Etalon spacers thickness.

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Etalon spacers thickness.

Post by Valery » Fri Mar 06, 2015 3:14 pm

Hi all,

Reading some posts here I noted that some etalons have different spacers thickness.

1. First note is by Mark Townley here: http://solarchat.natca.net/viewtopic.ph ... 7&start=25 post from Mon Jul 09, 2012 12:14 am

2. Second note is by Ken (aka Merlin66 ) http://solarchat.natca.net/viewtopic.ph ... 9&start=25 post from Mon Feb 16, 2015 1:09 pm
and from Wed Feb 18, 2015 12:59 pm

2. Third note is by Mark Wagner here: http://solarchat.natca.net/viewtopic.ph ... 9&start=25 post Wed Feb 18, 2015 11:28 am
and from Wed Feb 18, 2015 9:27 pm


All spacers are different thickness.

From 0,154mm to 0,4mm


The 0,4mm spacers give the tightest bandpass and the 0,154mm gives the widest bandpass.

The etalon with 0,4mm spacers will require very narrow blocking filter about 5 - 6A .


So, what real etalon spacers thickness what they are made of (mica, fused silica, BK7 what? ).


Thanks in advance to our etalon guys.


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Re: Etalon spacers thickness.

Post by Merlin66 » Fri Mar 06, 2015 4:42 pm

I believe they are quartz flakes, and for the Coronado etalons the spacing seems to be 0.21 mm.
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Re: Etalon spacers thickness.

Post by swisswalter » Fri Mar 06, 2015 5:32 pm

Hi Valery

we looked at the PST etalon spacers, they where most probably made of fused silica, between 0,2 and 0,3 mm in thickness
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Re: Etalon spacers thickness.

Post by Valery » Fri Mar 06, 2015 6:41 pm

Merlin66 wrote:I believe they are quartz flakes, and for the Coronado etalons the spacing seems to be 0.21 mm.
Thanks!

Does "quartz flakes" mean:

1. mica flakes?
2. pieces of natural quartz?
3. pieces of thin quartz polished in relatively large plate and then trashed on small parts?


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Re: Etalon spacers thickness.

Post by Valery » Fri Mar 06, 2015 6:43 pm

swisswalter wrote:Hi Valery

we looked at the PST etalon spacers, they where most probably made of fused silica, between 0,2 and 0,3 mm in thickness
Thanks, Walter!

Are you sure that they are 0,2mm and not 0,4mm (according to Mark T.) ?


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Re: Etalon spacers thickness.

Post by Merlin66 » Fri Mar 06, 2015 9:26 pm

They look very much like crystal flakes...roughly 3mm square....
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Re: Etalon spacers thickness.

Post by mdwmark » Fri Mar 06, 2015 11:11 pm

The spacers for air spaced etalons are not just some random sizes. They are polished very flat and have a thickness designed for the FSR. There will be a difference between the spacer thickness and the spacer index of refraction of the spacer. The spacer thickness that I gave was from the etalon calculator. You can go to the calculator and try the other thickness and see what comes out.
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Re: Etalon spacers thickness.

Post by Bob Yoesle » Fri Mar 06, 2015 11:39 pm

The spacers should be made from very low thermal expansion materials such a fused quartz, Zerodur, etc. Likely they are sourced for an outside vendor such as this:

http://lightmachinery.com/optics/optics ... in-wafers/

The small spacers are broken off these larger thin wafers, and then optically contacted to the etalon plates. Note the jewel boxes on the workstation in the background, and the open one behind Rikki's hand:
Etalon Spacer.jpg
Etalon Spacer.jpg (120.55 KiB) Viewed 2979 times
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Re: Etalon spacers thickness.

Post by Valery » Sat Mar 07, 2015 12:18 am

mdwmark wrote:The spacers for air spaced etalons are not just some random sizes. They are polished very flat and have a thickness designed for the FSR. There will be a difference between the spacer thickness and the spacer index of refraction of the spacer. The spacer thickness that I gave was from the etalon calculator. You can go to the calculator and try the other thickness and see what comes out.
Mark W.
I know that they are very flat and are on the optical contact with etalon plates. The question which confused me is why they are so different in the thickness.
Here I speak about air spaced etalons only.

The 0.395mm gap between the etalon plates (like in PST etalon, according to Mark Townley measurements) defines the FSR of only 5,5A . This mean we do need a very narrow blocking filter with very steep skirt of it's transmission curve. Otherwise the BF will pass also nearest peaks and especially blue-ward peak (if the incident light is not a parallel, say it has F/7 cone or so).

