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Secondary mirror attachment methods

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#26 Pinbout

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Posted 13 April 2020 - 12:22 PM

Here’s a cellular 2ndry - check out where your suppose to glue in threaded inserts

 

remember it’s a tapered 2ndry, 

 

I like to use this as a template for “concept” of support location

 

69E6D668-3A66-4566-8150-D523A1B47A01.jpeg



#27 kb58

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Posted 13 April 2020 - 12:26 PM

...There's little point in bonding a TCE matched button to the back of a secondary and then directly bolting it to a stiff plate or similar with a mismatched TCE...

That makes sense, or at least placing the junction further away. To minimize it, I fabricated most of the holder from the carbon sandwich material, and will screw it to a bit of Baltic birch which will in-turn be attached to the vanes. The mirror CG is on the centerline of the wood.

 

(The wood is just for simplicity and gives me wriggle room if I screw up alignment a bit.)

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  • focuser_plate4.jpg

Edited by kb58, 13 April 2020 - 12:44 PM.


#28 Ed Jones

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Posted 13 April 2020 - 12:32 PM

 

Ed, what is "thick"?

I always put a very small square of .020 inch thick tape down first as a spacer and then enough silicone for a nickel size spot.  I want to be sure it's thick enough for cte differences but mainly to be removeable with dental floss or a wire.


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#29 Dale Eason

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Posted 13 April 2020 - 02:18 PM

Toothpicks for spacer 

 

cut template for spacing

.

That is exactly what I did for mine except in place of the wooden holder I made one out of 1/8 inch wall Aluminum tubing with a wooden disk inserted to hold the threaded rod.  My mirror extends about the same distance past the support as well.  


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#30 tommm

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Posted 13 April 2020 - 10:52 PM

I always put a very small square of .020 inch thick tape down first as a spacer and then enough silicone for a nickel size spot.  I want to be sure it's thick enough for cte differences but mainly to be removeable with dental floss or a wire.

How did you determine that the 0.02" bond is thick enough for the CTE difference? FEA? Your own calculations of shear stress? Previous testing of mirrors bonded this way over a range of temperature?



#31 Ed Jones

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Posted 13 April 2020 - 11:09 PM

 

How did you determine that the 0.02" bond is thick enough for the CTE difference?

No calculations just no problems noticed in use.  Mainly to insure it's thick enough and blobs small enough to remove easily but never fail.  



#32 kb58

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Posted 14 April 2020 - 08:02 AM

But has it been established that the thin layer doesn't result in the mirror warping with temperature? I thought the whole point of a thicker layer was to allow for movement.


Edited by kb58, 14 April 2020 - 08:02 AM.


#33 Pinbout

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Posted 14 April 2020 - 11:55 AM

But has it been established that the thin layer doesn't result in the mirror warping with temperature? I thought the whole point of a thicker layer was to allow for movement.

It is - use a toothpick so there a bead that can “roll”



#34 kb58

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Posted 14 April 2020 - 12:28 PM

Sorry, I was responding to Ed's post about using 0.02", which seemed thinner than our precise standard reference toothpick. Kidding assign, there are flat toothpicks and round ones, with round ones being probably 3x thicker, so I guess that puts Ed in the flat toothpick camp.


Edited by kb58, 14 April 2020 - 12:31 PM.


#35 Pinbout

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Posted 14 April 2020 - 07:16 PM

Sorry, I was responding to Ed's post about using 0.02", which seemed thinner than our precise standard reference toothpick. Kidding assign, there are flat toothpicks and round ones, with round ones being probably 3x thicker, so I guess that puts Ed in the flat toothpick camp.

its a atm’ers world. But his thin layer isn’t standard issue 



#36 MitchAlsup

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Posted 14 April 2020 - 07:39 PM

But has it been established that the thin layer doesn't result in the mirror warping with temperature? I thought the whole point of a thicker layer was to allow for movement.

Modest sized secondaries expand and contract in the "couple" of thousandths of an inch department over human usable telescope conditions (less than 100ºF and greater than 30ºF).

 

A 0.020" or even 1/6" (0.0625") is large compared to 0.002", thus the (silicone) pad will shear and apply moderate forces on the back of the substrate. When the substrate is not thick enough or when the shear forces are great enough the secondary will impart astigmatic "correction" to the reflected light cones. 

 

Thicker pads also allow for the secondary to "sag" at different telescope orientations due to the lack of stiffness in the pad itself. Thinner pads will have less sag while potentially making the secondary take on astigmatism at some temperatures. Using silicone pads is about balancing the whole kit and caboodle.

