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Running Gui_Plop software for a 250mm x 40mm mirror with 3, 6, 9 and 18 point support produced the following: The colours in the graphics show deviation from perfect support. [Green]
Blue shows raised areas close to the support points.
While red shows the mirror sagging away from the supports.
The columns of figures alongside show the actual deviation from perfect support in figures These figures are important because they quantify the changes seen in their respective colour images.
Click on any image for larger graphics and text.
3 point RMS = 2.442^ -06
3 point P-V = 1.088^ -05
6 point RMS = 6.323^ -07
6 point P-V = 3.564^ -06
9 point RMS = 6.475 ^ -079 point P_V = 3.213 ^ -06
18 point RMS = 2.019^ -07
18 point P-V = 1.23 ^ -06
Note that 18 point support suggests a ~10x improvement in both RMS and Peak-to-Valley support over a simple 3 point support cell. 6 point support is 3 times better than 3 point but 9 point is very similar to 6 point. The central, red-coloured area is sagging below the desired level but is masked by the secondary. So 6 point support offers the best improvement with least complexity.
Once this information has been absorbed the decision can be made whether [or not] to invest in the far greater complexity of an 18 point support cell. The greater the accuracy of the mirror the greater potential to be gained from properly supporting the primary mirror. The thicker the mirror, the less it will distort.
I Copied the Plop graphic plots into PhotoFiltre to enlarge and move the tables nearer the plotted colour graphics to make them more legible.
Here is an 18 point cell from JMI. Chosen for the clarity of the website image. 6 triangles are balanced on three see-saw bars.[Whiffletrees] So that all support points are pivoted to share the applied loads evenly. The mirror blank usually rests on slippery plastic "buttons" to avoid stiction. Such complexity is usually reserved for larger and thinner mirrors.Click on any image for an enlargement.
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