Corona from 24 September 2022

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Re: Corona from 24 September 2022

Post by marktownley »

Very interesting, thanks!


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Re: Corona from 24 September 2022

Post by george9 »

Yes exactly, James! That yellow image in my first post is SDO 171A. It's been really helpful that SDO images the corona constantly, so that when you look in the scope and think you see something, you can go right on your smartphone and check if the satellite shows anything bright in the same area. If it matches SDO and disappears when you lower the filter temperature, then you are pretty sure you are looking at the corona.

This was especially important with my old converted Baader prominence viewer, which had some aberrations due to a faulty field lens coating (my fault for using that lens) that led to some subtle glows in the field of view.

There is also STEREO (https://stereo.gsfc.nasa.gov/beacon/), which shows you what the Sun will be up to in two days by letting you peak around the side of the Sun based on the satellite location. It helps with planning whether to drive to the mountain in two days. It is the green false colored image, which is 195A and shows the corona well enough. They also have 171A, but they don't show the current image for some reason.

We are getting to the point where we don't need the corona to be as bright to see it. Our goal is to image the corona all the way around the Sun.

George


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Re: Corona from 24 September 2022

Post by DeepSolar64 »

On a different note AIA1600 and CaK 3934a almost view the same layer of the Sun but with the former at a much shorter wavelength and views in the Carbon IV spectral line instead of Calcium. AIA304 images the Chromosphere but at a much shorter wavelength than Ha ( 6563a ) does. It images in the Helium II spectral line.

Anyway, back on topic. So I see a 540nm or broadband green filter is of no use in a coronagraph, right?

And to think during a solar eclipse the corona is much more easily and completely visible without a filter at all.

James


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Re: Corona from 24 September 2022

Post by george9 »

Right. I use 304A when I cannot tell which way my coronagraph is oriented and I don't want the Sun to drift out from under the cone. I get a shot in H-alpha, compare that to 304A to figure out my orientation, then when I shoot at 5303A, I can flip the 171A image around to the right spot without cheating (i.e., without forcing the 171A image to match my 5303 image).

Correct, 540nm won't be helpful.

To all, please correct the following as needed:

The corona looks white during an eclipse because you are seeing the K-corona near the Sun or F-corona further away, and they both have a broad spectrum. The E-corona or emission corona is dimmer than the K-corona but is limited to narrow wavelengths. For the coronagraph, there is still too much scattered light and while the K-corona may be brighter, the E-corona's energy is confined to those narrow bands and with our 1.2A filter, we can get a much better signal-to-noise ratio at 5303A than the K-corona. That is, a 1.2A filter eliminates all but 1.2/3000 of the background scatter yet much of the E-corona's energy is still passed by the filter in that strong band.

Klaus has been working on the K-corona and at least exploiting polarization, but no luck yet.

George


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Re: Corona from 24 September 2022

Post by DeepSolar64 »

George,
What would we do without those valuable SDO images :-)

The MLSO K-Cor Coronagraph uses polarization for it's magnificent corona images. It's among the best coronagraphs on Earth!

20221002_172801_kcor_l2_nrgf_cropped.gif
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Re: Corona from 24 September 2022

Post by Radon86 »

george9 wrote: Wed Sep 28, 2022 10:46 am Thanks very much. I will point out that I learned how to do it from Klaus160. And Christian's book was helpful with its easy-to-use formulas for the optical design. (And we all owe Lyot.) The hard part was debugging all the stray light in the system.

George
Where is this formula for the optical design.
I had difficulty finding this in the Solar Astronomy book.

What telescope are you using to apply the coronagraph ?

Thanks

Magnus


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Re: Corona from 24 September 2022

Post by Radon86 »

Is the corona harder to image than the Sun in H-alpha ?
A guide to this imaging sub speciality would be very helpful I think.

Are other solar imagers also interested ?

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Re: Corona from 24 September 2022

Post by george9 »

Hi. I am talking about Appendix 2 on page 234 of the Solar Astronomy book English version. I know there were additions, so not sure if the French version has Appendix 2. I know an update to that is planned.

I am using a Celestron FirstScope 80mm (910mm f.l.). I removed the objective and built a cell on the end of the dew cap because I am using a 50mm f/20 (1000mm f.l.) lens. It is an uncoated singlet with 10/5 scratch-dig (highly polished). I also removed the baffles from the OTA because I don't want light from the cone reflecting back down the tube.

