Year-end solar eye candy

[Philippe Fossier]

Spectroheliographs, such as Sol'Ex and the MLAstro SHG 700 decompose the sun's light into its component wavelengths and the solar disk is imaged by scanning across the disk using a small pixel camera and a very narrow ROI - typically 4000x200 pixels.

When imaging, using SharpCap, one only sees the Hydrogen-alpha spectral line at 656nm and the ROI getting illuminated as one scans along the RA axis. The full solar disk is reconstructed using specialized software.

Here is a sample animation of the solar disk imaged with the MLAstro SHG 700 on Dec 4th from Mt Tam, at 2,400ft using a Lunt LS100MT and ASI678MM.

Animation on Astrobin:

https://app.astrobin.com/i/72cbmd

This is a snapshot of the same (although the animation is way better):

[Surya Rao]

Wow! The animated sun's image is fantastic! 

Nice work Philippe!

Surya

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[Joe Lin]

Really neat. Why does the animated version wobble like that? The sun seems to kind of zig zag?

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[Philippe Fossier]

Good question.

The wobbling is not meant to represent the natural motion of the sun, just to show how the sphere can be rotated any which way you may wish.

The developer of he software calls this a wigglegram.

When in the actual JSol'Ex software, you can actually rotate the sun any way you want.

Hope this answers your question.

~ philippe

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[Akarsh Simha]

Absolutely spectacular. How do you get the "back side" of the sun? I can imagine that you could get the "back side" along the equator because of the rotation of the sun, but I don't understand how the back of the pole is seen.

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[Philippe Fossier]

Yes... well, I am chatting with the JSol'Ex developer via the JSolEx forum in Discord and this is what he tells me:

— 1:13 AM

how do you reconstruct enough of the solar sphere to allow some rotation and show what lies behind the horizon?

Ah ah, that is key to understand and the next release will have an animation to explain this. There's no "what lies behind the horizon", it's your brain lying to you. You need to understand that when we look at the sun from Earth, there is so much distance that the light rays arrive almost parallel. So what we see of the sun is not its true geometry, but an orthographic projection, a bit like a globe drawn on a sheet of paper. Everything we see from the sun is actually on a hemisphere (with the subtlety of filaments facing us that we cannot render properly). The algorithm simply remaps this 2d view onto a globe, as it should be. So the projection that you see actually shows the regions of the sun at their true scales. The fact that you have the impression that things go behind the horizon is your brain lying to you. Everything is visible, but squeezed so much they become invisible.

~ philippe

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[Akarsh Simha]

Ah, that makes sense. The software is accounting for foreshortening, bet still rendering just a hemisphere.

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[Akarsh Simha]

On Thu, Dec 18, 2025 at 1:00 PM Philippe Fossier <philippe...@gmail.com> wrote:

Spectroheliographs, such as Sol'Ex and the MLAstro SHG 700 decompose the sun's light into its component wavelengths and the solar disk is imaged by scanning across the disk using a small pixel camera and a very narrow ROI - typically 4000x200 pixels.

When imaging, using SharpCap, one only sees the Hydrogen-alpha spectral line at 656nm and the ROI getting illuminated as one scans along the RA axis. The full solar disk is reconstructed using specialized software.

Here is a sample animation of the solar disk imaged with the MLAstro SHG 700 on Dec 4th from Mt Tam, at 2,400ft using a Lunt LS100MT and ASI678MM.

This is very interesting, but why do you need the Lunt? Is the problem that the resolution produced by the grating isn't sufficient to show the prominences, and that's why you need the etalon on the Lunt for a narrow line-width?

Regards

Akarsh

 

Animation on Astrobin:

https://app.astrobin.com/i/72cbmd

This is a snapshot of the same (although the animation is way better):

[Akarsh Simha]

On Fri, Dec 19, 2025 at 2:03 AM Akarsh Simha <akars...@gmail.com> wrote:

On Thu, Dec 18, 2025 at 1:00 PM Philippe Fossier <philippe...@gmail.com> wrote:

Spectroheliographs, such as Sol'Ex and the MLAstro SHG 700 decompose the sun's light into its component wavelengths and the solar disk is imaged by scanning across the disk using a small pixel camera and a very narrow ROI - typically 4000x200 pixels.

When imaging, using SharpCap, one only sees the Hydrogen-alpha spectral line at 656nm and the ROI getting illuminated as one scans along the RA axis. The full solar disk is reconstructed using specialized software.

Here is a sample animation of the solar disk imaged with the MLAstro SHG 700 on Dec 4th from Mt Tam, at 2,400ft using a Lunt LS100MT and ASI678MM.

