telescope mirror support

About the development of the FEM module/workbench.

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astro_alex80
Posts: 1
Joined: Tue May 17, 2022 7:17 pm

telescope mirror support

Post by astro_alex80 »

Hi together,

I am hobby astronomer and happy to join the forum. Working a lot with freecad constructing a new telescope.

Like below a new mirror cell which is right maschined. The mirror is 355m in diameter and has 25mm edge thickness, material borosilicate glass.
18P_MirrorCell_final_40.png
18P_MirrorCell_final_40.png (85.77 KiB) Viewed 617 times

Such a parabolic telescope mirror has an accuracy down to a few nanometers.
Crucial for an optimal image quality especially for thin mirrors is the back support in the telescope. With so called PLOP Cell Designer a program exists using FEM to calculate the mirror deformation depending the back support.

Here an example plot from PLOP if the mirror would rest on 3 contact points only.
Plop3point.PNG
Plop3point.PNG (37.85 KiB) Viewed 617 times


Meantime I got quite familiar with FEM in freecad but for rather simple analysis. Wondering if and how such a simulation is possible in freecad. Thanks to the material editor I created borosilicate glass and gave it a try by putting 3 little fixed contact points on the backside of the mirror.
freecad_example.PNG
freecad_example.PNG (54.14 KiB) Viewed 617 times

However magnitude of deformation especially in the relevant Z axis is very different and varies by 10 times between PLOP and freecad. Most likely I am doing something wrong in Freecad and would be very happy about help.

Thanks a lot and kind regards,
Alex
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NewJoker
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Joined: Sun Oct 11, 2020 7:49 pm

Re: telescope mirror support

Post by NewJoker »

Can you share the FreeCAD file ? How does the mesh look like (it can be too coarse) ? How is the model loaded ?
thschrader
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Posts: 3129
Joined: Sat May 20, 2017 12:06 pm
Location: Germany

Re: telescope mirror support

Post by thschrader »

In this video at 1:13
https://www.youtube.com/watch?v=lJR80BFHUNE

the "Plop Run Controls" dialog opens.
There is an option to generate Z88 input files. Does Plop use Z88 as a solver?
https://en.z88.de/

If so, you could try this option and import the Z88 mesh into FreeCAD for a direct comparison.
plopZ88.JPG
plopZ88.JPG (45.57 KiB) Viewed 561 times
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