Analysis of Fabry-Perot mica spaced Ha filters

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Analysis of Fabry-Perot mica spaced Ha filters

Post by DSobserver »

Here an interesting new analysis made by C.viladrich

http://www.astrosurf.com/viladrich/astr ... lar/FP.htm


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Re: Analysis of Fabry-Perot mica spaced Ha filters

Post by marktownley »

Thank you for that link, it is very interesting and shows exactly my experience with the Quark.


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Re: Analysis of Fabry-Perot mica spaced Ha filters

Post by Bob Yoesle »

An excellent analysis!

I especially appreciated the citation of Gene Baraff's work on the importance of using properly optimized telecentric lenses matched to the focal length of the objective - something generally not fully appreciated with these filters -- and why DayStar rightly does not provide a FWHM figure for the Quark.


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Re: Analysis of Fabry-Perot mica spaced Ha filters

Post by swisswalter »

Hi Vincent

thank you very much for the link. Tons of good pages to read, very informative


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Re: Analysis of Fabry-Perot mica spaced Ha filters

Post by pbsastro »

Bob Yoesle wrote:An excellent analysis!

I especially appreciated the citation of Gene Baraff's work on the importance of using properly optimized telecentric lenses matched to the focal length of the objective - something generally not fully appreciated with these filters -- and why DayStar rightly does not provide a FWHM figure for the Quark.
Yes, very interesting. I will need to read it carefully, but that makes important to confirm the Baader telecentric ideal FL. Bob, you mentioned 800mm at CN. Can you confirm that? I will be using it with 1400mm which would make it only acceptable, not very good.
In any case I think the edge degradation should be determined on the field edge, not sun edge, as on most scopes (specially on the 1400mm one), we can't get the full sun.

Pedro


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Re: Analysis of Fabry-Perot mica spaced Ha filters

Post by mdwmark »

The data looks good to me. There is nothing magical about F/30. It was just convenient in the early days because most scopes where F/15. To get to .3Ang you are usually starting below that. But you really need to be F/45 or longer. You will not get to .3Ang at F/30 regardless what you start with. The mica etalon does have the advantage over an air spaced etalon for a larger (sweet spot) at narrower bandwidths because of the difference in index of refraction.
Telecentric versus collimated:
Telecentric may broaden out the filter but it will broaden evenly. A collimated beam is easier to design but there's a wavelength shifts from the center to the edge. The HW will be the same, the center wavelength changes(shifts blue). Which gives you the sweet spot you see with this design. The sweet spot will be less noticeable the broader the HW is.
This is just how etalons work.
Mark W.


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Re: Analysis of Fabry-Perot mica spaced Ha filters

Post by Bob Yoesle »

Bob, you mentioned 800mm at CN. Can you confirm that? I will be using it with 1400mm which would make it only acceptable, not very good.
Hi Pedro,

That is the design focal length Mark Wagner provided - and he should know as the Baader telecentric units are made to be used with the Solar Spectrum line of filters.

Christian has posted another update to his first study, which shows once again the power of double stacking etalons, and how the CWL shifts with tilt for a collimated system. As Mark has stated this shift changes across the filter due to field angle magnification: http://www.astrosurf.com/viladrich/astr ... aystar.htm

In an optimized telecentric system, you only deal with uniformly broadened band-pass based on the F/D ratio light cone. In a non-optimized telecentric system, you likely will face both band-pass broadening and CWL shifting.


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Re: Analysis of Fabry-Perot mica spaced Ha filters

Post by swisswalter »

Hi Bob

thanks for the additional link


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Re: Analysis of Fabry-Perot mica spaced Ha filters

Post by Valery »

mdwmark wrote:The data looks good to me. There is nothing magical about F/30. It was just convenient in the early days because most scopes where F/15. To get to .3Ang you are usually starting below that. But you really need to be F/45 or longer. You will not get to .3Ang at F/30 regardless what you start with. The mica etalon does have the advantage over an air spaced etalon for a larger (sweet spot) at narrower bandwidths because of the difference in index of refraction.
Telecentric versus collimated:
Telecentric may broaden out the filter but it will broaden evenly. A collimated beam is easier to design but there's a wavelength shifts from the center to the edge. The HW will be the same, the center wavelength changes(shifts blue). Which gives you the sweet spot you see with this design. The sweet spot will be less noticeable the broader the HW is.
This is just how etalons work.
Mark W.
Hi Mark,

What must be the F/D of a telecentric system that Solar Spectrum 0.2A etalon will really deliver these 0.2A ?


