> About the only "data" you see on this group is "It seems to work for me."
> I think this is frequently worse than no data since it implies that
> something is actually a workable technique when infact we have no idea
> what kind of test, if any, was performed and with what level of skill, or
> with what favorable conditions which might be different in another case.
Well, yes and no. Many of the posting here are "It seems to work for me" variety, but not all. If a very experienced amateur or professional says "it works", I don't think that I need scientific proof for his opinion. If I'm in doubt, I can always ask for a back-up or clarification.
Another matter here is different perception (very subjective thing!) between people. For example, someone will gladly accept 15 kg (30 lb) for an optical assembly for say 8" f/6 as quite light and acceptable. I don't. Simply as that. More weight means more vibrations (don't try to tell me otherwise !), and definetely less comfort in using the 'scope.
And back on discussion about plastic tubes. First I'll make clear that I _did_ build PVC tube (actually that was my first "real" scope, a 6" f/10 Newtonian). It was 3mm wall thickness, and it definetely _did_ flex. Maybe not much, but it was enough to see it in an eyepiece as a slight bounce back after nudging the scope. Perhaps not easily measurable directly, but definetely there. The other thing is dew. In my experience PVC tube will dew long before composite one, which on many nights won't dew at all while PVC will be runnig down with water! And there is thermal stability (not an issue for a visual telescope, but quite important for a photographic one). 1.5m long PVC tube will expand/contract several MILLIMETERS over temperature difference of 10 or 20 degrees. Definitely NOT the choice for an astrographic camera ! And at the end - the weight. Subjective matter as I said already. 10" PVC pipe with 1/4" wall thickness will be some 15kg (30 lb). Add to that mirror, spider, finder, mirror cell, focusser, end rings ... 20 kg easily. My Aluminium/Carbon composite 8" f/6 is less than 8kg total, easily carried in one hand. The choice is yours ...
Bratislav