 

Mounting of optics has everything to do with being sufficiently stiff in the directions one needs stiffness, and being absolutely free in all other directions. The pad should err more towards the stiff side than towards the free side. The structure the pad is mounted to needs some think of considerations to the thermals.


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#37 dave brock

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Posted 14 April 2020 - 07:46 PM

 

A 0.020" or even 1/6" (0.0625") is large compared to 0.002"

Typo....1/16" not 1/6"



#38 kb58

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Posted 14 April 2020 - 08:02 PM

Also need to fix the "0.002", as Ed wrote that he's using 0.020", or "20 thou." laugh.gif


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#39 tommm

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Posted 15 April 2020 - 10:59 AM

Some time ago I measured shear stress/strain using two 15/16"x15/16" ceramic tiles glued together (glazed sides) with GE SCS1000 silicone adhesive (acidic cure). One tile was epoxied to a piece of wood, the other bonded to it with GE 1000. The silicone was left in the house to cure for about 6 days before testing. The wood was held in a vise so that weight could be hung vertically from an edge of the top tile.  A thousandths indicator was fixed to the wood with its stem pressed against this edge of the top tile. A 2.35 lb (measured with scale) weight was hung from the top tile with a piece of monofilament fishing line looped around the edge of the tile. Measured displacement was 0.0078" (0.198mm). Silicone thickness was 0.1465" (3.72mm). The strain is then 0.198/3.72 = 0.053.

 

MoR measure.JPG

The two light thin lines down vertically from the tile are the monofilament line holding the weight.

 

The area of the top of a tile (area parallel to the applied force vector) is 23.8^2 = 567.0 mm^2 = 5.67x10^-4 m^2.  Using 4.448 N/lb and Pa = N/m^2, the shear stress is 2.35*4.448/5.67x10^-4 = 18.4x10^3 Pa = 0.0184 MPa. The modulus of rigidity is: G = Stress/Strain = 18.4x10^3/0.053 = 3.47x10^5 ~ 0.35 MPa.  This is about a factor of 6 lower than the 2 MPa value given by a couple of web sources as an average, but the average varies greatly with source. This one gives about 0.2 GPa for modulus of elasticity.  This one gives a range of 0.0003 to 0.02 GPa. That's about 3 orders of magnitude range, which is why I measured it. I think MoR is usually measured by a torsional measurement, but a linear displacement seems more appropo to bonding a mirror.

 

Say you use this adhesive to glue a supremax mirror to an aluminum backing plate using a 0.5mm bond thickness, and the pads are 60mm, 0.06m, apart.  The CTE of 6061 aluminum is 25.2 um/m-C = 25.2x10^-6 C^-1, Supremax CTE is 3.25x10^-6 K-1, so differential thermal expansion of aluminum - supremax is 22 um/m-C. With 0.06m spacing between pads and a 20 C temperature change the aluminum between pads expands 0.06*20*22 = 26.4um relative to the Supremax.

 

Seems that half of this should be applied to each pad.  Strain in the silicone is 13.2um/500um = 0.026. Stress in the silicone = strain*G = 0.026*3.47x10^5 = 9196 Pa. Strain in the Supremax = silicone stress/G for Supremax = 9196 Pa/27x10^9 = 3.4x10^-7 (the force along the top surface of the bond must be balanced by an equal but opposite force along the bottom surface). Multiplying by a mirror thickness of say 20mm gives the displacement of the mirror backside between pads relative to frontside of mirror: 20mm*3.4x10^-7 = 6.8x10^-6mm = 6.8nm, or about 1/80 wave. It seems that would not cause enough bowing of the mirror to notice astig.

 

If the higher values for MoR of silicone were used rather than my measured value for SCS1000 the mirror displacement would be far greater.

 

This seems to agree with Ed's comment that he hasn't noticed astig - if the MoR is close to what I measured. At least if using the mirror over a modest temperature range and/or at lower powers.  I would expect quite a bit of astig if the silicone used has MoR at the higher end of the range in the refs.

 

Benach also seems quite confident in the use of thin (<0.25mm) bonds, but they seem to be outliers with most others indicating more on the order of 2 - 3 mm, most of whom say debonding has not been an issue. Since debonding doesn't seem to be an issue if surfaces are degreased and cleaned thoroughly with IPA, it seems safer to go with the thicker range of bond, and to measure the MoR of the silicone you use.  As others have said, cleaning is critical for a good bond.  I couldn't pull the two tiles apart after completing the measurement.


