79 replies to this topic

[ckh]

How about silicone gluing one or more small disks with eyelets to the back of the mirror. Use short pieces of wire to connect the eyelets in the disks to eyelets in the mirror support. The wires are slightly slack. 

 

Silicone glue may be good to 350 psi.

 

Carl

[Starman1]

On a heavy mirror I once owned, I glued a nylon strap to the top edge of the mirror which was attached to a buckle on the back of the mirror cell.

It was slightly slack in use, but prevented the mirror from falling out of the cell when the scope fell below the horizontal.

It might even be useful if the scope falls to a stop quickly, even if the stop prevents the scope from going below the horizontal.

[Oberon]

Yes. There must be a zillion ways to do this, its simple and reliable, yet I've never seen it done or recommended.

For me the only critical unknown is selection of a glue that doesn't cause outgassing problems in a vacuum chamber. Presumably that means a silicon that can be cut off, or a thin layer of epoxy, but I don't know.
 

Another idea is to glue some sort of safety bracket to the mirror. Normally the bracket is free and unconstrained, not applying any force to the mirror. But should the mirror lift off its support, then the bracket would be caught by a constraining mechanism; anything, a loose cable tie would do.

 

[ckh]

Silicone can be cut with a razor, but getting all of it off might take a little work. A product called ORANGE-SOL is claimed to make is peel off after soaking and is non-toxic.

[tim53]

Double stick foam tape comes off with lacquer thinner or acetone. 

[Oberon]

Considering how much those are overexposed, I would expect even the ones with duplicated angles to be nearly invisible at the eyepiece.  How bad, for example, are Merope's diffraction spikes from the crescent clips that appear disappointingly bad in Maskulator on Sirius or other objects?  Those clips aren't aligned with your spider, right?

Took a look at bright stars a Cen' last night in poor seeing but transparent conditions. The spider cast thin straight spikes as expected, but looking closely for any other signs of diffraction I realised that there was a sort of spiky glow close around the star itself crudely reminiscent of the patterns generated by Maskulator and probably caused by the clips. I may also have some intrusion by the focuser. I could not detect a distinct pattern, there was no "cross" shape apparent as we see in Maskulator, so the result is inconclusive until I can experiment with and without.





Note that Saturn is not bright enough to show diffraction spikes from my wire spider when Saturn in the field

[Dick Jacobson]

Silicone can be cut with a razor, but getting all of it off might take a little work. A product called ORANGE-SOL is claimed to make is peel off after soaking and is non-toxic.

Just be careful in what solvent you use to remove glue. I heard of someone who used WD-40 (if I remember correctly) to get a mirror out of a tight cell, and the entire aluminum coating slumped off!

[Oberon]

Many if not most professional mirrors have stainless steel fittings permanently epoxied to their edge, so it would seem epoxy is OK in a vacuum chamber. 

[ckh]

I was curious about using epoxy, so I looked it up and found a special epoxy for high vacuum.  I believe it is intended for bonding vacuum chamber parts and parts that operate in a vacuum.  However, so little is used to bond attachment points, that a regular high-quality epoxy may be OK. Worst case is that you take the residue off with abrasive.

[mark cowan]

Epoxy is OK, so is a thin layer of fully cured silicon adhesive. Trouble enters if you try using petroleum based solvents for removing silicon adhesive, as those things can stick around and do damage to coating systems, never mind interfering with coating itself due to partial vapor pressure.

[Jeff Morgan]

Many if not most professional mirrors have stainless steel fittings permanently epoxied to their edge, so it would seem epoxy is OK in a vacuum chamber. 

 

Silly question perhaps, but here goes:

 

Why don't mirror blanks have a shallow groove around the circumference halfway between the flat surfaces such that clips could be engaged at that point?

[Starman1]

Jeff,

For the same reason that we don't let mirror clips press against the front of the mirror--they would cause astigmatism.

