Fixing lenses and mirrors into mounts.

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Fixing lenses and mirrors into mounts.

This topic has 20 replies, 9 voices, and was last updated 13 May 2020 at 18:28 by Mark Rand.

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12 May 2020 at 21:10 #471233
Mark Rand
Participant
@markrand96270
I have a need to mount two or three 50mm 1/10 wave, first surface mirrors into bored holes in aluminium mounts. I can think of a number of possible fixing methods:-

Nylon screws pressing on the perfiphery.
Shellac, either melted or alcohol born.
Beeswax.
RTV silicone rubber.

Do the telescope/microscope/camera… making community have any suggestions for the best low/zero strain option, including all the ones I've missed?

Regards

Mark.
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12 May 2020 at 21:10 #27356
Mark Rand
Participant
@markrand96270
How do I stick mirrors into holes?
12 May 2020 at 21:58 #471250
Michael Gilligan
Participant
@michaelgilligan61133
It may depend upon how stable you need the mirrors to be, Mark … and the relative weights of the components; and crucially, whether you need the mirrors to be removable from their cells.

Is whatever you are building subject to vibration … and do you need to isolate that ?

One that you didn’t mention is ‘double-sided-sticky’ foam pads.

MichaelG.
12 May 2020 at 22:34 #471262
Mark Rand
Participant
@markrand96270
Desired short term (2 hours) stability better than 25nm or .1 Seconds relative to the mounting at constant temperature…

Relative weights approx 750g for mount of a 50g mirror. 16mm thick alluminium and 10mm thick glass.

It would be nice to be able to remove the mirrors without damaging them (eg if they needed re-aluminizing).

Vibration only from careful handling.

Edited By Mark Rand on 12/05/2020 22:38:47
12 May 2020 at 22:47 #471267
Michael Gilligan
Participant
@michaelgilligan61133
Thanks for the additional info, Mark

Just realised though … I didn’t ask one important question:

Are those holes ‘through’ or ‘blind’ ?

… I will ‘sleep on it’

MichaelG.
12 May 2020 at 23:55 #471281
Michael Gilligan
Participant
@michaelgilligan61133
On second thoughts, Mark … I think I have the answer anyway

For the stability that you want, I would fight shy of any ‘soft’ mounting method

I believe that the arrangement used in these Ealing mounts is probably ideal : **LINK**

https://www.ealingcatalog.com/precisetm-mirror-mount-2-inch.html

Have a look at the PDF drawing, and you will see that the mirror is supported on a pair of parallel rods [acting as a vee-block] and restrained by a single screw at the top.

Frankly, I can’t think of a better method … But if it doesn’t appeal, then I will try !

MichaelG.

Edited By Michael Gilligan on 12/05/2020 23:56:04
13 May 2020 at 06:55 #471300
JasonB
Moderator
@jasonb
Mirror Adhesive? not far off your RTV option and in both cases would need to be able to "set" the mirror while it goes off. I use it on far bigger mirrors but can't see why it would not work and it is formulated not to attack any silvering.
13 May 2020 at 07:06 #471303
Clive Hartland
Participant
@clivehartland94829
Forget the beeswax, and the screws impinging on the mirror as they would set up pressure waves. Shellak is softened with methylated spirits, not alcohol. 3 dabs of RTV would do it and the easiest but not hard setting.
13 May 2020 at 07:17 #471304
Michael Gilligan
Participant
@michaelgilligan61133
Posted by JasonB on 13/05/2020 06:55:39:

Mirror Adhesive? not far off your RTV option and in both cases would need to be able to "set" the mirror while it goes off. I use it on far bigger mirrors but can't see why it would not work and it is formulated not to attack any silvering.

.

Perfectly reasonable, Jason … but I would just mention a couple of special features of Mark’s job

1. they are ‘first surface’ mirrors ; so the adhesive need not come near the silvering

2. they are relatively heavy, and the positional stability does need to be rather good

I suspect that any such ‘adhesive’ would remain too fluid to achieve [2]

MichaelG.
13 May 2020 at 07:55 #471308
JasonB
Moderator
@jasonb
But is it any different to RTV rubber that Clive suggests and from past posts he has quite a bit to do with optics.
13 May 2020 at 08:16 #471312
Michael Gilligan
Participant
@michaelgilligan61133
Posted by JasonB on 13/05/2020 07:55:55:

But is it any different to RTV rubber that Clive suggests and from past posts he has quite a bit to do with optics.

.

Probably not

But [with the greatest respect to Clive], I think the pseudo-kinematic mounting is much more likely to meet Mark’s requirement.

