There actually isn't a lot of definitive literature on the best spacing / thickness of the silicone blobs to support the secondary. It is more complicated than a primary mirror because a) it is elliptical, and b) the 45 degree cut of the glass means that there is more mass at one end of the elliptical mirror surface than the other. So, a support point (on the back surface) at one end of the ellipse would support more weight than the other end. I don't think PLOP will model this, but I saw reference to another FEM tool that might (though I haven't tried it). I think the saving grace for many secondary mirrors is the fact that they are smaller than primary mirrors and tend to be full thickness as well.
Here is an interesting website that discusses some aspects of mounting a secondary mirror:
http://www.cruxis.com/scope/scope110...darymirror.htm
Note however, that this site shows a cellular secondary which doesn't have the classic 45deg cut.
My approach is along the following lines:
- To have just three silicone support blobs - any more can introduce additional stresses on the mirror.
- Each silicone blob to be no more than 20mm diameter (my secondary has a minor axis of 100mm).
- From the Cruxis telescope site (based on his FEA), "Another important conclusion is that it's best to mount the mirror with the edge of the single support closest to the eyepiece".
This seems to make sense intuitively for a classic 45 degree cut in that this mounting point "overhangs" the actual mirror surface, and so the other 2 points (which will necessarily be closer together at the other end of the ellipse) will support a greater load (since this end of the ellipse on the back surface "underhangs" the mirror surface. I'm not really sure why this would be optimal for the cruxis secondary however.
However, I'm not sure which silicone to use?? Websites seem to only quote RTV silicone, but my understanding is that all silicones sold in the local hardware are RTV (Room Temperature Vulcanising)? I had thought a neutral cure would be best, but I see on the Protostar website they mention the acetic acid catalytic type? Dream Telescopes recommend a 1mm thickness, so I would assume that the silicone should have a low modulus (elasticity). Does anyone have an opinion here?