I’ve recently completed the first stage of modifying the mirror cell to convert it to use 6 point supports instead of 3 points.
By modelling deformations of the 350mm mirror in PLOP, I get the following figures for the current 3 points:
Support radius = 123mm
P-V: 1.65 x 10-4mm (~1/5λ at 500nm)
RMS: 3.81 x 10-5mm (~1/13λ)
For a mirror with a zygo report of 1/12λ P-V, this is less than impressive!
By using equally spaced 6 point supports, these figures are:
Support radius = 110mm
P-V: 1.07 x 10-5mm (~1/46λ)
RMS: 2.37 x 10-6mm (~1/210λ!)
So it can be shown that the deformations are potentially over an order of magnitude better using 6 points. Interestingly, allowing PLOP to run for a 9-point cell for the same mirror gives a slightly worse result than the 6 point, and though the 9 point is much better than a 3 point it’s harder to adapt this cell into.
![Orion Optics Mirror Cell with 6 point mod](https://www.chromosphere.co.uk/wp-content/blogs.dir/1/files/2015/01/mirror-cell-150x150.jpg)
My modification adds three “U” shaped supports that attach to the existing cell parts (being attached loosely via an M8 bolt to allow them to rock a little). There are two nylon mushroom head bolts that screw into these parts which the mirror will sit upon (these are the black cross heads in the image).
I think I’ll most likely need some mechanism to stop the bars rotating (but this might just be a bit of tape), and I’ll need new lateral nylon grub screws as 2 of the three current ones are well and truly stuck!
Orion Optics SPX – 6 Point Mirror Cell Mod by Graeme Coates is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.