetary views (primary is 0.99 strehl certified and secondary is 1/30th wave), slightly bettered only by my 10" CZ mirror I had a few years ago. I will not have 100% illumination at the centre if I use my 22mm LVW or lower mag EPs, need to swap with the 2.48" secondary I have as well. Recollimation is easy and fast with a cheshire and glatter laser, but I think I will be making a new UTA for it with 11.5" OD for my next project, and I need to do something in regards to the finder placement.. I use my left eye to view through it lol
All in all, the Geoptik is a very good alternative to the ultra premium structure, especially if you already have a good set of mirrors for it.