Comparison of Sepctroheliograph, Sundancer2 and Cak Lunt module

Use this section to discuss "standard" Baader/Coronado/ Lunt SolarView/ Daystar, etc… filters, cameras and scopes. No mods, just questions/ answers and reviews.
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Comparison of Sepctroheliograph, Sundancer2 and Cak Lunt module

Post by Bastelhannes »

Hi folks,

I am not sure, if this is the right section, because I am mixing up methods, DIY vs. commercial and differente wavelengths.



Comparison of three ways to observe prominences



On my balcony observatory I have three possibilities to view the Sun in dedicated wavelengths. 

Using prominences as an example, three fundamentally different solutions will be presented here and the imaging procedure will be described.





Solution 1: Spectroheliograph



For quick info, here are the details of the setup:

The basic telescope is a modified Skywatcher Explorer 150 PDS Newtonian, which has had its primary mirror decoated. So 97% of the incoming solar radiation is no longer reflected, but only 4%. Therefore the spectroheliograph does not need a solar filter like the astrosolar foil from Baader [Link]. 

(Attention: please never use this modified Explorer 150 PDS visually without a solar filter; for photography a ND3.0 filter is needed in front of the camera!)


Data of the Explorer 150 PDS:

Aperture: 150 mm
Focal length: 750 mm
Main mirror decoated

01_s.jpeg
01_s.jpeg (173.67 KiB) Viewed 2905 times
The spectroheliograph Sol'Ex is adjusted to the wavelength H-Alpha (656.28 nm) by rotating the diffraction grating (2400 lines/mm).

The light from the Sun is imaged through a 10 µm slit onto the camera sensor. The narrow strip of the H-alpha line is scanned across the sensor so that software can calculate a complete image of the Sun from it.

With Sharpcap (Firecapture, ToupSky and other software also work) the following steps are done:



1. this is what the image looks like to the camera sensor. The horizontal stripes are the individual absorption lines of sunlight, the prominent dark line is that of H-alpha. The vertical stripes are those of impurities or irregularities of the slit, but also structures on the solar surface.
Spektrum01_s.jpeg
Spektrum01_s.jpeg (48.25 KiB) Viewed 2905 times
2. the area of the H-alpha line is selected using ROI function:
Spektrum02.jpg
Spektrum02.jpg (80.51 KiB) Viewed 2905 times
3. tracking is turned off and thus this line is scanned over the sensor. A SER or AVI movie is saved


4. the software INTI calculates the different images, e.g. prominences and sun surface
Sol01_s.jpeg
Sol01_s.jpeg (11.8 KiB) Viewed 2905 times
So the sun can not be seen live with this method, it has to be calculated first.


The images have a very high contrast and any desired wavelength can be set.

The spectroheliohgraph can be built by the user and increases the understanding and experience enormously. Due to its pure reflecting optics, the Newtonian also allows investigations outside the visible spectrum in the UV range.



The prominence of 30.08.2022 in the spectroheliograph:
Sol_Prot_s.jpeg
Sol_Prot_s.jpeg (11.03 KiB) Viewed 2905 times




Solution 2: A dedicated H-alpha filter



A Sykwatcher Evostar 120 is equipped with a DERF 110 mm (energy reflection filter) and a SolarSpektrum Sundancer 2. Since this is a monochrome application, an achromat is quite sufficient; an apochromat is not necessary here.
Teleskope_s.jpeg
Teleskope_s.jpeg (104.82 KiB) Viewed 2905 times
Evostar 120 [https://www.teleskop-spezialisten.de/sh ... ::233.html]

DERF110 [Link not available currently]

Adapter DERF110 auf Evostar 120 [https://www.teleskop-spezialisten.de/sh ... :5686.html]

Solar Spectrum Sundancer 2 [https://www.teleskop-spezialisten.de/sh ... :5331.html]



Data Evostar 120

Aperture 120 mm (but stopped down to 110 mm aperture by DERF110)
Focal length 900 mm (the Sundancer extends this to 2700 mm).


The probuberance of 30.08.2022 in the Solar Spectrum Sundancer 2

Sundancer_Prot_s.jpeg
Sundancer_Prot_s.jpeg (11.17 KiB) Viewed 2905 times






Solution 3: CaK module from Lunt



A TS-Optics 102mm f/11 ED refractor (it went bad in the exhibition and had to go away there) is equipped with a CaK module from Lunt.



Setup:

TS-Optics 102mm f/11 ED refractor [https://www.teleskop-spezialisten.de/sh ... :4153.html].
CaK module [https://www.teleskop-spezialisten.de/sh ... :4416.html]



Data TS-Optics 102mm f/11 ED refractor :

Aperture: 102 mm
Focal length: 1122 mm

A few notes about this setup:

The CaK module can be used up to 100 aperture of the telescope without objective filters. DA the CaK wavelength is 393 nm, the resolution is much better than with H-Alpha. A 100 mm telescope aperture is quite sufficient to see fine details on the solar surface.
Teleskope_s.jpeg
Teleskope_s.jpeg (104.82 KiB) Viewed 2905 times
Now we are dealing with the white, rear telescope.



