Designing a new etalon for halpha

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Designing a new etalon for halpha

Post by Bastelhannes »

Due to the lack of available halpha filters I started the designing of an etalon and also started discussions with etalon manufacturers. As usual lessons are learned on the way and I want to ask you experts for some ideas, your opinion and want to start some discussions. I am going to invest a lot of money in this when I see the silver lining…


I learned form you guys (especially Thank you Christian Viladrich for your excellent explanations and the great Solar Astronomy book - Thank you all authors) a lot.

Okay, let's start:

The central line of H-alpha already is a minimum in the spectrum with an intensity of about 17% . A normal etalon has a transmission line of approximately 90%. Bandwidth FWHM is around 05. - 0.7 Angstrom. (I am not sure, if I am allowed to use pictures form the book of Christian's website here.) The problem is the bandwidth at the intensity of 10%. Due to the Lorentzian curve of the filter charactersistics a lot of the photosphere will be visible, too. The bandwidth widens up to 3xFWHM, this means 2.1 A for a 0.7A FWHM etalon.

It is possible to make the curve “steeper”, this means making the FWHM and 10% width smaller. But at costs of the transmission intensity. It would be about 30% of a “normal” etalon.

Would this be still useful? I am full of doubts, because exposure times would explode…

I hope we will have a deep discussion here. I do not want to fight against Lunt, Daystar, Coronado and Solar Spectrum. I just want to make filters available again and use the chance to play around with the figures. The first etalon is about 20k in productions, the series will be much more affordable. Just to let you know the risk I am going to take…

Bastelhannes


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Re: Designing a new etalon for halpha

Post by marktownley »

Bastelhannes wrote: Sat Sep 10, 2022 4:47 am It is possible to make the curve “steeper”, this means making the FWHM and 10% width smaller. But at costs of the transmission intensity. It would be about 30% of a “normal” etalon.
This is what double stacking does.


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Re: Designing a new etalon for halpha

Post by Bastelhannes »

Sure, but what about putting it in one etalon instead of two?


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Re: Designing a new etalon for halpha

Post by marktownley »

Bastelhannes wrote: Sat Sep 10, 2022 1:56 pm Sure, but what about putting it in one etalon instead of two?
Why? What does that achieve?


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Re: Designing a new etalon for halpha

Post by Bastelhannes »

One etalon instead of two, only one adjustment of wavelength, no ghost images, no reflections.

But I get the feeling I should save my money and people will wait for etalons or h-alpha telescopes longer...


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Re: Designing a new etalon for halpha

Post by rsfoto »

Bastelhannes wrote: Sat Sep 10, 2022 1:56 pm Sure, but what about putting it in one etalon instead of two?
Hi,

I am no expert in building etalons but there must be somewhere a catch that two etalons give a narrower bandwidth as compared to one because if there would be no catch we would already have the narrower bandwidth, or not ?

BTW, IMHO the biggest problem are theose ridiculous tiny golden adjustment wheels for changing the etalon angle in case of this type of etalon and I think implementing my remote adjustment solution using stepper motors is a good investment. The price compared of my solution, to the cost of an etalon, is more then affordable ...

Grüße Rainer


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Re: Designing a new etalon for halpha

Post by Rusted »

I keep wondering if one of the "miracle" carbon based materials will finally provide us with simple, narrowband filters at all desirable wavelengths.

Well, they work for desalination. :D


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Re: Designing a new etalon for halpha

Post by Bastelhannes »

Then we should declare the photons being salty...


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Re: Designing a new etalon for halpha

Post by Bastelhannes »

Okay, I decided to stop this project.

So we will stay with Lunt, Daystar, Coronado, Solar Spectrum etc..., if they will ever deliver any filters or scopes with good quality....

I will produce only some for my private fun...

How can I close this thread?


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Re: Designing a new etalon for halpha

Post by christian viladrich »

Hello,

Very interesting and challenging project ! Still, I understand the project is already closed ?

