I hope Chris doesn't mind my continuing this discussion as a slight digression on his build, but this is a topic that I really think warrants some further consideration for such massive mirrors in general, and may help others considering building such huge scopes with their mirror cell designs.
I trust that as long as we keep the conversation respectful and on the topic of the behavior of such supports with respect to collimation effects, that no offense will be taken.
Regarding the following statements,
.....First the flexions on the triangles will probably never be homogeneous, as accuracy needed to achieve perfect balancing and triangle support positioning on the base plate are pretty high, and if finally they are, the edge supports to mirror contact points will probably move with these (hypothetically homogeneous) flexions....
it is my position that the deflections of the all the various triangles needn't be exactly equal in magnitude to maintain tight collimation tolerances.
I trust it is already understood that the forces on the glass will equalize among the supports, or in the case of unequal force designs, will be closely represented by PLOP.
The point that I wish to make here is that a large amount of mechanical averaging is occurring among all the cell elements, even if some of the components such as these triangles may flex as much as a millimeter when the scope is pointed at zenith. Although such a large heavy mirror as this one may require a little more focus adjustment on this cell for different altitude objects than would be required if an incredibly stiff cell were used, I wouldn't expect the views to be degraded in comparison.
I realize that slight variation in material properties and triangle dimensions will result in some variation in how much each triangle deflects. The same is true for the triangle supports. The amount this variation will affect primary collimation, however, will depend on how much the AVERAGES OF DEFLECTION of all the elements attached to each of the three rearmost pivots vary.
I understand that these fast scopes have primary collimation tolerances of only a fraction of a millimeter, and tilt effects of the mirror are amplified a few times to the focal length. The amount these components flex under load will affect the amount of scope refocus required for different postures. The amount that variation in stiffness among the components will affect collimation due to tilt, however, will depend on the differences in the average deflections of all the components in each third of the the entire cell. This averaging effect will greatly diminish the amount of variation from component variation.
I think this is important with respect to cell design for these monster scopes. I don't want others to get discouraged by overdesigning the structure, and then choosing not to build a scope due to excessive cell weight in addition to mirror weight, or material costs.
As long as the variation of tilt keeps the aim of the mirror within collimation tolerances, and the edge support remains close enough to the center of gravity of the mirror, I expect such a cell to yield phenomenal results. I think the tendency is to overbuild to err on the side of caution by many, and sometimes at significant cost in materials and weight.
As for huge scopes that may flex their cell components significantly when aimed at high altitudes, compensating edge supports that remain at the center of gravity plane may be especially important. Howie Glatter's sling on linear bearings is one neat solution that I've seen on addressing this issue.
The main point I'm trying to make here is that a tremendous amount of dimensional averaging occurs within these large cells that reduces overall variation in collimation. Keeping components light helps to minimize variation in forces on the mirror from any torque from components whose centers of gravity and pivots aren't exactly coincident.
Approximating the forces to the mirror to calculated values is what really matters- -as long as collimation is kept within tolerances.
My hat is off to you, Chris! Keep building!
Edited by jtsenghas, 30 December 2016 - 12:40 AM.