The following image is a raw image(!) directly from Sharpcap:
CaK_s.jpeg
CaK_s.jpeg (55.74 KiB) Viewed 2905 times
When the contrast, exposure and a few other small parameters are adjusted appropriately, the prominences become visible in the image shown above (image detail is from top left):
Cak-Prot.jpeg
Cak-Prot.jpeg (11.33 KiB) Viewed 2905 times

Bastelhannes
www.teleskop-spezialisten.de
Last edited by Bastelhannes on Thu Sep 01, 2022 9:32 am, edited 1 time in total.


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Re: Comparison of Sepctroheliograph, Sundancer2 and Cak Lunt module

Post by Bastelhannes »

The history of the Spektroheliograph:

The detailed development of this homebuilt project is described in these reports:

part 1: https://www.teleskop-spezialisten.de/sh ... anguage=en

part 2: https://www.teleskop-spezialisten.de/sh ... anguage=en

part 3: https://www.teleskop-spezialisten.de/sh ... anguage=en


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Re: Comparison of Sepctroheliograph, Sundancer2 and Cak Lunt module

Post by Montana »

This is great summary :hamster: however, the links don't work :( All you need to do is add the link to the text and it will work fine, no need to add within brackets.

Thanks for the summary. Do you sell the SolEx ready made? this would be of interest to the readers :)

Alexandra


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Re: Comparison of Sepctroheliograph, Sundancer2 and Cak Lunt module

Post by Bastelhannes »

Christian Buil created the Sol'Ex. I reached out to him asking if we could offer ready built Sol'Ex or the printed parts. I unfortunately never got an answer. He made such an effort creating this project, I really do not want to mess up with him.

Hm.. the links in the report are not working. I forgot to activate them...


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Re: Comparison of Sepctroheliograph, Sundancer2 and Cak Lunt module

Post by marktownley »

That's an interesting comparison Karsten!


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Re: Comparison of Sepctroheliograph, Sundancer2 and Cak Lunt module

Post by thesmiths »

Bastelhannes wrote: Thu Sep 01, 2022 7:08 am The basic telescope is a modified Skywatcher Explorer 150 PDS Newtonian, which has had its primary mirror decoated. So 97% of the incoming solar radiation is no longer reflected, but only 4%.
I've been thinking of installing a SHG on a Sky-Watcher Heritage-150P Flextube with de-aluminised mirror. What kind of exposure and gain can you get with the 178MM camera?


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Re: Comparison of Sepctroheliograph, Sundancer2 and Cak Lunt module

Post by Bastelhannes »

Beside the problem of the very unstable focuser of the Heritage 150P (this will NOT carry the SHG stable enough) the exposure times are about 12-15 ms, gain 210. Usually I do not use gain, then the exposure times are about 30 ms.


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Re: Comparison of Sepctroheliograph, Sundancer2 and Cak Lunt module

Post by thesmiths »

Bastelhannes wrote: Thu Sep 01, 2022 5:07 pm The exposure times are about 12-15 ms, gain 210. Usually I do not use gain, then the exposure times are about 30 ms.
Oh that's a really long exposure. I'm used to using exposures around 1-3 ms with no gain. Thanks for sharing the info. I'll have to reconsider.

I know about the focuser issue. It also rotates. I was planning to install a helical focuser.


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Re: Comparison of Sepctroheliograph, Sundancer2 and Cak Lunt module

Post by Bastelhannes »

The 4% reflection would lead to short exposures, but the small slit and the deployment of the flux to the complete grating loses a lot of light.

You will not need to replace the focuser itself only, but the whole construction. The little L shaped construction is too weak. Some customers are complaining even using a planet camera only (the cable misaligns the focuser already).


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Re: Comparison of Sepctroheliograph, Sundancer2 and Cak Lunt module

Post by iseegeorgesstar »

Thank you for the write up. Very helpful. The single-line spectrograph in the first one was really cool.

I have a question.
A Sykwatcher Evostar 120 is equipped with a DERF 110 mm (energy reflection filter) and a SolarSpektrum Sundancer 2. Since this is a monochrome application, an achromat is quite sufficient; an apochromat is not necessary here.
I've seen this said before. Even though you're looking at one limited wavelength (halpha) you don't need multiple lens, wouldn't though the polish of lens make a difference? A lens with a higher strehl or RMS wave wouldn't give a sharper/crisper image?


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Re: Comparison of Sepctroheliograph, Sundancer2 and Cak Lunt module

Post by Bastelhannes »

That is a good question we are discussed internally some days ago.