The design of a Ha (or Ca II) filtration involves a number of interwinned issues. The different options considered by David Lunt (Coronado) at the beginning of the end of 90s/ beginning of 2000s is quite illustrative of this. David Lunt had a huge background on coatings (originally from the “Star Wars program). I don’t have all the historical details, but here are some landmarks :
- It seems to start with rear-mounted etalons. They were of three types : AS1 (=air-spaced etalon), SMn (medium index etalon), VHn (high index etalon).
- The SMn took a lot of developing time and only a handful were ever produced. VHn was a failure. Making the narrow band BF was also challenging.
- Later on, David Lunt moved to the air-spaced etalon in front position we all know.
- David Lunt also filed a number of patents (hybrid etalon, compression tuned etalon, etc.).

Here are some of these issues (for sure, the list is not complete) :
- Front or rear-monted etalon ?
- Diameter of the free aperture ?
- Air-spaced, mica-spaced, fused silica-spaced etalon ?
- Temperature stability ?
- CWL tuning by temperature, compression, tilt ?
- Telecentric or collimating mount ?
- Peak transmission ? Uniformity ?
- FSR ? This is related to the performance of the BF (soft-coating, hard-coating, single/double/triple cavity ?, peak transmission ?)

Regarding the profile of a Fabry-Perot etalon. The profile is the same whatever the cavity (air, mica, fused silica, etc.). So, we have a bandpass of 3 x FWHM at 10% peak transmission. If we want a steeper curve, we need to go double-stack. There is no way out.

At this point, we can mention high performance hardcoated filters (such as Alluxa or Materion). Currently, two-cavity filters (equivalent of double-stack) are feasible with a FWHM of 1.2 A. This vastly outperforms Ca II mica-spaced etalon. Still, the price of such filters is probably very high and accuracy of CWL still challenging.

However, for Ha etalons, the only solution is the classic Fabry-Perot etalon (possibly double-stacked to have a steeper profile). My personal feeling, if we forget about cost, is that the best solution is a fused-silica etalon (or BK7 ?): peak transmission is much better than mica-spaced etalon, acceptance angle is much larger than air-spaced etalon because of the larger index. Issue such as tuning of CWL should be looked into carefully, as well as the design the associated BF. In any case, uniformity is very challenging, and … there is no free lunch.

Just some thoughts …


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Re: Designing a new etalon for halpha

Post by Bastelhannes »

Dear Christian,

Thank you for all the hints. Most of them are already in my mind. I will evaluate some ideas, but it lis likely going to be a private project now. Investing some bucks in to this and I am in discussion with some etalon manufacturers.

First I thought to make this project to create a new source for solar telescopes in the future. But it could be too much for me alone and Mark stepped on my brakes.


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Re: Designing a new etalon for halpha

Post by MalVeauX »

Hi,

Just thinking out loud here, I'm no expert on this at all, so hopefully someone can correct anything that is false here:

Etalons are used because despite them being finicky and sort of expensive to make, they're still one of the cheaper solutions as a system to generate ultra narrow bandpasses. They're economic, if you will. And are from a time when "coatings" were less well known (20 years ago vs today) and so other potential filter options were not really options for a consumer due to immense cost and high challenge of getting exactly what you want. Even now, narrowband coating single cavity filters are an option, but are costly, but they're not perfect. They are done in batches and some are close and some are poor. But, if you wanted to design a new filter system for ultra narrowband and not just re-invent the wheel with etalon uses, there's two ways to approach it that are not terribly expensive...

One way is to use a single etalon system and then use a very narrowband blocking filter (instead of 5~6A, consider if it were 1A).

Another way is to avoid etalons in general and just look at creating single cavity filters with coatings for HA and stacking several of them. If they're close to 1A, you can generate a working system. These exist, they're just expensive, and usually difficult to make but we are better at it now than 20 years ago.

