wow bob just looked that paper over, what a beautiful piece of work, i love it when things work out so nicely. thanks for the reference, i will keep it folded up in timoshenko where i will not lose it. now i know what tom johnson had up his sleeve when i went down to the old shop in 1965 to buy a pair of 4 inch sct's to make into a pair of binocs. he let me test every one he had in the shop to get the best ones. of course in those days, his secret was carefully guarded, and he didnt breathe a word of it. i remember the 20 inch celestron sct sitting over in the corner of the room, the one that was appearing in celestro\n pacific ads. i got a good look at it, definitely only one was eveer made, and that was it. good for publicity tho. anyway, the difference between a schmidt corrector plate deflection and the drum, was it mylar or something , is that the theoretical treatment of the corrector plate is that it resists the load entirely by internal bending stiffness in the glass, and the edges are simply supported and unrestrained laterally, ie plate theory is pure bending in this case, and deflections are so small that deflected curvature does not effect analysis. on the other hand, the mylar drum head resists the load in pure tension with no bending resistance, and for very small deflection theory, it would not make any difference whether it was a sphere or a catenary shape. for larger deflections, say visablly depressed, the atmospheric load will have to be considered perpendicular to the mylar surface at each point, or some error will be introdued into the solution. let me summarize, glass corrector analysis is pure bending of a circular plate with uniform load and simply supported edges-----mylar drum anslysis is pure tension analysis of circular membrane subject to a uniform loas applied to the surface and restrained only at the edges.-----the plate and mylar can both be subject to small and large deflection theory, but in this case, small deflection theory is appropriate for the corrector plate, while it is prudent to choose large deflection theory for the mylar, because it is more rigerously correct. ----gad do i talk a lot, tom leech