Our local conditions are not bad from a seeing perspective but our general location in central Arkansas is not the best. I have tentatively concluded from both my reading and limited empirical observation that 12.5" is probably at least beginning to pushing the outer limit. In other words, it would be a rare night that, visually if not with ccd, that we would gain meaningfully from a larger instrument and for the reasons you state as well as the greater difficulty of makeing a quality instrument as aperature increases, there would be more nights when performance might be worse. Certainly, as others have now stated, anything beyond 16" is likely on the otherside of the fence, especially I suspect for visual use. When the night you got to use an instrument to good effect on a planet is an event worthy of being recorded in the family bible you have probably gone to far in aperature for the purpose at hand.
However, there are factors that remain to be explored and I have not decided where the cut off point is. One question is at what size can one get a so called web backed mirror? How beneficial are these in the thermal mass area? Likewise I understand that the Gleanning in Sky and Tel a few years back published a study of differant tube materials and concluded that thin aluminum was the material of choice for an enclosed tube (a design highly recomended by Texerau) largely do to its ability to radiate latent heat. I was wondering about covering the interior of the tube with a sheet of foam insulation material perhaps covered with the black velvet recomended here for light absorbtion. this would hender the equalibrium process but would tend to force the heat radiation to the exterior of the tube. Do you or anyone have any experiance or thoughts on this? How about using a fan to blow air over the mirror on such a set up to replace some of the ability to disipate heat. This could be done before an observing session and even periodically during it is suppose. Texerau reports that you cannot acutally use a fan while observing. Finally, Texerau says one should allow at least two inches between the tube wall and the light path. That hurts when it come time to select a secondary. Any thought on his recomendation?
Thanks for the past and future input!!
Bruce McMath