The 0.25mm gap between the etalon plates defines the FSR of 8A which is significantly wider.

However if the manufacturing tolerances are the same, etalon transmission peaks will have different FWHM. In my designs 0.4A for 0.395mm spacers and 0.6A for 0,25mm spacers.

According to my estimations PST etalons ( I now tested four of them) have 0.3A < FWHM < 0.4A. Three are narrow and one is very poor - something like 1A.
My SM40 has 0.55 < FWHM < 0.7A - very much like 0.6A.

In the same time measured FSR for CORONADO SM60 is 13A ! which gives the spacer thickness of about 0.17mm and estimated FWHM about 0.7A.

The question for me is that 0.4mm spacers are easier to make and give much narrower FWHM, but what about BF necessary band pass?
Available in free selling blocking filters (Lunt, Meade-CORONADO) are 10A bandpass and are not good match to such a narrow etalon like in PST.

I got several very precise flats ( 1/5 relative thickness) 120mm diameter and are willing to make a 100 - 110mm clear aperture etalon(s).
Of course, I would like to have the narrowest possible FWHM and easiest to make spacers. The problem: where to find a narrow enough blocking filter with about 5-6A FWHM with steep skirt.

Any thoughts?

Thanks,

Valery.
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Re: Etalon spacers thickness.

Post by Valery » Sat Mar 07, 2015 1:46 am

Bob Yoesle wrote:The spacers should be made from very low thermal expansion materials such a fused quartz, Zerodur, etc. Likely they are sourced for an outside vendor such as this:

http://lightmachinery.com/optics/optics ... in-wafers/

The small spacers are broken off these larger thin wafers, and then optically contacted to the etalon plates. Note the jewel boxes on the workstation in the background, and the open one behind Rikki's hand:



See:
http://astronomyandlaw.com/tag/lunt-solar-telescopes/
Hi Bob,

Thank you for your info, very helpful.

I already sent them the questions about the possibility to get spacers from them


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Re: Etalon spacers thickness.

Post by marktownley » Sat Mar 07, 2015 7:44 am

Valery wrote:Are you sure that they are 0,2mm and not 0,4mm (according to Mark T.) ?
Anything I measured would have just been with a cheapo pair of calipers not really designed for accurate sub millimetre measurement, so, take as a guide rather than a definite...
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Re: Etalon spacers thickness.

Post by marktownley » Sat Mar 07, 2015 7:51 am

Valery wrote:The problem: where to find a narrow enough blocking filter with about 5-6A FWHM with steep skirt.
If you can get a 5-6a blocker then you could double stack them to get the skirts steeper?
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Re: Etalon spacers thickness.

Post by mdwmark » Sat Mar 07, 2015 8:03 am

You are missing three other important factors.
1) what % mirror they are using.
2) how flat the surfaces really are.
3) how much tilt between the plates.
You can use the same spacer distance and have different final HW by changing any of these.
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Re: Etalon spacers thickness.

Post by marktownley » Sat Mar 07, 2015 11:27 am

mdwmark wrote:You are missing three other important factors.
1) what % mirror they are using.
2) how flat the surfaces really are.
3) how much tilt between the plates.
You can use the same spacer distance and have different final HW by changing any of these.
Mark W.
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Re: Etalon spacers thickness.

Post by Valery » Sat Mar 07, 2015 4:00 pm

mdwmark wrote:You are missing three other important factors.
1) what % mirror they are using.
2) how flat the surfaces really are.
3) how much tilt between the plates.
You can use the same spacer distance and have different final HW by changing any of these.
Mark W.
Thanks. I know this. I just took the realistic figures and then remain only the space as a freedom factor and I see that my
largest problem will be the blocking filter(s). We can make flats of etalon quality and in much larger size than all these
4" or 5".
One positive factor which works for me is that with the same precision of the spacers thickness uniformity, as larger the
diameter of the spacers circle, the more parallel both flats are.

Let see what the manufacturer of the quartz wafers will tell us about the possibility to make custom thickness spacers.


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Re: Etalon spacers thickness.

Post by swisswalter » Sat Mar 07, 2015 4:22 pm

Hi Valery

wouldn't it be more economical to have the etalons custom made by e.g. lightmachinery ?
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Re: Etalon spacers thickness.

