Helium imaging -- surface features and prominences (July 16)

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Helium imaging -- surface features and prominences (July 16)

Post by thesmiths »

Montana wrote: Fri Jul 15, 2022 6:09 pm Can I book my Helium D3?
Wavelengths of some of the Fraunhofer spectral lines
Wavelengths of some of the Fraunhofer spectral lines
fraunhoffer lines.JPG (28.12 KiB) Viewed 1441 times

To research the topic of imaging at the wavelength of Helium, I first watched the YouTube video by Christian Buil: https://youtu.be/TuF6R9K5cqg. Some other good tips are given by Peter Zetner in several old SolarChat posts: viewtopic.php?t=14044, viewtopic.php?t=19411, viewtopic.php?t=25437. On my second attempt, I was able to see something.

The first task: finding the Helium line. It is relatively close to the Sodium doublet, a little bid towards the blue. Near the limb, Helium can be seen as an emission line (rather than a typical dark absorption line). Here is a screenshot taken from a scan with our SHG, as the slit approaches an E/W limb (edge):

Helium emission line at the top, the two Sodium absorption lines at the bottom.
Helium emission line at the top, the two Sodium absorption lines at the bottom.
He line 2.JPG (14.18 KiB) Viewed 1441 times

We needed to learn how our software would treat the various lines and how best to position them. Basically, we need one strong Sodium line to be entirely within the ROI of the scan (and of course, the Helium line). This leads to a rather larger width of ROI than usual (around 300 pixels vs 150 pixels), so the file size is about twice (or more) than normal.

Spectral line of Sodium (right) -- the Helium line is on the left somewhere.
Spectral line of Sodium (right) -- the Helium line is on the left somewhere.
Test_102917_spectral_line_data.png (813.15 KiB) Viewed 1441 times

This graphic is the normal output from our software and shows the D2 sodium line being successfully fitted by the software and the D1 sodium line is partially visible at the right edge. This graphic actually corresponds to the scan image above it, so we know the Helium line is around 1/6 from the left edge, or around the 50 pixel mark. The detected Sodium line is around the 230 pixel mark, so we can guess the Helium line should be around 180 pixels to the blue from the fitted line. (Note that in the graphic the scales are not the same so the line appears extremely curved.)

The fitted curve (a cubic polynomial) is what generates the centre line images in normal imaging (like H-alpha, H-beta, Ca-K/H, etc). From the centre of the line (assumed to be the darkest points), we normally do small pixel shifts (like -10 to +10), to see how the image changes with small wavelength displacements. In this case, we want a very large offset to the blue, so we input a pixel shift range of -190 to -170.

The software then generates a series of pixel shifted images. It turns out that looking at the "protus" images (the black disk image that highlights prominences), the exact location of the Helium line can be fairly easily recognised. In this case, it turned out to be at -179 pixels.

The second task: how to display the Helium absorption? We took the image at -179 pixels and another one close by (we chose -170). What is normally done is to subtract (or divide) the "helium image" from the "continuum image" to visualize the effect of absorption of Helium at the Sun's surface. When I played around with this way of visualising, I was not very satisfied. The reason is that the effect is so small that the differential image must be amplified considerably afterwards and any small amount of noise creates a lot of artefacts. I found it was more revealing (and potentially less misleading) to create a gif that would switch between the two images:

GIF animation of -179 (Helium) and -170 (continuum) pixel shifted SHG images.
GIF animation of -179 (Helium) and -170 (continuum) pixel shifted SHG images.
102917_shift=-170_shift=-179.gif (507.36 KiB) Viewed 1441 times

This way of displaying the data highlights how weak an effect Helium absorption is -- it is barely visible on top of the features of a normal continuum image, but the location of the helium absorption clearly corresponds to active solar regions. See, for example, the H-alpha and Ca-K images from Arne Danielsen taken on the same day viewtopic.php?t=37135. Generally speaking, the darkening caused by Helium occurs near where both H-alpha and Ca-H/K show relative brightness.

What about Helium prominences? They are quite weak compared to H-alpha but about the same strength as Ca-H/K prominences. To see them better, we took another SHG scan that was very overexposed -- the exposure time was increased until the D2 sodium line was just barely visible (1.14ms vs 0.38ms for the continuum, both with gain of zero). The sodium line needs to be visible or the software will not work.

The -179 pixel shifted "protus" image, rotated left by 90 degrees.
The -179 pixel shifted "protus" image, rotated left by 90 degrees.
104007_shift=-179_He_protus.jpg (55.55 KiB) Viewed 1441 times

Fortunately, on July 16 there was a huge filamentary prominence lifting off, visible in many H-alpha images (including our own viewtopic.php?t=37131). The image above is this giant feature in the famous yellow light of Helium emission, and the image below shows the prominences in H-alpha (on the same day).

