Coronado etalon finesse peaks far from H-A

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piekielrl
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Coronado etalon finesse peaks far from H-A

Post by piekielrl » Wed Jan 13, 2016 12:56 am

In the recent past, I've seen some posts here where members with access to a spectrograph have shown the finesse graphs of Coronado etalons. The range has always been close to the Ha band. For those of you who have done this or seen these, how far do the peaks go into the visual spectrum? Do they go all the way to the blue-violet? The reason I'm asking this is that if these other peaks coincide with other significant solar-viewing bands, i.e.: HeD3, Na, CaK, etc., it should be possible to use one of these etalons (with proper blocking and trimmer filters) to make a tunable solar scope in other wavelengths besides Ha.
Can anyone enlighten us here about the overall finesse of these etalons?

Thanks!
Bob Piekiel

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Merlin66
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Re: Coronado etalon finesse peaks far from H-A

Post by Merlin66 » Wed Jan 13, 2016 1:18 am

Bob,
In theory you are correct!
The "comb-tooth" band passes extend across the whole spectrum.
I uploaded a wide bandwidth image (spectroscopic) a couple of years ago based on the SM60 etalon.
There's probably a technical method of determining the resonance spacing which could give a "first approximation" to the "target" wavelengths - getting suitable blocker is a different challenge.
(I'm hoping that the spectroheliograph will supersede the need for etalons/ blockers and give detailed solar images at ANY nominated wavelength and ANY nominated bandwidth)
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Valery
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Re: Coronado etalon finesse peaks far from H-A

Post by Valery » Wed Jan 13, 2016 4:48 am

Merlin66 wrote: (I'm hoping that the spectroheliograph will supersede the need for etalons/ blockers and give detailed solar images at ANY nominated wavelength and ANY nominated bandwidth)
The only problem remains to solve is an image quality par with an etalon.
All the pictures taken with SHG I saw, were of low resolution.


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Spectral Joe
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Re: Coronado etalon finesse peaks far from H-A

Post by Spectral Joe » Wed Jan 13, 2016 5:50 am

With good conditions they can do pretty well.
112115_dcv_usm.jpg
Variable seeing during the scan can be a real problem. On the other hand, no (affordable) etalon system gives comparable spectral purity to a properly constructed spectroheliograph.

As for peaks far from the design wavelength, the coatings on the etalon are optimized for the design wavelength, the reflectivity drops off quickly as you move away in wavelength and with it the finesse.
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Re: Coronado etalon finesse peaks far from H-A

Post by Merlin66 » Wed Jan 13, 2016 6:03 am

As Joe says, the SHG can give some creditable images....
I think the key to future improvements is the fast scanning rate (of cameras like the ASI174) and then combining a series of scans (taken within seconds of each other) to improve the SNR and allow stacking/ wavelet enhancement.....
Watch this space.
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Re: Coronado etalon finesse peaks far from H-A

Post by marktownley » Wed Jan 13, 2016 6:40 am

I've tried before with my double stack etalons removing the Ha ERF and blocker and using the CaK filter at the rear of this; it passes CaK light, but offers no tighter bandpass than just using the CaK filter on its's own with no etalons. The downside was reduced transmission and longer exposure.
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