Thermal Equilibrium. The OTA must be as thermally equalized as possible. To my non-mechanical-engineering way of thinking, this means: all critical parts of the OTA should thermally equalize at the same rate. If they didn't, the two mirrors of the Newtonian would become closer together and less accurately aligned as the temperature goes down. If all parts of the OTA lost their heat at the same rate, the mirrors would stay aligned during the thermal process. I could be wrong. But assuming I'm not wrong, the implication of this is: all parts of the OTA should be of the same proximate material: aluminum, or composite. This means 1) aluminum tube, mirror cell, spider, and focuser. OR 2) carbon tube, carbon cell and spider and carbon focuser. There is ONE exception I can think of at the moment: Blacklite tubes (fancy cardboard) probably have C.T.E. close to that of carbon. So an OTA of 3) Blacklite tube / carbon cell, spider, focuser might be good. Economically, the carbon solution is orders of magnitude more expensive than aluminum, right now. But I'll investigate both. There is one company I know of making carbon primary cells, I have to find that company and email.