Post by Valery » Sat Mar 07, 2015 4:30 pm

swisswalter wrote:Hi Valery

wouldn't it be more economical to have the etalons custom made by e.g. lightmachinery ?
Hi Walter,

I don't know about the LM service. I suspect it will be quite expensive and no challenge and interest.
My scopes already are of 6" size and will be larger soon. I do not intend to make these 100-110mm
front mounted etalons for myself. If to make for myself, then better to go with 8". ;)


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Re: Etalon spacers thickness.

Post by swisswalter » Sat Mar 07, 2015 6:44 pm

Ah I see ;)
Only stardust in the wind, some fine and some less fine scopes, filters and adapters as well. Switzerland 47 N, 9 E, in the heart of EUROPE :)

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Re: Etalon spacers thickness.

Post by fjabet » Sat Mar 21, 2015 12:07 am

Having large etalons is very tricky. They have to be thick as the parallelism between plates must be as good has the surface flatness (L/100, 1A roughness), that mean also either huge spacers, and possibly a central spacer as for Coronado.
As for the spacer height, it is not very important as the spectrum beat is a modulo of it, the reflectivity is the real metric for the finesse, and the flatness for homogeneity and the FWHM as well.
The real challenge would be an affordable (let us say about 10K€) Lyot etalon as it brings a real added value in term of field angle tolerancing.
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Re: Etalon spacers thickness.

Post by Valery » Sat Mar 21, 2015 2:31 am

fjabet wrote:Having large etalons is very tricky. They have to be thick as the parallelism between plates must be as good has the surface flatness (L/100, 1A roughness), that mean also either huge spacers, and possibly a central spacer as for Coronado.
As for the spacer height, it is not very important as the spectrum beat is a modulo of it, the reflectivity is the real metric for the finesse, and the flatness for homogeneity and the FWHM as well.
The real challenge would be an affordable (let us say about 10K€) Lyot etalon as it brings a real added value in term of field angle tolerancing.
The etalons I do mean are air spaced and front mounted.

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Re: Etalon spacers thickness.

Post by Bob Yoesle » Mon Mar 23, 2015 6:50 am

Two essentials:

If unobstructed (no central spacer) it will be critical to block all possible IR prior to the etalon.

Even better - central spacer, and block all possible IR prior to the etalon ;-)

See Fig 7 and Item 3.b. in http://www.sunsteinlaw.com/media/Sidori ... ilters.pdf
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Re: Etalon spacers thickness.

Post by marktownley » Mon Mar 23, 2015 7:19 am

Had not read that paper for a while, thanks for the link Bob! :)
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Re: Etalon spacers thickness.

Post by swisswalter » Mon Mar 23, 2015 4:45 pm

Hi Bob

thank you very much for that paper
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Re: Etalon spacers thickness.

Post by antonello » Thu Apr 23, 2015 7:34 am

Like many of you, I too have read the Etalon theory and I thought I would try to build one. The idea came to me because I have two optical windows with 40 mm of diameter machined to lambda/10 , and I have the possibility of covering the two adjacent sides (Etalon mode) with a treatment that reflects 95% of light.
I have also read that the pecisione request for flat surfaces of the Etalon must be lambda/50-Lambda/100, but I have also read that a BF for to work well must cost many hundreds of dollars (about $ 1,800 for 18 mm of diameter) and instead I use a 25 mm filter Omega Optical from only $ 100 with an etalon Lunt 60mm and works perfectly ... (to see http://solarchat.natca.net/viewtopic.php?f=9&t=15211), and then I would try to make the etalon although the accuracy is only Lambda/10.
The problem is at issue here. Someone has found the solution to have three identical layers with thickness calibrated 0.41 mm or 0.205 mm (for example)?

Valery, have Lightmachinery responded to you?

Obviously, my idea is to use this second etalon "homebuilt" in double stack mode after a telecentric system....

Thanks

Antonello

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Re: Etalon spacers thickness.

Post by antonello » Fri Apr 24, 2015 8:34 am

Hello

Yesterday I found the time to play with this program for etalon design:

https://lightmachinery.com/optical-desi ... -designer/#

and I discovered many interesting things. It is possible see immediately the theoretical results that correspond to certain characteristics of the discs semireflecting of Etalon.

Those interested can check the importance of optical correction of optical windows (starting from lambda/10), the percentage of transmission / reflection of the two sides and the error of flatness of the thicknesses ... It seems that the value most important and determinant it is the Mirror Tilt Error...

Here it is possible to see visually what corresponds to a tolerance of of 10 nm (see bellow)

https://lightmachinery.com/optics/optic ... in-wafers/

Antonello

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