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eliptical flat mounting with silicone


jonstarrysky
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Have ordered a premium 1/10 wave secondary and want to make sure the method of mounting is optimal. For 63mm minor axis it is a rather thin 10mm plate glass, so may distort slightly under its own weight. My holder is the basic stalk/ post design that holds the mirror beneath it with silicone, rather than holding the mirror at the edge within a casing.

Dogma is use 3 small blobs of silicone, and use 1-2mm removable spacers whilst the silicone is setting. Easy.

What is less clear is *where* to put the blobs on the back of the mirror. For a 3 point *primary* mirror cell 70.7% radius gives the most equal distribution of mass across 3 support points. PLOP will say 40% of the radius is better - the deformation will be greater than 70.7% but is mostly compensated at the focusser.

But we dont have a PLOP model for secondary mirrors -which are obvioulsy eliptical and orientated at 45 degrees.

Can anyone advise on where to best place the 3 blobs of silicone to minimise the effect of the mirror deforming under its own weight ?

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I have just done this under the supervision of a talented professional optician. He stressed not three blobs but a ring of silicone much smaller than the outer rim of the mirror. This stops expansion of the post distorting the mirror. Previously it was held by three blobs at the edge of the flat and introduced astigmatism due to thermal distortion from the post. The optical improvement is uncanny, though he did recommend some other mods as well. If you only attach via a smaller ring most of the post expansion is not going to affect the mirror. To avoid pressure build up we pierced the ring once it had set using a needle.

Olly

Edited by ollypenrice
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Oh I might add when you put your ring of silicone on place 3 plastic tooth picks under the mirror if possible in a equilateral triangle let the silicone set up for about 15 to 20 min then remove the tooth picks this will aid in weight dispersion

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I have just done this under the supervision of a talented professional optician. He stressed not three blobs but a ring of silicone much smaller than the outer rim of the mirror. This stops expansion of the post distorting the mirror. Previously it was held by three blobs at the edge of the flat and introduced astigmatism due to thermal distortion from the post. The optical improvement is uncanny, though he did recommend some other mods as well. If you only attach via a smaller ring most of the post expansion is not going to affect the mirror. To avoid pressure build up we pierced the ring once it had set using a needle.

Olly

Olly, this is very interesting. If you have more details and time, can you add this or pm me. My plans considered the expansion issue, so this is very interesting. I was theorising - you are using actual observations. It is really an issue then.

The post will be aluminium - at least 2x the thermal expansion coefficient of plate glass and about 4x that of pyrex. I was planning on customising the post with a lower titanium section, which is closely matched to glass re. expansion coefficient. Or Kovax - an alloy that essentially has no thermal expansion. I am suprised at what you say as the silicone when set should have enough flexibility to accommodate the expansion issue....

I had never heard of the ring of silicone idea. Only 3 points, usually nearer the edge than the centre, to best distrubute the mass across the mirror between each support point. And 3 points will always lie in a plane. A circle - in theory if a section of this contracts differently to the resr this will impart a stress on the mirror. 3 points is always a plane.

I can however see some of the logic of a small ring. The expansion differential will be smaller and wont concentrate the stress at defined foci....

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I have just done this under the supervision of a talented professional optician. He stressed not three blobs but a ring of silicone much smaller than the outer rim of the mirror. This stops expansion of the post distorting the mirror. Previously it was held by three blobs at the edge of the flat and introduced astigmatism due to thermal distortion from the post. The optical improvement is uncanny, though he did recommend some other mods as well. If you only attach via a smaller ring most of the post expansion is not going to affect the mirror. To avoid pressure build up we pierced the ring once it had set using a needle.

Olly

this is great info. will be using this soon hopefully.

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Hi,

My mentor says this; he once tried the three blobs method on an catadioptric he made and found that once installed there was some pinching of the secondary. When he switched to a ring at the same diameter as the three blobs the pinching disappeared.

He says that the silicone ring thickness should be in a proportion of about 1/6 to the thiickness of the glass of the secondary unless the secondary is exceptionally thick.

In my case the post was of PVC which, to my surprise, has a higher index of expansion than aluminium, it seems.

I can see that three blobs will always be in a plane but all I can say is that I have had a great result from the ring and that when my man talks of pinched optics he is talking about testing with the highest standard of professional kit.

I am only the mouthpiece in all this, you understand!

Olly

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Hi,

My mentor says this; he once tried the three blobs method on an catadioptric he made and found that once installed there was some pinching of the secondary. When he switched to a ring at the same diameter as the three blobs the pinching disappeared.

Very interesting.

At a guess, I would think that the ring would introduce a more 'even' distortion of the secondary, which maybe changes the overall radius of curvature of the mirror. That's equilivent to changing the focus position, but that's obviously not an issue as we have focusers. The three points give a higher frequency distortion (astigmatism), which affects the image quality more. :D

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Very interesting.

At a guess, I would think that the ring would introduce a more 'even' distortion of the secondary, which maybe changes the overall radius of curvature of the mirror. That's equilivent to changing the focus position, but that's obviously not an issue as we have focusers. The three points give a higher frequency distortion (astigmatism), which affects the image quality more. :D

I agree TeaDwarf (btw I am in oxon too). This is why PLOP predicts a 40% radius support for a 3 point primary mirror cell, despite the fact that 70.7% radius gives the best overall distribution of mass. Howver the 40% radius support gives a type of deformation which can largely be compensated at the focusser. This is not the case for 70.7%. The silicone ring idea for secondary mounting may be similar conceptually - the deformation is compensated at the focusser. So what is the optimal radius for this ring ????? There is a question for the modellers. It will need to take thickness into consideration....

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  • 2 months later...

I'm about to start a big building my Newtonian, the silicone ring sounds very interesting. My secondary is 140mm ma, 15mm thick. Is the diameter of the ring critical ? The quoted 40% looks very small.

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