Recontacting Etalons

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kornfeld1

Recontacting Etalons

Post by kornfeld1 »

I disassembled my PST etalon assembly in order to add a centering ring as mentioned in
this thread.

In a moment of sheer stupidity, I picked up the etalon itself by the foam. The foam had stuck to the etalon, so it stayed stuck just long enough for the etalon to lift up in the air...and then unstick and crash back down onto my work surface and decontact.

Two of the spacers are still contacted to one plate of the etalon. The third spacer is completely free.

I've tried several things to get them recontacted. Both surfaces have been cleaned with isopropyl alcohol, and I've brought the two plates together at an angle and tried rotating them relative to one another.

I've heard a few unexpected clicks that sound like things stuck back together, but when I check, they are definitely not stuck.

I've come to terms with this. It's an expensive toy; but it's still just a toy.

So, any tips on things I should try with getting it back together?

And anyone want any measurements of anything?


colinsk

Re: Recontacting Etalons

Post by colinsk »

I am by no means an expert in this subject but I have been reading experiences. If you want to decontact an etalon you place a drop of distilled water on the spacers. This will drive the etalons apart. To contact them you need to clean the surfaces with a solvent and angle them together until you hear the click.

It sounds like you are close. David G. has a little experience with contacting and perhaps he will comment.

PS Thank you for making this its own thread.


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Re: Recontacting Etalons

Post by marktownley »

Sheeeesh! Sorry to hear this :(

Hang fire and between us we'll work out how to get it back together. I've never decontacted and etalon and don't want to!


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Re: Recontacting Etalons

Post by DavidG »

There is another method that I was told about which allows one to center things up. As you know the surfaces must be perfectly clean. Make sure that the solvent that your using is reagent grade or better yet spectroscopic grade. The IPA you get at the drug store has water in it or worse perfumes and oils.
You want to use a high purity solvent like acetone which evaporates fairly quickly. You very lightly wet the surfaces of the spacers with it and place to two plates together. The wet surface allows you to move things around for a few seconds and get them aligned. Then you warm the plates with a source of warm air like a hair dryer. If all goes well the solvent will evaporate quickly and the optical contact bond will form.
I would first try to get the spacer that is totally free, contacted back onto the plates that has the other two spacers in place. Once it is in place, then try to get the two plates to bond together.
- Dave


Engineering = Take what you have and making what you need
kornfeld1

Re: Recontacting Etalons

Post by kornfeld1 »

Davidg...awesome post. I'll definitely give that a shot. I'll look into getting some spectroscopic grade acetone.


kornfeld1

Re: Recontacting Etalons

Post by kornfeld1 »

I've never decontacted and etalon and don't want to!

Yeah....I really didn't want to either. I was working super carefully, being meticulous about not touching optical surfaces, moving slowly, etc. I have absolutely no idea why I picked the etalon up by the foam that was stuck to it. As soon as it dropped, I wasn't even upset, I was just staring at my work surface in complete disbelief at what I had done.

It's been a long while since I've done something so boneheaded. I just keep reminding myself that I'm fortunate that my thoughtlessness lead to this breaking, and not to any bodily injury or anything like that.


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Re: Recontacting Etalons

Post by swisswalter »

Hi Phil

very interesting thread. I had a decontacted LS50 and was not able to recontact it but I made mistakes. So take care that all s as clean as possible, I mean really clean, that is one of the clues. Good luck. Keep us informed.

Details of my adventure with the LS50 can be read on my homepage


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colinsk

Re: Recontacting Etalons

Post by colinsk »

Hi Phil

very interesting thread. I had a decontacted LS50 and was not able to recontact it but I made mistakes. So take care that all s as clean as possible, I mean really clean, that is one of the clues. Good luck. Keep us informed.

Details of my adventure with the LS50 can be read on my homepage

It took me a few minutes to find your link here do to my lack of german:

http://www.wastronomiko.com/images/ausr ... repair.pdf


colinsk

Re: Recontacting Etalons

Post by colinsk »

OK, I just read through the document. Great work! The suggestions I would make are to use a zero expansion material for the spacers and to calculate the exact thickness. Making spacers is not an easy task. Mica is a good choice if you can temperature control. Shoot for red and tilt shift blue.

