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alacant

loose newtonian mirror cell

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Hi everyone. I had a bit of side to side movement on my 208mm mirror. Whilst I can still rotate the mirror quite easily in the cell, the side to side has been taken up by inserting a piece of card on two of the three clips bearing against the edge of the mirror. Adding the same to the third clip locks the mirror, something I believe will not allow for the necessary expansion. 

Do we have any guidelines on holding the mirror? It's the common same-as-every-other-mirror-cell-you've-ever-seen Chinese mass produced type.

TIA and clear skies.

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Tighten the clips sufficiently to hold the mirror in place. Don't worry about pinched optics, thermal expansion etc, a slipping mirror will do far more to harm views. Don't test by rotating the mirror beneath the clips as you may scratch the mirror edge. To check that your mirror is sufficiently well held, tighten down the clips until they seem to be holding the mirror without unduly pressing, collimate the scope, and check that it maintains collimation at all angles of altitde. An overly loose mirror will go out of collimation when the scope is tilted, a sufficiently rigid mirror is one that maintains collimation. (It is of course possible for a tilted scope to miscollimate because of larger mechanical issues such as inadequate truss tubes, but let's leave that aside for now...)

Edited by acey
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A guideline that I read about some time ago was that there should be room to slide a thin piece of card (eg: a business card) between the mirror clips and the mirror surface. I don't think they are actually suggesting that you start showing business cards in there but it illustrates the tension (or lack of it) that you should be aiming for.

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1 minute ago, John said:

slide a thin piece of card (eg: a business card) between the mirror clips and the mirror surface

Hi. Yeah, I saw something similar. The thing is that if the clips are not in contact with the mirror, it slides. Hence my putting card -of a similar type to buisiness cards- between the side of the clips and the edge of the mirror; the card sits parallel to the optical axis at the edge of the mirror.

14 minutes ago, acey said:

pinched optics,

Unfortunately, tightening the clips sufficiently to prevent movement pinches and I get triangular stars:(

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12 minutes ago, alacant said:

 

Unfortunately, tightening the clips sufficiently to prevent movement pinches and I get triangular stars:(

Yes, it will. Your mirror needs to be able to move freely without being "held" in it's cell, at the same time you don't want it to move....

The three mirror clips serve a primary purpose of preventing the mirror falling out of the cell, not holding it in, if that makes sense? The weight of the mirror bearing on any two of its edge supports should be enough to hold it steady. If you need to reduce play within those edge supports, try using thin layers of a compressible material, like cork, to take up the slack, but at no time to you want to wedge the mirror in.

On my dobs and newts I like to be able to be able to freely rotate the mirror within the edge supports (clips removed of course), but not to have any side to side play or movement. This way even fast scopes will maintain good collimation through a session whilst allowing the mirror glass to do its thing without being restrained against it's will! If it is too tight you stand the risk of introducing astigmatism or other aberrations.

In an ideal world the edges of the mirror would be held by properly spaced rollers on a whiffle tree.

HTH

 

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21 minutes ago, Tim said:

thin layers of a compressible material

Yeah, the 'compressible' is a good idea, although the side support is lined with what looks like velvet so offering something of a buffer. My 2 card shims on 2 of the 3 clips seem to fit your guidelines; the third retains the velvet and the mirror remains mirror without it moving laterally.

I'm secretly hoping that this will be the final tweak. I've added extra springs -instead of locking screws- and a longer, wider dovetail. The collimation is almost constant and now evident only after a knock or e.g. a meridian flip to a low dec.  

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