Recently at the amateur level this was done with Calcium, stacking non-etalon near 1A filters to generate a system that could easily show the disc and the chromosphere limb in calcium with spicules and prominences. Only a few were made because it was expensive, but that was for a single person purchasing all of it. As a company with more buying power and investment ability, a much larger batch could be done to reduce cost and do hard coated filters. This idea can be applied to any wavelength of interest.

Something to keep in mind is that you don't really need a filter system that produces 0.3A or 0.5A or something. All you actually need is to suppress the wing transmission profile and keep the center wavelength higher transmission. You can use whatever filter type system you want. But this concept is all that you really need to think about mostly... how to knock down the off band transmission.

Very best,


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Re: Designing a new etalon for halpha

Post by Bob Yoesle »

The realities of commercial production of etalons for consumer-grade recreational solar astronomy etalons is what I believe the issue really is. I and a few others are or have been lucky enough to own some of the better commercial etalon examples with outstanding performance, but these were not cheap in their day, and even less so now. These days it has become more quantity over quality, as these are more profitable. Good to excellent but low volume etalon examples are not.

The laws of etalon physics, coatings, and of optical fabrication are very well established, and as Christian's detailed explanation states, there's no free lunch. Etalon plate flatness, smoothness, gap uniformity, substrate uniformity, refractive index, acceptance angle optimization, temperature stability, etc. are all parameters which are optimized in professional research-grade etalon systems. Production methods are usually an order of magnitude more sophisticated and/or time consuming and difficult, and can include using state-of-the-art methods like fluid-jet polishing and coating technologies, and therefore they are very expensive for the average person.

Moreover, we're not doing science. What would be the point of supplying such a high level of etalon quality - even if it was affordable - to us folks who's purpose mostly seems to be posting pretty pictures on solar forums for personal enjoyment? And heck, many of these images appear to be obtained by folks who have never really looked at the Sun or learned what an on-band H alpha image should look like.

You will discover all of this once you get up to speed with those who are more knowledgeable and qualified in the "art" of etalons. For better or for worse, right now you're dreaming of castles in the sky held up by unobtainium.

Let us know when you get to Pandora ;-)

Bob


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Re: Designing a new etalon for halpha

Post by Bastelhannes »

I love to dream. And I do not accept limits juste because these are there. Currently I am checking the costs to calculate the investment. This is done by setting up some parameters and requirements and I talk to manufacturers.

I love Marty's idea of stacking filters. But they will be tilt-tuned, or am I getting it wrong?


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Re: Designing a new etalon for halpha

Post by MalVeauX »

Bastelhannes wrote: Mon Sep 12, 2022 2:14 pm
I love Marty's idea of stacking filters. But they will be tilt-tuned, or am I getting it wrong?
Yes, they could be tilt tuned, unless you design them to work in a pressurized cell, both are feasible. Outside of the challenges of production, which is totally possible, it's more also working out reflections while keeping transmission high. I had a nice opportunity to test a single etalon along with two 1.7A (approx) stacked and was able to get close to removing the double limb with a definitive increase in contrast. Getting it down below 1.5A or down to 1A would have removed the double limb (definitely at 1A). It's possible to do this without an etalon, just very costly to have made.

Check out this thread:

viewtopic.php?t=30810

I think if you could work out a fairly reliable process of getting ~1A filters made via coatings that could be further tuned with minor tilt and/or pressure in a chamber, that you could further attenuate the bandpass by suppressing the wing transmission profile with similar filters. Or, just use a single etalon and then attenuate with narrower blocking filters stacked like in my examples above, but imagine just having 1A or even 1.2~1.5A filters.

As you can see there, the bandpass didn't get more narrow. The system was just more selective for the CWL of HA. The 1.7A and 6A blocking filters just further suppressed photosphere light allowing higher contrast on the CWL of HA. This was with wider bandpass filters. If I had 1A filters, two of them stacked should be able to produce a no double limb high contrast result, without the complexity of tuning two etalons with jacquinot spots, etc. This greatly simplifies things as you can see with rather promising results.