If you want to sell the etalon I would love to play with it. ;)


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Re: Recontacting Etalons

Post by DavidG »

An easy way to tell what is going one with the uniformity of the air gap is to look at the interference fringes formed between the two Etalon plates. It is the exact same approach one uses when testing two optical flats. You need a source of monochrome light which can be simply a standard CFL light bulb and to look thru a red or green filter. Even a piece of red or green plastic from a candy wrapper will work to filter the light. You should easily see fringes and they should be straight and evenly spaced. If they are a bullseye pattern and centered that indicates that one of the surfaces is not optical flat. If the pattern is made up of arcs or a bulleyes that is not centered then that indicates that the air spacing between the plates is not uniform. Only when you get straight fringes will the Etalon work like it is suppose to.

- Dave


Engineering = Take what you have and making what you need
colinsk

Re: Recontacting Etalons

Post by colinsk »

I am not sure if I understand that. I only have collimated point sources for monochromatic light. I would expect a perfect etalon to have a centered bullseye. I would think lines or arcs to represent tilt. At least this is how it works when I build a Michelson interferometer. I am thinking it should be the same?


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Re: Recontacting Etalons

Post by robert »

This is a view of a low energy bulb in a glass lamp shade through an LS75 without the ERF (3 obstructing spacers visible). The rings are centred on the camera lens. Is this what you mean?




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Re: Recontacting Etalons

Post by colinsk »

This is what I would expect from a good etalon. Do you see the faint secondary set of rings? They appear to be a third reflective surface. Perhaps the glass to coating interface on the etalon mirror.


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Re: Recontacting Etalons

Post by DavidG »

What I'm describing is looking at the pattern caused by the reflected light between the surface of the air gap. So the Etalon is placed on a table with the light above it vs looking thru it. As I said it the same technique used in testing optical flats against each other. http://www.edmundoptics.com/learning-an ... manual.pdf
The reason why you want to test by the reflected light is that it would difficult if the Etalon plates are free from each other and you tried to hold them together to see what the transmitted pattern looked like. So when you are trying to recontact an Etalon when you place the two plates in contact and you can easily tell if one of the spacers is not at the same height as the rest from the shape of the interference pattern. It should be perfectly centered if the air gap is uniform. Most likely there is dirt on that surface and it only takes a speck of dust to throw the pattern to one side.

- Dave


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colinsk

Re: Recontacting Etalons

Post by colinsk »

Now I understand. I am thinking of testing with a gap and you are placing the plates together.

Karow's "Fabrication Methods for Precision Optics" is a good reference.


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Re: Recontacting Etalons

Post by DavidG »

You do the test with the gap which is from the spacers but in reflection and not transmission. Since the plates are optically polished very flat, you should get interference fringes that are uniformly spaced and centered. If not then the air gap is not uniform. By the way since you know the wavelength that your testing at, by counting the fringes, they will give you the spacing between the plates. If the spacer thickness is too large you will not see the fringes unless you use a simple Fizeau interferometer. The interferometer is just a source of collimated light and slight off to one side. You place are eye at the focus of the lens also. You can make one from a large plano convex lens were the light source is placed at the focus of the lens. For the size of the Etalons we are talking about a 3" lens is fine and with focal length of around 15". It also doesn't need to be of high quality. The test setup can be found in "How to Make a Telescope" by Jean Texereau in the chapter on making diagonal mirrors.
When I looked at SwissWalter's write up on his attempt to recontact his Etalon, he has a picture of his his plates being tested by interference. The patterns is off center which indicates that the air gap is not uniform and explains why his Etalon is showing non uniformity in H-alpha detail in the image.

- Dave


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colinsk

Re: Recontacting Etalons

Post by colinsk »

Based on my observations all airspaced etalons would be slightly off center. My PST would be at least a fringe off center making only 1/3 of the etalon on band.

How does the axis of the light source and the observer effect the fringes? I typically use a spatial filtered 658nm laser.


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Re: Recontacting Etalons

Post by DavidG »

The larger the air gap the more critical the viewing angle becomes. A simple Fizeau interferometer removes the viewing angle problem. I'm sure that the off center pattern that your seeing in the PST Etalon is caused by the viewing angle and not a non uniformity in the air gap.