[ed_turco]

It seems like most beginners worry too much about the clips impinging upon the mirror's surtface and the subsequent diffraction.  This effect is so small that the additional diffraction will be very hard to find in a visual telescope.

 

When I was building my Definitive Newtonian Reflector (see below) this diffraction was a different story as I was trying to eliminate any diffraction that could be gotten rid of. 

 

I came up with a solution that can be applied to any mirror clip.  I use double-faced foam tape to fasten the mirror to the mirror cell.  As for those vlips  that creep up on the sides of the mirror, I did this:  I filed off the top of the clip so that there was no impinging "bend" that would normally cause that small amount of extra diffraction.  I applied some of that same tape to the sides of the clip and then applied it to the mirror.  A little tightening of the screw that tightens the clip causes it to rest against the side of the mirror.

 

It is a firm hold but not one so tight that the mirror's figure would be effected.  And yet, I've held a mirror in its cell upside-down with no danger of anything shaking loose!  I think that's pretty good.

 

For any normal Newtonian, I would avoid doing anything except to use the clips to hold the mirror in place in a normal fashion.   The extra diffraction caused by these clips is so minimal that it is not worth the trouble, unless one wants to be a crank and build a Newtonian that will equal the performance of an apochromatic refractor of equal aperture, namely me.

 

My article cited below is now being refereed by an optical journal, so we'll see what's what.

[Oberon]

I'm not sure how a mirror can be fastened with double sided taped to the mirror cell without inducing astigmatism due to differential expansion; the mirror should be free to move. All we want to do is limit its movement so that it doesn't fall out.

 

btw I'm thinking in terms of Dob cells not Eq, and it seems you're doing the opposite. Your solution could not be applied to my cell for example, but then my cell wasn't designed for Eq. Although OTOH it could be modified easily enough and still remain an exception. 

 

However a solution that involved a constraint glued or even double sided taped to the mirror that only prevented the mirror from falling out would work for both Dobs and Equatorial scopes.

[Oregon-raybender]

Clips or rings normally are outside the CA of any optic, so it does not infer. There is two solutions, one mount the mirror with a ring or make a ring to fit over the clips (reduce the CA) Second, for high tech optics, we glue the mirror or lens using 3-4 holes on the side of the cell, using silicon, to remove we drill out the hole. The best book for solutions is Paul Yoder's  Mounting Optics in Optical Instruments, SPIE press.   It has all the math and ideas you need.

 

Starry Nights

 

https://www.amazon.c...L40_&dpSrc=srch

[Diego]

Clips or rings normally are outside the CA of any optic, so it does not infer. There is two solutions, one mount the mirror with a ring or make a ring to fit over the clips (reduce the CA)

Exactly, just make a ring/mask a few mm smaller than the mirror's diameter and eliminate the mirror clips. If well made and rigid enough, there is no way the mirror can fall through.

Unless someone is psychology affected by the fact that your mirror will be 3 to 6 mm smaller in aperture, then don't go with this approach.

[Starman1]

Clips or rings normally are outside the CA of any optic, so it does not infer. There is two solutions, one mount the mirror with a ring or make a ring to fit over the clips (reduce the CA) Second, for high tech optics, we glue the mirror or lens using 3-4 holes on the side of the cell, using silicon, to remove we drill out the hole. The best book for solutions is Paul Yoder's  Mounting Optics in Optical Instruments, SPIE press.   It has all the math and ideas you need.

 

Starry Nights

 

https://www.amazon.c...L40_&dpSrc=srch

This indicates that gluing the mirror to its support introduces more RMS error into the wavefront than other means of support:

http://www.cruxis.co...ecalculator.htm

Gluing the side of the mirror is even worse for error in the wavefront.  It isn't even mentioned for that reason.

 

However, the thicker the mirror, the more tolerant of a variety of support compromises.

A 6:1 mirror can be mounted many ways a 10:1 or 16:1 mirror cannot.

Edited by Starman1, 08 November 2018 - 02:50 PM.

