Againm, if what I read is correct, somewhere in the 10" - 16" area you are running out of room to improve through aperature and the only solution is to maximize the theoretical capacity of the instrument through quality. In short, below 10" the easiest improvement comes from simply making it bigger but at some point after that, quality that allows one to capitalize on the available seeing becomes paramount. The question becomes on how many nights in your area would the seeing be such that say a good 16" would have an advantage over a near perfect 12.5".
Further tube currents can ruin the seeing even on a good night can they not?
Part of my probem is that I have never had access to a reasonably large quality instrument and hence have little personal experiance to go on. I do know that I have briefly looked through a few large aperature instruments (over 20") and always been disappointed though that may have been as much a quality issue as a seeing one.
In this vien I wonder whether CCD images are really the acid test here, as has been suggested? Parker and others are using multiple fractional second exposures, and image manipulation techniques some of which are expressly intended to overcome seeing. I wonder how many images he records to produce one for publication? What is the resolution he is achieving by the way? Is it as good as one sees with the eye through the same instrument, or could see if the instrument performed at an optimum level for its diameter? I don't know the answer but am curious to hear from those that do.
Final question, would the use of velvet to reduce reflections in an open tube design, prolong or worsten the tube currents in an aluminum tube by slowing the equlibrium process?
Thanks for all the input. PS I like frank opinions.
Bruce McMath