- Dave


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Re: Recontacting Etalons

Post by colinsk »

I have not made a Fizeau interferometer before. Can one use one flat of the etalon to measure or does one need a full size reference flat.


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Re: Recontacting Etalons

Post by DavidG »

Colin,
You don't need a reference flat since the two etalon plates are the reference surfaces to each other. As we know the Etlon couldn't work unless both inner surfaces of the Etalon aren't flat to close to 1/100 of wave. So the flatness is not an issue, what is, is the shape of the interference pattern formed between the two inner surfaces since that is a direct measure of both the thickness and uniformity of the air gap. The same is true of the bullseye pattern formed when you look thru the Etalon at monochrome light. But like I said it would be extremely difficult to look thru an Etalon were the two plates are no longer in optical contact and get any meaningful information about the uniformity of the spacers and air gap.
What I would do is test the Etalon by reflected monochrome light using a simple Fizeau interfreometer by place the plates together with the spacers between them. If the interference pattern is centered and uniform, that means the spacers have uniform thickness and all the surfaces are clean. If that is the case then one can go ahead and try to optically contact them. What I have been told is one method to make the optical contact is a drop of solvent like Acetone is placed next to each spacer so it wicks under both the top and bottom surface of all the spacers, then the assembly is warmed up to evaporate the solvent with some mild pressure added and the optical bond forms.
You can also see interference fringes between the surfaces of each of the spacers and the Eatlon plates before they are optically contacte. Again by looking at the uniformity of the interference pattern you can determine is the surfaces are clean.

- Dave


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kornfeld1

Re: Recontacting Etalons

Post by kornfeld1 »

I'm currently trying to see if I have any contacts that can donate a small quantity of acetone to me....I'm having a hard time finding it commercially in quantities under a liter, and I definitely don't need that much.

The discussion going here is great, but I don't have much to contribute, so thank you for all of the great ideas!


colinsk

Re: Recontacting Etalons

Post by colinsk »

Old school nail polish remover is acetone. Don't get the non acetone version or the scented versions. A hardware store should be able to supply a L of acetone for less than $3. It is a very useful solvent. Your body produces it naturally so it is able to remove it if you absorb it through your skin. However it will take other chemicals through your skin as well so non-pure acetone requires gloves.

Dave, Have you ever heard of the dry method for contacting an etalon? I am told if everything is right you hear a "click" what it bonds.


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Re: Recontacting Etalons

Post by kornfeld1 »

After reading here and elsewhere online, I was under the impression that store bought acetone (even of the unscented variety) was still not terribly pure; and that even reagent grade acetone (at 99.5%+ purity) might not work as well as spectroscopy grade acetone.

I guess it can't hurt to give it a shot though. If it works, great; if not, then I'm only out a few bucks!


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Re: Recontacting Etalons

Post by DavidG »

Being a research chemist and engineer for the last 27 years I can tell you that any Acetone purchased for the public use is not very pure. You can use many different types of organic solvents, like methanol ethanol, Isopropyl alcohol, toluene etc. It just needs to be fairly pure, especially free from water. When you make an optical contact bond, it requires the surfaces to be very flat and clean so the surfaces can come very close together and allow a molecular bond to form. It only takes a few residue molecular to hold the surfaces apart and the bond won't form. Using a few drops of solvent allows the surfaces to be positioned but also cleans them and as the solvent evaporates it can take with it any small amount of residue.

Colin,
I have seen the dry contact method done first hand one time. It was done using two small optical flats about 1 1/2" in diameter. The surfaces were first cleaned with Methanol and inspected to be sure that there was not a speck of dust on either surface. Then the two plates were lined up edge to edge and held at an angle and slowly brought together starting at one edge. When the plates were almost fully touching each other I heard a click as they pulled themselves together. Once the bond formed I could not pull them part. I've heard other say that you bring the plates together and give them a slight twist and you'll here a click when the bond forms. The key seems to be that you need to have motion that allows the air to be squeezed out from between the interfaces so the bond can form.

- Dave


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Re: Recontacting Etalons

Post by DavidG »

After reading here and elsewhere online, I was under the impression that store bought acetone (even of the unscented variety) was still not terribly pure; and that even reagent grade acetone (at 99.5%+ purity) might not work as well as spectroscopy grade acetone.