The corona is way harder than H-alpha because it is 100 times dimmer.

George


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Re: Corona from 24 September 2022

Post by DeepSolar64 »

Yes, the corona should be much dimmer than the chromosphere and therefore much more difficult.


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Re: Corona from 24 September 2022

Post by Montana »

This is truly remarkable and the best you have displayed here :bow :bow :bow and thank you for the information, if only I was brainy enough to do this, had the spare cash and had a high mountain nearby ;)

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Re: Corona from 24 September 2022

Post by DeepSolar64 »

Alexandra, You do have a starting point. A Baader Prominence viewer! Draft Mr.Townley to help you convert it into a true Coronagraph! He's an expert modder and tinkerer!


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Re: Corona from 24 September 2022

Post by george9 »

Alexandra, as James points out, you have a great start. Adding a field stop should feasible. You can iterate on the rest as needed.

The big challenge is the cost and temperature regulation of the 5303A filter.

But there are other fun things you can do with a just a little tinkering. For example, image prominences at other wavelengths, like H-beta, H-gamma, helium D3, other helium. Even image prominences with no filter (broad band for true color). The latter requires clear skies and improved scatter of the Baader prominence viewer, similar to coronal viewing.

I have seen the corona at sea level, just about 20 yards from the sea, in fact. It was a very clear fall day. So you may not need a mountain. But mountain does help a lot. The image in this post was actually not my usual 3200-foot mountain but a 2300-foot (700 meter) star party field.

George


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Re: Corona from 24 September 2022

Post by DeepSolar64 »

The true magenta-pink combined color of H-Alpha and H-Beta prominences is absolutely beautiful!!


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Re: Corona from 24 September 2022

Post by george9 »

Thanks so much for the SPOD, Alexandra! George


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Re: Corona from 24 September 2022

Post by DeepSolar64 »

Congratulations on the SPOD, George. Well deserved!! :bow


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Re: Corona from 24 September 2022

Post by Sumpy »

Just another voice in the chorus here. I find this entire topic absolutely fascinating, and your image results are just absolutely incredible! :bow

And congrats SPOD!


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Re: Corona from 24 September 2022

Post by DeepSolar64 »

Same here. Congratulations on the SPOD!!


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Re: Corona from 24 September 2022

Post by george9 »

Here is some follow up on the fringes.

I reproduced it shining a flashlight into the coronagraph without an objective.

The fringes seem to be a property of each filter. One has a straighter fringe and one has a more curved one. The dual fringes in my 9/29 post above is the two filters, not Newton's rings. Each filter shows its fringe without the other.

ASI1600 and ASI290 both showed the fringes.

No amount of manipulation changed them. I tilted one slightly within the cell and no change. I tilted a few degrees before or after the filter, and no change. I still need to tilt before and after the filter.

If I just shine a flashlight into the bare filters, I don't get a fringe. Only in the telecentric beam. So maybe something there can be done. I could try the collimated beam, but I expect the same.

Otherwise I am just stuck figuring out post-processing. Most likely I can shift the temperature up or down and use that as a base. I am not sure what I subtract versus divide versus both. I am wondering if I subtract and then divide.

I don't see the fringes in the eyepiece. Only in the camera, which is more sensitive.

George


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Re: Corona from 24 September 2022

Post by george9 »

An addition. When the Sun is off center, peeking around the cone, there is no fringe pattern. It's not like Newton's rings. There seems to be some fringe component separate from the main stream of light coming through.

I need to figure out under which circumstances light causes a fringe pattern and which do not. E.g., why is there a sharp demarcation of the fringe in that image from 9/29.

This also points to a need for subtraction, not division.

George


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Re: Corona from 24 September 2022

Post by DeepSolar64 »

Better things come through research.


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Re: Corona from 24 September 2022

Post by george9 »

Better things do indeed come through research.

I did not solve the fringe problem, but I did figure out the half-fringe phenomenon. As the Sun goes off axis the internal reflection in the objective lens that is normally blocked by the coronagraph's Lyot spot moves to the side of the objective lens and gets around the Lyot spot.

In the attached image (Sun about 3-5 degrees off axis, not behind the cone), see how bright the fringe half is compared to the non-fringe half? That's how much light the internal lens reflection adds to the background scatter, and that's why Lyot bothered with it.

And the Lyot stop (vs spot), which addresses diffraction around the edge of the lens, is even 10-1000 times brighter than this source. So a simple cone blocking the Sun never would have worked by itself.

George
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