Hello Eric, nice to hear, that your ambitious project makes progress. Did you introduce it somewhere?
In the meantime I procceded to Microgrit 25 my, meditative smooth sound now.
See two new videos:
I found this interesting discussion about meniscus mirror stiffness from 2018:
where was sayed:
"At constant weight, the stiffness of a thin meniscus mirror is smaller than that of a thin, flat-back mirror of the same size"
-Opto-Mechanical Systems Design, Fourth Edition, Volume 2 Pg 81
Paul Yoder, Daniel Vukobratovich
This confuses me. I would expect, that a meniscus mirror is stiffer than one with a flat back of the same weight. Hey this is the reason, why I am doing this.
Approximating the vertical case (mirror looks to the zenith) a 600x21 mm meniscus with PLOP automated cell design with:
Diameter: 600 mm
Edge thickness: 21 mm
Focal lenth: -400,000 mm (to get a sagitta of allmost 0)
Central obstruction: 0
Cell: 18 point, allow angles to vary
Result: RMS = 4 nm which corresponds to ~1% Strehl loss
The mass is 13.2 kg
A mirror with a flat back of the same mass would have an edge thickness of 26.6 mm and would have a deflection of RMS= 6.7 nm, so 67% worse.
To get the same deflection of RMS=4 nm with flat back I have to chosse 32.8 mm edge thickness, this would have a mass of 17 kg, so 29% higher weight.
Is this way of using plop right? If so, why the above mentioned literature comes to a different result? What means "self-weight deflection" in this content?
How can we approximate the not vertical position? For example, how does a meniscus behave looking at 45° elevation? Here the lower support feet will apply some shear forces and thus some bending momentum to the glass. How to calculate or at least estimate this?
How does a meniscus behave in the mirror test stand at 0° (looking to the horizon)? Here I would assume, that it bends more than one with a flat back, but hope, that the disadvantage is not too serious.
Edited by Stathis_Firstlight, 22 April 2020 - 02:57 AM.