This is very interesting, but why do you need the Lunt? Is the problem that the resolution produced by the grating isn't sufficient to show the prominences, and that's why you need the etalon on the Lunt for a narrow line-width?

Makes me wonder if anyone has successfully set up an echelle grating and obviated the need for an expensive solar telescope to image prominences.

[Philippe Fossier]

Good question. 

The Lunt LS100MT is a 100mm f/7 refractor which can be used with or without etalons. With the MLAstro SHG, I do not use etalons.

~ philippe

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[Philippe Fossier]

This the Lunt without etalons and with the MLAstro SHG.

I can attach etalons for Ha, a Herschel wedge for white light, a Ca-K module for that light type or nothing at all and do night-time observing.

Here the setup with just the MLAstro SHG:

With Herschel wedge (white light):

~ philippe

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[Joel Lee]

Awesome images Philippe! I recall seeing one of your images at Chabot earlier in the year where I think you were part of a talk there? It was great listening to you and the others give the talk and seeing your solar image on the big screen!

I recall from a couple of videos that the Spectroheliographs were sometimes subject to warping when the image was finally composed. I was wondering how did you get around that? Is that a seeing conditions related issue or a mount related issue? How would you compare them vs etalons single and double stack? 

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[Philippe Fossier]

Solar scope vs spectroheliograph:

With solar, we're constantly working with tradoffs:

Lunt makes great equipment (consistent quality, easy to engage and work with) and I am happy with my setup but....

- Lunt telescopes with a smaller aperture (60-80mm) are best to capture the entire solar disk but more challenged in getting sufficient details when zooming onto surface features 

- The LS100MT shines when zoomed onto surface features but gets finicky when trying to capture the entire solar disk due to the need to align the central monochromatic region of the 2 etalons,

   known as the Jacquinot spot. On the edges of that sweet spot, light intensity gradients appear which make the image unbalanced.

Spectroheliographs make capturing the entire solar disk in all it glory rather easy, particularly with the help of the JSolEx program which generates a plethora of 

additional images: doppler eclipses, negative images, annotated maps of the suns with all active regions, doppler images, etc...   it feels like being in a candy store...

However, you cannot zoom in and get an image of the same fine detail quality as a Lunt or a good Coronado.

On the plus side, they are very inexpensive - clearly less than $1000.

On the minus side, learning to use them correctly will take time and practice.

  

Using a spectroheliograph:

Imaging with a spectroheliograph can require a fair amount of patience:

- Three collimation knobs must be adjusted on the unit itself:

1.    spectral line tuning
2.    camera/slit collimation
3.    ROI & vertical dust lines collimation

- telescope focuser

- ensuring a minimum "tilt" which requires aligning the SHG with the vertical axis of the telescope very precisely

- X/Y ratio, the most elusive tuning, which consists of adjusting the camera frame rate to the slew speed

- Another challenge is that the SharpCap histogram is of little use to help determine the right exposure

  it takes some trial an error (JSolEx has an exposure calculator but I have found it unreliable).

You then manually scan along the RA axis and record while keeping JSolEx running to check the geometry, tilt and X/Y ratio

of the data just captured.

Some automation can be had via a python script running in SharpCap to capture the data and move the mount for you.

Ideally, you want to get 10 clean runs in as short a time as possible, stack them and get a decent set of images.

Warping can happen when either tilt and/or X/Y ratio are off.

Of course "seeing" and wind must cooperate.

This is what you see in SharpCap when you scan along the RA axis:

Below is the result of about 10 scans taken on the shores of lake Tahoe this past September.

Cool morning air, cloudless sky, looking east as the sun rises over cold water - perfect conditions.

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[Joel Lee]

Thank you so much for the excellent breakdown Philippe! This opens quite a lot more considerations for solar than I previously thought. I had thought a 100 is a can-tackle-everything scope but it's just like deep sky with many tools for the many different tasks. 

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[Philippe Fossier]

Not sure my email response to your questions went through as I got an error while sending it. It might show up under "quoted text".

-Philippe 

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~ philippe

[Joel Lee]

It went through and I was able to see them on my side! Thanks again! I think the reply all replied to some other post for some reason. 

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[Shashi Sathya]

Wow.  Simply superb image Phillippe.  The video was so cool.  

Shashi.

[Philippe Fossier]

Thank you. Cedric Champeau, the JSol'Ex developer has a lot to do with it; His software reads the solar scan and produces many different renditions, including the animation.

~ philippe

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