Edited by tommm, 15 April 2020 - 02:56 PM.

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#40 sky-man

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Posted 27 December 2020 - 11:18 AM

I'll tell you how I re-glued the observation mirror in winter, otherwise in the cold the mirror jars with the factory Chinese plaster.
I re-glued the diagonal mirror to Henkel Neutral Germany one-component silicone sealant. Working temperature of the hardened sealant -40 ... 120C, elasticity 25%, hardening 2mm per day:
https://a.radikal.ru...fc0862948d1.jpg

The Chinese plaster around the edges has already darkened and began to harden, turning into resin.
198750dc1453.jpg

As we know, such a material, when the temperature changes, will pull the mirror and make astigmatism. As I understand it, the Chinese do not glue on the sealant because you need to wait 24 hours for drying. In a conveyor environment, fast adhesive plasters cannot be replaced.

Observations were planned at -15C, and this is a temperature change of minus 40 degrees, and this will definitely warp the mirror to astigmatism. The patch-resin, in contrast to the sealant, is less elastic and, when the mirror expands and contracts, holds it stronger and bends, while it is not polymerized and can adhere to the new shape of the mirror and retain astigmatism after temperature fluctuations.
It was decided to replace the plaster with a sealant.
The old plaster was cut with a 0.3mm fishing line. Before cutting, circle the base with a marker on the glass so that later it is easy to glue in the same place. The residues are removed by rolling with your fingers. You don't need to scrape with a knife.
There was a good dot on this mirror and it hadn't been jarred yet. If your mirror has already warped, after cutting, let it lie down for 3-4 hours to straighten it, this is noted in tests of one optical laboratory.
Before bonding, wipe both surfaces with Galosh gasoline.
https://a.radikal.ru...0f7acc75735.jpg

As advised, I put matches and made a round spot 25mm in diameter for a 58mm mirror. Left to dry for 7 days.
https://c.radikal.ru...acb129b818e.jpg

https://d.radikal.ru...2804a61d7bb.jpg

A penny of sealant is visible from the side. Previously, the plaster was 1mm thick, now the sealant is 2mm (matchstick thickness). Therefore, the 3 secondary adjusting screws must be unscrewed by 1mm.
https://a.radikal.ru...20f7346c618.jpg

This sealant is thick and tough like a cake. It does not spread, you need to spread a cake with a diameter of 25 mm above the height of the matches, also smear a circle on the base with a thin layer, put this base on the matches on the marker marks and press down slightly so that it lies and rests on the matches. The flatbread grows slightly in size. Do not touch for 7 days.

To me, the repairmen glued the sink to the wall with such a sealant, it stands rooted to the spot.

If something is not glued evenly, the silicone can also be cut off with a fishing line, cleaned with a blade and glued again.

Weight of the glued mirror with a metal base 179 grams:
https://b.radikal.ru...47452da4357.jpg

Features of this sealant (characteristics):
It has excellent adhesion to various surfaces: glass, tiles, ceramics, metals (aluminum, zinc, copper, etc.)
Resistant to atmospheric and chemical influences.
UV resistant.
Waterproof.
Elastic.
Cannot be painted.
Does not corrode metals, does not react with alkaline surfaces (concrete, plaster, etc.), suitable for sealing baths, showers, sinks, mirrors.
Application temperature: from +5 to + 35 ° С. Cure rate (23 ° C or 73.4 ° F, 50% RH) 2 mm per day. Heat resistance of the hardened sealant: from minus 40 to plus 120 ° С (withstands periodic exposure to negative temperatures up to minus 50 ° С).

A simple example of the clamping of diagonal mirrors with a dried plaster, after peeling off, it straightens itself in the test in the laboratory. The material of the diagonal mirror mounting is very important for the telescope optics:
https://a.radikal.ru...4b113973ef0.png

https://c.radikal.ru...5ac55218db3.png

520f7346c618.jpg

 

 

 


Edited by sky-man, 27 December 2020 - 12:34 PM.

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#41 sky-man

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Posted 27 December 2020 - 11:34 AM

I ran a test.  Such a sealant of this size and thickness withstood 15 kg of weight for a long time.  Which is equal to the 90 70mm mirrors that we glued.
 

4f2954e997ea.jpg

The weight that could break this connection was 20 kg.  These are 120 mirrors of 70mm.  I hope you don’t plan to stick more than one mirror?

e170ff76c0cd.jpg
 


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