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Re: Analysis of Fabry-Perot mica spaced Ha filters

Post by marktownley »

I have to say Christians work on this paper is one of the best resources available for pulling things together mathematically in a way that has been discussed loosely on the web for sometime now. The more I digest it the more it makes me think.


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Re: Analysis of Fabry-Perot mica spaced Ha filters

Post by pbsastro »

Bob Yoesle wrote:
Bob, you mentioned 800mm at CN. Can you confirm that? I will be using it with 1400mm which would make it only acceptable, not very good.
Hi Pedro,

That is the design focal length Mark Wagner provided - and he should know as the Baader telecentric units are made to be used with the Solar Spectrum line of filters.

Christian has posted another update to his first study, which shows once again the power of double stacking etalons, and how the CWL shifts with tilt for a collimated system. As Mark has stated this shift changes across the filter due to field angle magnification: http://www.astrosurf.com/viladrich/astr ... aystar.htm

In an optimized telecentric system, you only deal with uniformly broadened band-pass based on the F/D ratio light cone. In a non-optimized telecentric system, you likely will face both band-pass broadening and CWL shifting.
Thanks Bob. Then I think I will go with an optimized telecentric for my 1400mm refractor. The Baader will be good for my TV102.
I m also convinced to go double stacking, thanks to the double limb topics you have raised. Thanks! f I have been talking to Mark to see if can get me a double stack filter, he will check what he can do, in terms of reflections control, but it seems I will need two ovens/controllers.
Anyone wants to join me in this waggon :)

Pedro


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Re: Analysis of Fabry-Perot mica spaced Ha filters

Post by mdwmark »

To get to .2Ang you are really starting below that. They are in the .1Ang range. The focal ratio should be up in the F/50- F/60 range. Unless you are doing Doppler work I don't see any reason to be that narrow. That is why I changed to .3Ang specs. At these narrow HW filters the substrates need to better then 100th wave. It becomes a matter of how nature made the mica and the type of mirrors you have on the substrate.
Unlike an air spaced etalon you only have one shot at it. You can't go back and re-polish the flats to a better surface.
The .3Ang filters, I try to have them in that range at F/45-F/50 range. But really a .3Ang filter at F/35 still shows good detail.
To get the image size back down you really just need to use a long focal length eyepiece. I have even used a old 130mm telephoto lens backwards as an eyepiece.
The one advantage of the mica etalon is that you can use it as a variable HW filter, just by changing the focal ratio( going faster you need to increase the temperature also because it has shifted blue).
Mark W.


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Re: Analysis of Fabry-Perot mica spaced Ha filters

Post by marktownley »

mdwmark wrote:The one advantage of the mica etalon is that you can use it as a variable HW filter, just by changing the focal ratio( going faster you need to increase the temperature also because it has shifted blue).
I'd not thought of it like this before... You have my tinkering mind working in overtime ;) Thanks Mark.


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Re: Analysis of Fabry-Perot mica spaced Ha filters

Post by Bob Yoesle »

Christian has updated his analysis with a good treatment of air-spaced etalons, and illustrates some practical applications:

http://www.astrosurf.com/viladrich/astr ... P-Coro.htm

Fantastic information - should be added to the Library I should think (hint Mark ;) )


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Re: Analysis of Fabry-Perot mica spaced Ha filters

Post by swisswalter »

Thank you Bob


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Re: Analysis of Fabry-Perot mica spaced Ha filters

Post by Bob Yoesle »

Christian continues to update his analysis of filter performance. Here's his latest, which gets to a question Ken had raised about how blocking filters affect bandpass, as well as mixing different primary FWHM's:

http://www.astrosurf.com/viladrich/astr ... ntrast.htm


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Re: Analysis of Fabry-Perot mica spaced Ha filters

Post by marktownley »

Another great read from Christian!


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Re: Analysis of Fabry-Perot mica spaced Ha filters

Post by swisswalter »

Hi Bob

thank you very much, most interesting


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Re: Analysis of Fabry-Perot mica spaced Ha filters

Post by Bob Yoesle »

Christian just let me know he's added an update to the above Blocking Filter analysis - very interesting results using an DayStar 0.6 A with an Omega 4.5 A used as a BF... http://www.astrosurf.com/viladrich/astr ... ntrast.htm


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Re: Analysis of Fabry-Perot mica spaced Ha filters

Post by marktownley »

An interesting addition. I believe this is the principal of how I get the contrast gains that I do with my quark and naked PST etalon.


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Re: Analysis of Fabry-Perot mica spaced Ha filters

Post by Valery »

One more excellent addition to Christian's book.


Valery


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