I guess it can't hurt to give it a shot though. If it works, great; if not, then I'm only out a few bucks!

I thought of another method that just might work to make the surface super clean. Scotch Tape, the real stuff not the cheap stuff. I know it sounds a little wacky but I remember a cowork using this method to get metal films to adhere to glass surface and it worked great. If you place a small tab of it were the spacer goes and remove it, it will clean the surface to the molecular level. Try doing the area were the one spacers is fully removed and see if you can get it to make contact again. If it works you can then carefully use the tape to clean each of the upper surfaces of the spacers and then the areas on the Etalon plate were spacers will touch.

- Dave


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Re: Recontacting Etalons

Post by colinsk »

When avoiding water make sure to not use any of the alcohols. They are hydroscopic. Better than 95% water free is really hard.


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Re: Recontacting Etalons

Post by DavidG »

I called a friend up who has done optical contacting and asked what he does. Like we know the surfaces must be perfectly clean. He places the plates and spacers together and looks at them under monochrome light. You should see even interference fringes on the top of each spacer. If not there is a speck of dust. He saids that the dust will easily show up as a tiny black dot under the monochrome light. Once everything looks good he then press straight down with his two thumbs on the upper plate. He said it takes a fair amount of pressure and you'll hear click when the bond forms.
When I remember back to when I saw it done first hand, I do remember the plates being squeezed with hand pressure. Maybe the amount of pressure is what has been missing to make this work.

- Dave


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colinsk

Re: Recontacting Etalons

Post by colinsk »

That matches what i have been told as well. Thank you Dave!


colinsk

Re: Recontacting Etalons

Post by colinsk »

Here is an interesting link about contacting large art pieces! Surfaces this large are amazing!

http://www.zartwerks.com/optical_contacting.html


kornfeld1

Re: Recontacting Etalons

Post by kornfeld1 »

Great post Colin, thanks. This has all unfortunately fallen to the back burner as my work computer was stolen from my workplace, which lead to me trying to do as much as I could to make sure my identity wasn't subsequently stolen. Funny how life ends up getting in the way of our hobbies. :)

I should be able to give this some more work this weekend though, so hopefully I'll have a good update. Thanks again to everyone for the input.


colinsk

Re: Recontacting Etalons

Post by colinsk »

I now have a decontacted SM40 so I will have some things to try as well.


etatsolarchat

Re: Recontacting Etalons

Post by etatsolarchat »

Stupid question why exactly is decontacting them a serious problem?

Is it just that they need to be put back super tight to each other?

Do they just need to be in perfect contact or does rotation and offset need to be perfect as well??


colinsk

Re: Recontacting Etalons

Post by colinsk »

The surfaces need to be perfectly flat and clean in order to make a glueless molecular bond. It is said to be an art. I have an SM40 to try it on.


etatsolarchat

Re: Recontacting Etalons

Post by etatsolarchat »

So is it just the bond that's needed what about rotation and offset?


brianb11213

Re: Recontacting Etalons

Post by brianb11213 »

So is it just the bond that's needed what about rotation and offset?
Silly question really but isn't the bond between the spacers and the plates?

The performance of the etalon is dependent on the free surfaces of the plates being very very parallel (to the order of 1/100 wavelength) but NOT in contact .... if the surfaces are "perfectly" figured then offset & rotation of the plates shouldn't be an issue but I guess that in practice this is fairly critical as, at the microscopic level, the plates probably are going to be figured to be parallel rather than flat.

The issue with assembling the etalon is that a speck of dust even 1/100 wavelength in size (6.5 nm) trapped between spacer & plate will destroy the performance of the unit.


