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8 point mirror cell

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#1 SteveV

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Posted 15 July 2020 - 09:04 PM

Hi:  Been quite a while so kind of hunting for a reality check.  Have a couple of 8" mirrors to finish up from light years ago.  Then it's onto a 14.5" x 1" plate glass porthole that's rough ground and wanting to be f/4.  

 

Pondering that 14.5 f/4 at the moment.

 

So running some PLOP cells 6 point auto-run yields a cell with RMS 1/117 wave and PV of 1/26 wave. 

 

8 points 1/152 RMS and 1/32 PV.    

 

12 points (Nils and Cowan) yield 1/344 and 1/63 RMS and PV respectively for this mirror!  

 

Here's a shot of the 8 and 12.  Any reason not to make the 8 point?  Seems to comply with Lewis' suggested minimums or should I go bigger?

 

Here's the file if anyone wants to run it.

 

Thank you!

 

  ;8 pt cell

diameter 368
thickness 25
density 2.45e-06
modulus 6000
poisson 0.22
f-ratio 4
n-mesh-rings 20
n-mesh-depth 0
rel-support-radii 0.606254
num-support 8
support-angle 202.5
basis-ring-size 4
basis-ring-min 0
obstruction-diam 79
optimize rel-support-radii 0 0.01
part bar 4 point 0 0 point 0 1
part bar 1 part 0 2 part 0 3

Attached Thumbnails

  • 14.5 8pt.jpg
  • nils 12pt.jpg

Edited by SteveV, 16 July 2020 - 12:43 AM.


#2 mark cowan

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Posted 15 July 2020 - 09:55 PM

Looks like it's right in the linear progression - a 6 point on steroids.  Why not, it provides a great opening when somebody asks "How many points is that cell..." :waytogo:


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#3 BGRE

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Posted 16 July 2020 - 12:01 AM

????? Surely PV is always larger than RMS???????
IS the surface deformation due to the support actually measured in waves?
If so its way too large for useful imaging.

#4 dave brock

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Posted 16 July 2020 - 12:18 AM

I think he's just not bothered putting the 1/ in front of all his numbers.


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#5 SteveV

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Posted 16 July 2020 - 12:55 AM

Sorry edited to make them fractions.

 

Looks like it's right in the linear progression - a 6 point on steroids.  Why not, it provides a great opening when somebody asks "How many points is that cell..." waytogo.gif

Thank you for the reply and reality check.  Can't really think of any issues other than no need to build that beautiful 12 point.  Yet anyway...  


Edited by SteveV, 16 July 2020 - 01:00 AM.


#6 MitchAlsup

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Posted 16 July 2020 - 08:20 AM

It seems to me that the mirror is adequately supported with a 6-point cell. Barely, but adequately.

 

On the other hand, it is YOUR telescope, so do what you please.


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#7 mark cowan

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Posted 16 July 2020 - 01:39 PM

To be fair, the 12 point design lends itself really well to support without a central obstruction (i..e, an offset axis design)...


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