I would love to test a system with no etalons, there's already working models out there with calcium. But this above model already is close with 1 etalon and two stacked blocking filters. But I think it would be great to try no etalon and use today's tech to produce better single cavity filters with coatings and make selective systems.

Very best,
Last edited by MalVeauX on Mon Sep 12, 2022 2:48 pm, edited 1 time in total.


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Re: Designing a new etalon for halpha

Post by rsfoto »

Bob Yoesle wrote: Mon Sep 12, 2022 1:54 pm The realities of commercial production of etalons for consumer-grade recreational solar astronomy etalons is what I believe the issue really is. I and a few others are or have been lucky enough to own some of the better commercial etalon examples with outstanding performance, but these were not cheap in their day, and even less so now. These days it has become more quantity over quality, as these are more profitable. Good to excellent but low volume etalon examples are not.

The laws of etalon physics, coatings, and of optical fabrication are very well established, and as Christian's detailed explanation states, there's no free lunch. Etalon plate flatness, smoothness, gap uniformity, substrate uniformity, refractive index, acceptance angle optimization, temperature stability, etc. are all parameters which are optimized in professional research-grade etalon systems. Production methods are usually an order of magnitude more sophisticated and/or time consuming and difficult, and can include using state-of-the-art methods like fluid-jet polishing and coating technologies, and therefore they are very expensive for the average person.

Moreover, we're not doing science. What would be the point of supplying such a high level of etalon quality - even if it was affordable - to us folks who's purpose mostly seems to be posting pretty pictures on solar forums for personal enjoyment? And heck, many of these images appear to be obtained by folks who have never really looked at the Sun or learned what an on-band H alpha image should look like.

You will discover all of this once you get up to speed with those who are more knowledgeable and qualified in the "art" of etalons. For better or for worse, right now you're dreaming of castles in the sky held up by unobtainium.

Let us know when you get to Pandora ;-)

Bob

Hi Bob,
... or learned what an on-band H alpha image should look like.
This has always intrigued me and I ask now, where can I see a 100% on band H-alpha image ?

Thanks and regards Rainer


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Re: Designing a new etalon for halpha

Post by christian viladrich »

rsfoto wrote: Mon Sep 12, 2022 2:46 pm
Hi Bob,
This has always intrigued me and I ask now, where can I see a 100% on band H-alpha image ?

Thanks and regards Rainer
Hello Rainer,
In you have a spectrograph, you can tune your filter system right on Ha. This is what I have done with my double stack system and a Sol'Ex SHG. Here is an example of result right on Ha :
http://astrosurf.com/viladrich/astro/so ... -B1920.jpg

and many others here (check the images with the DayStar PE 0.6 A and the SMN-35) :
http://astrosurf.com/viladrich/astro/so ... _2022.html

Of course, apart of the question of being centered right on Ha, you will have different results depending on the bandpass of the filter used, and on the number of filters stacked.
In any case, Sol'Ex SHG is a very nice tool to check this out.


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Re: Designing a new etalon for halpha

Post by christian viladrich »

Bastelhannes wrote: Mon Sep 12, 2022 2:14 pm
I love Marty's idea of stacking filters. But they will be tilt-tuned, or am I getting it wrong?
Tilting is just one of the options. In comes to a cost : banding, broadening of the FWHM, reduction of peak transmission.

Another option is to thermo-regulated the filter(s), which leads to the interesting question of the CWL drift with temperature. If the CWL is too stable, you won't be able to change it with a temperature offset.

Compression is only valid for air-spaced etalons.

And as said by Marty, if you stack filters, you have to deal with ghost images, which could be a nightmare. It could be worth to have a look at professional papers studying this.

Among the many questions related to a filter system design, there is also the question of "how to check whether the etalon is up to its spec". If you don't have a spectro, there is no way to do this.


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Re: Designing a new etalon for halpha

Post by Bob Yoesle »

This has always intrigued me and I ask now, where can I see a 100% on band H-alpha image ?
Hi Rainer,

I was referring to a CWL (central wavelength) of the filter system being tuned to be on-band, and this is not the same thing as a 100% on-band image.