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Re: Recontacting Etalons

Post by DavidG »

The optical contact bond, in this case is between the spacers and the inner surfaces of the Etalon plate. The thickness of the spacers determine at what wavelengths there will be contrustive and destructive interferance. The uniformity of the spacing and the flatness of the two inner surfaces of the plates determines the how uniform the band pass will be across the plates. So the plate surfaces need to flat to around 1/100 of wave and the space between the plates uniform to this level as well.
When one of the spacers becomes decontacted the spacing between the plates is now wedged. You no longer have interference taking place at the correct wavelengths and also the wavelengths are not uniform across the plates.
A friend that as done optical contacting told me that they would shiny monochrome light on the etalon with the spacers between the plate. You could see interference fringes between the spacers and the plates. I'm taking about the area directly above each spacer and not the unobstructed area in the middle of the plates. The interference fringes need to be the same in number and in shape. The shape should be straight lines. If not then the very tiny air gap bewteen the spacers and plate is not uniform and caused by speck of dust. Once they got things to show uniform fringes, you pressed straigth down with a good amount of force. The fringes would get fewer in number and wider until there was only one visible which made the surface between the spacer and the plate look black. Again I'm talking about the area directly above each spacers and not the unobstructed area in the middle of the plates. When the optical contact bond would form and the surface would then go clear.


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Re: Recontacting Etalons

Post by colinsk »

There may be a slight wedge in the plates and then the rotation would be an issue. I have not measured a wedge but will try when I have the SM40 apart. You would match the thick sides on the same side of the rotation. This would be a way to not have the outsides of the etalons making interference fringes. Another would be to have really good AR coating on the outside surfaces. David has implied both methods are in use.


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Re: Recontacting Etalons

Post by Wah »

Is there anyone know the price of repairing a decontacted etalon?
Or is there anyone here can provide recontacting etalon service? B) B)


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Re: Recontacting Etalons

Post by swisswalter »

Hi Wah

send me an email. I have some information for you

walter ät nanosys dot ch


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Re: Recontacting Etalons

Post by kornfeld1 »

Bump! Been a while since I've been around here. I've all but given up with the recontacting of my etalon.

On the upside, I found a lead on a PST with a rusty objective. What would you all say is a reasonable price to pay for one? I was thinking $250, but I have no idea if that's in or out of line.

Thanks in advance for the help!


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Re: Recontacting Etalons

Post by Merlin66 »

If the etalon is good, and you actively want to consider a Stage 1/2 mod then IMHO anything less than $300 is a good price at the moment.


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Re: Recontacting Etalons

Post by kornfeld1 »

Excellent, I was hoping you'd see my post. If anyone else agrees or disagrees, please post up.

I still have my other entire PST, which has the newer blue objective and and a good blocking filter and ERF (or whatever the lens under the red blocking filter is. I can just put the other etalon into this assembly and it should be good to go, correct? Or is there an additional lens from the rusted PST I need to incorporate in with the new stuff?


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Re: Recontacting Etalons

Post by Merlin66 »

No, you just have to swap over the etalons...


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Re: Recontacting Etalons

Post by marktownley »

Then unscrew all the remnants of the etalon-less rusted PST - black box, ITF, blocker, gold tube, rubber knurled ring and sell them all individually on flea-bay and I reckon you will be surprised how much they go for ;)


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Re: Recontacting Etalons

Post by colinsk »

I am looking to purchase any decontacted etalons. colink at designer in light dot com

:thanx:


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Re: Recontacting Etalons

Post by swisswalter »

Hi Phil

yes, go for it, 250$ is a good price if the etalon is in good shape


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Re: Recontacting Etalons

Post by kornfeld1 »

Great, thank you all for the input. I should find out today if the deal will go through.


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Re: Recontacting Etalons

Post by Merlin66 »

Colin,
How did you go with the SM40 decontacted etalon you were playing with?
I'm still working on George's etalon and re-reading this thread for ideas...


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Re: Recontacting Etalons

Post by colinsk »

I have a plan but I have not attempted the etalon yet. I am close to moving and I have been waiting on this project.

1. Make a small laminar flow enclosure as I need it to coat holographic plates anyways.

2. Use a drop of water to decontact the etalon on the contacted spacers.

3. Clean the contacting points with pure reagent grade alcohol. Any variety seems to be fine.

4. Carefully place the glass together in the exact right positions.

5. Press firmly and measure.

6. Repeat if needed.

So far that is all I have. I will start a thread on laminar flow enclosures when I am building mine.


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swisswalter
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Re: Recontacting Etalons

Post by swisswalter »

Hi Colin

good luck with your projects.

The purity of the alcohol is important. Get purris pa. Depending on the country, alcohol can contain all sorts of denaturating agents.


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