I've seen all sorts of thermal problems, but never an edge induced issue. Hmmm.
TL;DR: absence of evidence is not evidence of absence. Especially since I have
seen edge support induced issues (and it's not like there is a dearth of information about them, including Mike Lockwood's little page with experimental results).
There will be 'creep', 'give', etc much of which is different to a glued support point.
Doesn't really matter. The end result is a force that exactly counteracts the mirror's tendency to slide down due to gravity, since in equilibrium you're telling us the mirror isn't moving anymore. There is F, there is m, so unless something cancels F through F=m.a a is non-zero. Since you told us that eventually a is 0 in equiilibrium...
One might even posit that the PLOP3D simulator fits felt pads that apply static friction better than it models glue points, in general (as you quite rightly point out).
A glued point would deform a little and transmit the force to the bottom of the mirror. But under such forces my felt pads will slip. So under some circumstances at least, glue and felt aren't equivalent.
Correct. Which is why I said exactly what the PLOP edge support simulator does in this case.
I strongly suspect my felt pads (about 5mm thick) are compliant enough to not transmit any significant forces
If there isn't a "significant force" due to static friction, then the mirror slides down under the effects of gravity when the mirror is slanted 45°.
Yet you are telling us it doesn't. I still fail to see how felt pads would keep the mirror from sliding down without applying forces, compliant though they be. They'd have to be magical or telekinetic at the least, not just compliant.
Perhaps my felt is thick and robust enough to have worked OK.
No, you're moving the scope while inclined low enough until the mirror wins against the felt pad's static friciton and rests on the edge support despite
the friction of the pads, and then you're lucky they're compliant enough not to make the mirror stick somewhere it should not later (but I've seen scopes where you'd not want to bump the scope while pointed closer to zenith -- including the former build of my current scope). Tat's not good design, even though it may 'work'.
If by now you aren't convinced that you need to design a good edge support and that people are just 'making a fuss', you're just making an argument based on personal incredulity. There are sound reasons to worry about this, for the simple reason that relying on sheer luck or 'I just shake the scope a bit when I start observing and mentally sacrifice a goat' (which seems to work with your edge support and your felt pads but which I would not really advise if you had a sling!) doesn't always
work reliably for all
scopes. I've seen enough scopes in my life with edge support issues to make an argument with a single counterexample where it does work rather unconvincing.
To me, the simple fact that you can slant your mirror 30° and it will still stick to me means there is far too much static friction for the design to be what I call 'good'.
Edited by sixela, 28 July 2021 - 11:32 AM.