In terms of mounting a largeish secondary mirror (in my case 100mm minor axis / 142mm major axis and 25.4mm thick) I'm not aware of a better method of mounting, although you need to apply some solid basic principles. If you were to use retaining clips on the periphery to mount the secondary that will introduce a larger wavefront error (though still within acceptable limits I believe, ~1/20 wave PV ballpark) as well as diffraction effects.
Principles that I have applied:
- Use only 3 silicone blobs supporting roughly equal areas (taking into account the 45 degree cut angle) at around 70% radius, being ~18mm diameter. More than three can result in astigmatism if the supporting cell deforms due thermal effects, etc; whereas three will act more or less the same as a standard three point support cell. I'm amazed when I hear about commercial manufacturers using 4 or more blobs, or worse, rings of silicone around the whole mirror - a recipe for astigmatism.
- The support "cell" should have a similar cte as the mirror substrate and be extremely rigid. In my case I have a pyrex secondary (cte ~3.3 x 10-6 / deg C) and I made a solid carbon laminate plate ~7.6mm thick which would have a cte closer to zero. FYI - aluminium has a much higher cte ~ 23 x 10-6 / deg C.
- Note that the silicone "blobs" that I have formed are only ~ 0.3mm thick and I do not notice any distortion in my star images (CCD); though I have not performed any high-magnification visual star tests.
David