Author Topic: Mirror cell design question.  (Read 1083 times)

Prana

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Mirror cell design question.
« on: December 03, 2016, 10:01:06 PM »
I built a six point cell for a six inch primary, seen here.  I've been thinking of eliminating the three supports with six pads, and instead having six springs, sketched in red showing a new simpler single upper section.  Does anyone see a problem with six springs instead of the three supports and pads?  Essentially, there would be six individual spring loaded pads supporting the mirror.
« Last Edit: December 03, 2016, 10:10:40 PM by Mark »

mccarthymark

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Re: Mirror cell design question.
« Reply #1 on: December 04, 2016, 08:56:35 AM »
My logic as a non-engineer and a novice ATM:

Usually the mirror rests, either on the floating support points or silicone.  Its weight keeps it in even contact with the pads / supports during scope movement, holding collimation.  If the supports are springs, each of them presses up on the mirror with its own force, and the mirror would bounce around during scope movement -- probably would not hold collimation.  The springs might also somehow cause astigmatism -- putting uneven forces on the bottom of the mirror, or pressing the mirror against the retaining clips.

Your original cell looks like a great (tried and true) solution. 

--Mark
Mark

John Pierce

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Re: Mirror cell design question.
« Reply #2 on: December 04, 2016, 09:39:21 AM »
I wouldn't think a 6" mirror would need a 6-point suspension unless it was really thin... my 10" uses 3 points (really, 3 quarter-sized blobs of silicone about 1/8" thick) and is just fine.   the "plop" program calculates optimal suspension points for various configurations.

Don Pensack

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Re: Mirror cell design question.
« Reply #3 on: November 06, 2017, 02:55:17 PM »
PLOP optimizes a 3-point cell for contacts at 40% of the radius from center, so the 3 points would have to be fairly close in (1.2" from center of mirror).
The 6-point design is a bit more forgiving.  For a 6" mirror (likely 1" thick or less), you could also simply cement it to a triangular plate.
But under no circumstances would you want a spring to press directly on the mirror or even on a pad the mirror sits on.
And keep the 3 collimation springs--don't use 6.

If you experiment with PLOP, you'll find there is a point where increasing the number of points of support makes almost no difference.
With my thin 12.5" mirror, that point was with 18 points of support (which is what the mirror has).  If you put in the details and get a significant reduction in error
with a change from 3 to 6 points, use 6.  Obviously, you designed just such a cell.  My only comment there would be that you need to significantly reduce the size of the contact patches unless the cell is used on an equatorial mount.  If used on a dob/alt-az scope, the contact points could be reduced to maybe 1/4" diameter.