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Should imaging mirrors be held down the same way as Dob mirrors?

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#1 bokemon

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Posted 09 May 2021 - 06:35 AM

Hello folks, 

I spent some time a few months back reading about mirror support systems like 3 point triangle pivoting contacts with hard plastic pads, side supports half way up, two bottom supports at 45 degrees, etc.  But this was all for BIG mirrors in a Dob telescope where one side is always facing down, and you are expected to collimate the scope every time.  The mirror is allowed to slightly slide around and rotate, but keeping the original shape is of paramount importance when using high power eyepieces.

 

An imaging scope has different requirements where the mirror better not move as the scope slowly swings across the sky, and any side could be "down".  However, I assume that the shape-keeping requirements are less stringent since most coma correctors are going to make spot sizes of > 10 um anyway on a sensor with few micron sized pixels.

 

My first imaging newtonian was a little 6" mirror, and I modified the primary cell to have the three plastic contacts.  But the side supports were only the bare metal of the cell at two points near the bottom of the mirror, and a third point of rubbery foam I jammed in so the mirror won't rattle around.  Clips were rubber and barely touched the top surface of the mirror so that it would have no room to bounce up and down.  This mirror setup gave good images and survived multiple car trips with no need to recollimate, so overall I am pleased.

My next scope has a 10" mirror, but that was a conical mirror with only a connection at the back and no side supports or clips or anything.  Very easy.

 

But now the current mirror is a 12" in a GSO 9 point cell.  The way it came stock was that all the three contact points on one pivoting triangle had double sticky foam of 1mm, with the outer two points stuck down, and the inner point with the wax paper still on so it did not stick.  The side supports were the same three rubber clips that also lightly touch the top surface.  So the questions are:

1) for the bottom supports, the triangles have a raised cylinder of 1mm height and 12mm diameter at the corners.  I could either glue on nylon washers and sand them flat, or just sand them flat and put a piece of tape over them.  I mean... tape is slippery, right?

2) For the side supports, the default is the three rubber clips pushing laterally against the side of the mirror.  There's only about a 1 mm gap between the side of the mirror and the cell, so if I wanted "hard" supports, I can only glue / jam in a piece of plastic on two sides and rubbery foam on the third.

3) For the top support, I don't think I have much choice but to make the rubber clips barely touch.  I don't want the mirror flopping around when it's standing on edge, during transport or mount balancing.

gso_12_cell.jpg



#2 jgraham

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Posted 09 May 2021 - 06:55 AM

I like to stabilize my mirrors with strategically placed gobs of clear silicone adhesive. This holds the mirror securely with zero force and can be easily removed if needed. I had a 6" f/20 Cassegrain with Novak cells that was downright squirrelly until I secured the mirrors with a few spots of silicone. Now it's stable and a delight to use.

Food for thought.

#3 MitchAlsup

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Posted 09 May 2021 - 09:58 AM

When I built my 13" F/3 I "stabilized" my mirror's position by making the side/edge support adjustable. Here is an image of me adjusting the edge supports so the mirror could move only 0.004":

 

assembly01.JPG

 

And here is me adjusting the space between the bevel on the face of the mirror and the "can't fall out the front" clips to that same 0.004". The clips are not visible and the mirror will not fall out the front from any position.

 

assembly02.JPG

 

So, this visual only DOB satisfies your requirements.



#4 bokemon

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Posted 11 May 2021 - 03:35 PM

What I would like to know, is why you are ok with the full weight of the mirror x sine(angle) resting on just two hard side supports, yet you won't use a third soft or springy support so the mirror doesn't slide around at all.




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