I should get a really good single lens optimized for h-alpha and do some tests...


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Re: Comparison of Sepctroheliograph, Sundancer2 and Cak Lunt module

Post by Bob Yoesle »

I've seen this said before. Even though you're looking at one limited wavelength (halpha) you don't need multiple lens, wouldn't though the polish of lens make a difference? A lens with a higher strehl or RMS wave wouldn't give a sharper/crisper image?
Not really.

Polish is one thing (naturally - but most likely most pertinent to a coronagraph), strehl is another.

1) Daytime seeing is seldom going to be at a level where a strehl over 0.8 (1/4 wave) would be readily identifiable.

2) For a singlet lens, a custom designed one-off lens with antireflection coatings would likely cost more than a well-corrected off-the-shelf long-focus doublet. Additionally, if one can find out or determine the optical formula for the doublet lens, simply re-spacing the lens elements can improve the correction to become diffraction limited at about any wavelength.

3) For any wavelength other than the e line where most telescopes have their optimum correction, spherochromatic aberration will begin to dominate and decrease the strehl at that wavelength. Therefore a long-focus f15 refractor with a strehl of 0.8 would more than likely surpass an f7 APO with a strehl of 0.9 or better. The spherochromic correction for CaK is even more critical than for H alpha.

Spherochrom aberration.png
Spherochrom aberration.png (487.5 KiB) Viewed 2653 times
telescope-optics.net


And with regard to comparing SHG imaging to traditional optical filters; a SHG is a wonderful instrument / DIY project and allows almost any wavelength to be imaged at very narrow band-passes. It can also be employed for filter testing and tuning processes. On the other hand, the traditional continuum, CaK, and H alpha wavelengths seem to me to be the most revealing and interesting for both visual observation and imaging.

The other aspect is the limited slit-width resolution that is generally available with a SHG, and the rather complex software manipulation that is required to synthesize images of higher resolution.

Even with the best SHG images I have seen, seldom (if ever) could one achieve the resolution of even modest apertures, as seen below in these CaK views with a small but well-corrected ED100/900 instrument:

CaK w ED100-900.jpg
CaK w ED100-900.jpg (448.79 KiB) Viewed 2653 times

A SHG is wonderful for doing a broad range of wavelengths at high spectral resolution, and can be employed for spectroscopic studies. Optical filters will pretty much be superior for higher spatial resolution imaging, as well as visual observation.

Bob


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Re: Comparison of Sepctroheliograph, Sundancer2 and Cak Lunt module

Post by thesmiths »

Bob Yoesle wrote: Tue Sep 13, 2022 3:08 pm The other aspect is the limited slit-width resolution that is generally available with a SHG, and the rather complex software manipulation that is required to synthesize images of higher resolution.

A SHG is wonderful for doing a broad range of wavelengths at high spectral resolution, and can be employed for spectroscopic studies. Optical filters will pretty much be superior for higher spatial resolution imaging, as well as visual observation.
Hi Bob, I disagree with you that software manipulation is these days very complex. Your link to Phillipe Rouselle's webpage from 2003 is very out of date. The SHG reconstruction software that we and others have worked on in the last two years is highly automated (see e.g. viewtopic.php?t=37066).

Of course SHG is not a visual technique but with the recent development of workflow to do stacking, the spatial results are not that different than filters. See the image below of a full disk image taken at the Ca-H wavelength, 72mm aperture, f10. A stack of 9 frames.

CaH SHG, 72mm, stack of 9
CaH SHG, 72mm, stack of 9
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Re: Comparison of Sepctroheliograph, Sundancer2 and Cak Lunt module

Post by Bob Yoesle »

Hi Doug,

I stand corrected - that's a spectacular full disc image and the best I now have seen.

Thanks for the link (it didn't come up in a google search), and I'm glad to hear the software processing is much improved. Looking forward to seeing how far you can take angular resolution...

Bob


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Re: Comparison of Sepctroheliograph, Sundancer2 and Cak Lunt module

Post by thesmiths »

Bob Yoesle wrote: Tue Sep 13, 2022 5:42 pm Looking forward to seeing how far you can take angular resolution.
Fundamentally, I don't think the SHG angular resolution limit is that much different than with a filter. Apart from the main imaging lens (telescope), there are additional optical components in an SHG (collimator and camera lenses) but narrowband optical filters are also quite complex optical systems.

One area I would say impacts the practical resolution is the fact that the data acquisition rate is quite slow, requiring more stable atmospheric conditions. It's not possible to take 10% of the best 1000 frames; it more like taking 50% of the best 20.

I would say, however, that the increased contrast (resulting from the narrow bandwidth) gives the appearance of increased spatial resolution. I think the same thing is seen with double stacking optical filters. As with a microscope, the issues of spatial resolution and contrast in image formation is a somewhat complicated discussion.


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