Mirror Cell: Clamp vs Glue

[RobSay]

I'm doing the slow thinking on a truss Dob project - planning on a 12" primary, plywood construction and building my own cell. Current thought based on research is to use 9 point support - although my impression is that 12" seems to be the limit for this. Will use various modelling programs to verify before diving in. One thing I've failed to turn up is any comparison of the relative merits of mirror fixing with adhesive vs clamping.

Various Dob construction guides advocating adhesive as sufficient (up to 12") - others indicate edge clamps as the most important thing and go into great detail on bevelling concerns whilst avoiding stressing the mirror.

What's the trade off that the gluers aren't mentioning? Or what benefit is being realised through the greater effort of clamping?

thanks

R

[John S]

This is my favourite ultralight at the moment and it uses adhesive for the primary, not adviasble for the secondary, his fell off oops.

A homemade 20 inch dob lightweight telescope, DS-5

John.

[acey]

Clips cause diffraction (and possibly stress), glue avoids this. But whenever the mirror needs resilvering you need to be able to detach it from its cell. That's the trade-off.

[callump]

Not sure what you mean by 'clamping' - but a mirror should never be 'clamped' (tight) to the cell.

I used a 3 point glue (silicone sealant) for a 10" and it was fine - and for a 12" would do something similar - not sure if 9 points is optimal or not - run a few designs through PLOP.

I also glued my secondary (as many do) on the 10" - never any sign of it detaching. With a bigger, heavier secondary (like on a 20") that might be a different decision.

/callump

[Peter Drew]

When glueing a mirror to the backplate with silicone, place three coins or washers at 120 degrees near the edge. Once the silicone has set remove them, this gives ventilation to the back of the mirror and clearance for a hacksaw blade when the mirror needs removing for re-aluminising.