MichaelG.
13 May 2020 at 10:10 #471331
Nigel McBurney 1
Participant
@nigelmcburney1
i looked at the Ealing link and thought ,thats similar equipment to what I made while working for my first employer,a good part of production at the time was cast iron optical benches and the variety of mounts for various optics, the lens holder could take various diameters having 3 adjustable arms with what could be described as circular vee blocks at the business end which held the lens,the arms were lightly sprung loaded. The Ealing holder relies on two rods similar in effect to a vee block,the third location is some type of screw,I think that with just one size of mirror two balls would give better kinomatic mount,and the third mount could be a ball ended plunger lightly sprung to give a constant pressure ,the plunger being made from nickel silver, a nylon screw would be ok if the Ealing holder was just used for experimentation and the mirrors /lenses often changed,a nylon screw would possibly relax after a time .A lot of the products relied on kinmatic 3 point mounts ,one job I made was an instrument for measuring the radius of lenses, the leses to be measured were suspended on 3 balls precisely spaced and mounted.
13 May 2020 at 10:57 #471341
Michael Gilligan
Participant
@michaelgilligan61133
Posted by Nigel McBurney 1 on 13/05/2020 10:10:00:

[…]

The Ealing holder relies on two rods similar in effect to a vee block,the third location is some type of screw

[…]

.

Just for info, Nigel … I am almost certain that the Pylon [sic] tipped screw on the Ealing page is just a typo.

The smaller version of the mount mentions a Nylon tipped screw.

Admittedly not fully kinematic, but [to me] this looks like a very practical solution to Mark’s puzzle.

MichaelG.
13 May 2020 at 11:45 #471353
Howard Lewis
Participant
@howardlewis46836
My first thought, if it was practicable, was to turn a shallow groove in the ID of the mount, and to retain the mirror with a snap ring.

Elements in Camera lenses, and filters, are retained by rings screwed into the mount.

No chemicals, which might attack the mirror, and hopefully, zero stress in the mounting likely to cause distortions.

Just a thought.

Howard
13 May 2020 at 12:47 #471370
Robert Atkinson 2
Participant
@robertatkinson2
Mirrors and other optics on scientific equipment I've worked on were almost always secured to aluminium alloy mounts in a recess and 3 or 4 small spots of adhesive. Typically this was a filled opaque epoxy probably with thermal expansion between glass and alloy. Some use toughed acrylic like Loctite 330 (sometimes repackaged as optical adhesive with much inflated price tag)

There is excellent guidance here

https://wp.optics.arizona.edu/optomech/wp-content/uploads/sites/53/2016/08/24-Mounting-optical-components-1.pdf

Robert G8RPI.
13 May 2020 at 13:51 #471396
Clive Hartland
Participant
@clivehartland94829
Epxy adhesives can set up severe stress zones around optics, I have seen prisms split due to this factor.

Seating the mirroe in a tube with another thin wall tube to keep in place is a standard method. this thin wall tube fixed with a dab of shellak or held in place by a threaded ring, again locked with shellak.
13 May 2020 at 15:27 #471430
Dave S
Participant
@daves59043
The ones on the Laser Cutter are mounted from the back, so the first surface outside rests on a step in a bore, then 3 arms, slightly springy, press it forward. You'll potentially knacker the outer edge of the mirror for mirror purposes.

Finally using an Autocollimator ?

Dave
13 May 2020 at 15:48 #471435
Michael Gilligan
Participant
@michaelgilligan61133
Posted by Dave S on 13/05/2020 15:27:54:

The ones on the Laser Cutter are mounted from the back, so the first surface outside rests on a step in a bore, then 3 arms, slightly springy, press it forward. You'll potentially knacker the outer edge of the mirror for mirror purposes.

Finally using an Autocollimator ?

Dave

.

Do you have insider knowledge, Dave ?

… or did I miss something in Mark’s posts ?

MichaelG.
13 May 2020 at 16:07 #471440
Dave S
Participant
@daves59043
A long time ago Mark was rebuilding something – maybe the grinder – to 'good enough for now'. Probably to do something else like the ML7 bed. There was IIRC talk of doing something like a surface plate calibration with an autocollimator to finally do a proper job on the grinder.

Something like that anyway.

So adding 2+2 and getting 5 – first surface mirrors, especially very flat ones are used for that sort of thing

I'm fairly new here, but used to hang out on the Uk.Rec.Models.Engineering newsgroup, amongst other places.

Dave
13 May 2020 at 18:14 #471477
Neil Wyatt
Moderator
@neilwyatt
Aquarium silicone which is free from nasties is popular for telescope mirrors but I have used EvoStik mirror adhesive, that includes a 10" diameter by over an inch thick mirror, and secondaries comparable to your flats.

I would do something to ensure a good mechanical bond to the aluminium, such as multiple holes.

Neil
13 May 2020 at 18:28 #471481
Mark Rand
Participant
@markrand96270
Have a house point that boy!

These are mounts for mirrors to use with the autocollimator.

The 1976 surplus-from-work surface grinder was used to rough grind the bed plate of the 1952 Hardinge HLV when I was rebuilding it. That needed spot-grinding to get it flat enough (too hard to scrape). Since then I've rebuilt/re-scraped most of the Beaver milling machine (still got some more work to do on the head).

In the process of having a tidy-up in the shed, I realized that the collection of scrap hard disks from work would take far less space in the form of ingots.

Once I'd done that, I made some castings for mirror mounts.

Now I'm working on joining the mirrors to the mounts…

I think I'm heading towards the "sort-of kinematic design", but that's not fixed in stone yet.
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