A 100% on-band image would have no parasitic continuum leaking through the transmission profile wings. Some double or triple stacking schemes can come close.

This will be readily observable by the underlying photosphere becoming invisible and thus the absence of a "double limb." Bray and Loughhead defined this "pure chromosphere" as being within +/- 0.65 A of 6562.8 A. I have conservatively rated my Tucson era SM90 double stacked system as being pretty close to achieving this when centered on-band.

Surge prom.jpg
Surge prom.jpg (183.35 KiB) Viewed 4703 times

Note there also is a total absence of small sunspots and their penumbrae, as well as mottles and rosettes which begin to show up when the CWL is shifted off-band, or any underlying photosphere granulation.

Also note FWHM means essentially nothing, and transmission profile is everything. If you had an etalon system with 4-5 cavities, good peak transmission, and essentially a square-wave profile, it could have a FWHM of 1.0 A and essentially be perfect. This is also why a double stacked filter system with a FWHM of 0.5 A will be far superior to a single filter with a 0.3 A FWHM.

With regard to getting tuning on-band, you can remove the eyepiece and view the etalon(s) through just the blocking filter. This will essentially be the same as Christian's collimator etalon test, and when the image is brightest and most uniform you will almost certainly be on-band. It will also reveal any etalon gap uniformity issues.

Here's my simulation.

Etalon uniformity.jpg
Etalon uniformity.jpg (101.45 KiB) Viewed 4703 times

I use this tuning method quite frequently after the etalons have settled down and thermally stabilized...

Bob


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Re: Designing a new etalon for halpha

Post by Bastelhannes »

And as said by Marty, if you stack filters, you have to deal with ghost images, which could be a nightmare. It could be worth to have a look at professional papers studying this.
Hm... that was my fear. Additionally : we have more surfaces, if we use more than three stacked narrowband filters (compared to an etalon with a block filter).

But... we had some problems with filters for the RASAs due to the incident angles of the light. You can desging a filter, which has its CWL under a specified angle. Would be an interesting device...


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Re: Designing a new etalon for halpha

Post by Dennis »

"Exploding exposuretimes" is surely not what i want for imaging, since this would set us back to the stoneage with lucky imaging. I actually would be grateful for more transmission instead, to be able to reduce the gain levels of the cam.


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Re: Designing a new etalon for halpha

Post by christian viladrich »

Dennis wrote: Wed Nov 02, 2022 10:53 am "Exploding exposuretimes" is surely not what i want for imaging, since this would set us back to the stoneage with lucky imaging. I actually would be grateful for more transmission instead, to be able to reduce the gain levels of the cam.
This could be acheived by using a double-stack fused-silica etalon.

Transmission of mica-spaced etalon is rather low, because of the polarizing filters needed, and also because of mica transmission.

On the opposite, air-spaced and fused-silica spaced etalons have good peak transmission.

The advantage of fused-silica versus air-spaced etalon is the larger index of the fused-silica (meaning lower FWHM broadening in telecentric beam).


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Re: Designing a new etalon for halpha

Post by Radon86 »

christian viladrich wrote: Wed Nov 02, 2022 7:05 pm
Dennis wrote: Wed Nov 02, 2022 10:53 am "Exploding exposuretimes" is surely not what i want for imaging, since this would set us back to the stoneage with lucky imaging. I actually would be grateful for more transmission instead, to be able to reduce the gain levels of the cam.
This could be acheived by using a double-stack fused-silica etalon.

Transmission of mica-spaced etalon is rather low, because of the polarizing filters needed, and also because of mica transmission.

On the opposite, air-spaced and fused-silica spaced etalons have good peak transmission.

The advantage of fused-silica versus air-spaced etalon is the larger index of the fused-silica (meaning lower FWHM broadening in telecentric beam).
Very interesting...!!

Magnus


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