[Oregon-raybender]

True, I was using 6:1, however, now folks want thin lite weight for various reasons, cooling, etc... The idea is that silicone is very forgiving, if the thickness is enough not to cause flex transferred from the support to the glass. I would not suggest it to be so thin that the silicone or any glue or tape transfers the mount flex to the glass. I think it's great that we have these wonderful programs to assist us in making decisions for all aspects for optics in this case thin ATM mirrors. I offer a word of caution, just because we can calculate to the highest decimal does not mean we expect it. I had a number of engineers and scientists try to force the performance of the optical design into manufacturing, "Why can't mech engineers, opticians and glass manufacturers make my optics to 7th or 10th decimals in mm and support it perfectly!  These should be used as guide and a check as Paul has suggested, many other factors to deal with,  How far do we take it to? What is it doable?

The sling works fine.

 

Starry Nights

Edited by Oregon-raybender, 08 November 2018 - 08:00 PM.

[Jeff B1]

Oberon, very interesting study and presentation.  Bob Cox, long gone curmudgeon, published several articles in S&T and other magazines about this very subject, so the story goes way back in time.  While most diffraction “spikes” never bothered me; however,  they can be irritating for some people and also imaging.  I have never been inflicted with the perfectionist synonym that my father had, but it drove me to drink never the less.   My primaries usually have very little of the clips sticking out onto the mirror surface, so diffraction is minimal.  What little I see is just mixes in with dust and other debris on the optics, so I drink less than perfectionists.  

 

Unfortunately there are people who suffer benign mental disorders and they can be fixated on fine points of any endeavor; such as telescope making.  While it is interesting, and productive, some use it to embellish their place in life.  After observing planets, and some DSOs, in all types of instruments for many years some of us have seen it all and take to good with the bad and use stuff efficiently so not to ruin our observing.  The hobby can be fun too.  

Edited by Jeff B1, 09 November 2018 - 08:14 AM.

[Jon Isaacs]

After observing planets, and some DSOs, in all types of instruments for many years some of us have seen it all and take to good with the bad and use stuff efficiently so not to ruin our observing.  The hobby can be fun too.

 

 

My attitude:

 

I am an observer.  Rarely have used a telescope so bad that if I were a better observer,  I would not see more .

 

My attitude comes from my years as a cyclist .Some people look to the bicycle in order to go faster but it's the rider that makes the difference. 

 

I have a lot of fun. 

 

Jon

[Jeff B1]

Man, Jon – I envy you with the bike; I just can’t get on one now days.  It really irritates me after a lifetime of athletics and physical activity to be so couch potato-ie.  

[MeridianStarGazer]

Exactly, just make a ring/mask a few mm smaller than the mirror's diameter and eliminate the mirror clips. If well made and rigid enough, there is no way the mirror can fall through.

Unless someone is psychology affected by the fact that your mirror will be 3 to 6 mm smaller in aperture, then don't go with this approach.

How does this affect air flow?

[Jon Isaacs]

Exactly, just make a ring/mask a few mm smaller than the mirror's diameter and eliminate the mirror clips. If well made and rigid enough, there is no way the mirror can fall through.

Unless someone is psychology affected by the fact that your mirror will be 3 to 6 mm smaller in aperture, then don't go with this approach.

 

So.. this boils down to a psychological question ..

 

Which bothers you more.. the potential for diffraction spikes from the clips or the loss of aperture from the ring?

 

Myself, the clips are the easiest and most robust.  I go with that. 

 

Jon

[Diego]

So.. this boils down to a psychological question ..

 

Which bothers you more.. the potential for diffraction spikes from the clips or the loss of aperture from the ring?

 

Myself, the clips are the easiest and most robust.  I go with that. 

 

Jon

I'm not bothered by either. Clips are obviously much easier to do and agree more robust. Unless you can improve a tde with the ring, clips are fine. 

[MeridianStarGazer]

Some people grind a channel around their mirror so a sling can go in there. Why not make it deep enough some short clips can reach in too? Then you have all the safety and nothing in front of the mirror.