H-alpha SHG image, inverted, July 16.
H-alpha SHG image, inverted, July 16.
160722-Ha-700mm-2400g-14x-9_19-inv-rotate.jpg (209.24 KiB) Viewed 1120 times
Last edited by thesmiths on Fri Oct 14, 2022 3:48 pm, edited 2 times in total.


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Re: Helium imaging -- surface features and prominences (July 16)

Post by Montana »

:hamster: :hamster: :hamster: not quite the same as Peter's discs but the inverse dark plage is quite prominent, the features in Helium D3 are fascinating :bow

Alexandra


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Re: Helium imaging -- surface features and prominences (July 16)

Post by thesmiths »

Montana wrote: Sun Jul 17, 2022 11:23 am not quite the same as Peter's discs but the inverse dark plage is quite prominent
After having done the whole process myself now (for the first time), I'm concerned that a lot of the He D3 images out there have a lot of artefacts. The issue is when you try to subtract a very strong background in order to retrieve a weak signal, you have to be sure that you are doing an almost perfect subtraction. Since the backgrounds are non-identical, I think it is impossible to do a perfect subtraction followed by a large increase in gain (to amplify the small He signal. Maybe Peter can chime in here at some point as I'm sure he recognises the problem I'm talking about.


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Re: Helium imaging -- surface features and prominences (July 16)

Post by p_zetner »

Yes, I'll definitely chime in!

To begin with, I'll post an image from July 2015.
HeD3_ChroTel_montage_smth.jpg
HeD3_ChroTel_montage_smth.jpg (333.57 KiB) Viewed 1385 times
The left part of the image is a spectroheliogram I took in HeD3 (587 nm) light. The right side of the image is from the Chromospheric Telescope (ChroTel) on Tenerife, operated by the Kiepenheuer-Institut für Sonnenphysik captured on the same date. This is an image taken in infrared He light at 1083 nm. The He 587 nm and 1083 nm absorption lines originate on the same helium energy level so images at these two wavelengths equivalently probe the same helium absorption in the solar atmosphere. This is much like the situation with the Ca H / K doublet ... different Ca spectral lines, originating on the Ca ion ground state, which image the Sun in essentially equivalent ways. If you compare my spectroheliogram on the left with the ChroTel filtergram on the right, the resemblance is pretty striking.
I'm concerned that a lot of the He D3 images out there have a lot of artefacts.
I'm not sure whose images you are referring to. Apart from mine, I don't know of many such images being posted by amateurs. There are certainly some minor artefacts visible in the spectroheliogram above but none too severe to call into question the fact that most of the image unambiguously reveals the helium chromosphere. I'm sure you agree that it would be ludicrous to suggest that both my image and the ChroTel image suffer from the same artefacts! Unfortunately, the KIS ChroTel group no longer seems to post current He 1083 nm images so I can't make regular comparisons, although there are archived images until as recently as September 2020.
The issue is when you try to subtract a very strong background in order to retrieve a weak signal, you have to be sure that you are doing an almost perfect subtraction.
It's true that you have to be very careful about subtracting the continuum component. Subtraction of nearly equal "signal+background" and "background" images is problematic and can easily reveal optical deficiencies in the spectroheliograph. Some gratings, for example show a distinctive "pattern noise" which I can't easily remove.
Since the backgrounds are non-identical, I think it is impossible to do a perfect subtraction followed by a large increase in gain (to amplify the small He signal.
Not quite sure what you think a "perfect" subtraction entails. The image above shows that it can certainly be done well enough. Since this 2015 image my processing technique has improved. Presently, each frame of the SHG video is processed using a spectral analysis in which a scaled average disk spectrum is subtracted in each video frame, column by column (for vertical dispersion direction) to reveal deviations from the average disk spectrum. These deviations correspond to the He absorption signal and constitute the signal in the spectroheliogram.


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Re: Helium imaging -- surface features and prominences (July 16)

Post by marktownley »

Fascinating!


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Re: Helium imaging -- surface features and prominences (July 16)

Post by highfnum »

all very interesting
I wish more details on software steps
I find He the most challenging


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Re: Helium imaging -- surface features and prominences (July 16)

Post by hopskipson »

highfnum wrote: Sun Jul 24, 2022 2:04 pm all very interesting
I wish more details on software steps
I find He the most challenging
I’m very new to sgh.
I agree since very few amateurs are doing shg (or at least posting) . I’m trying to perfect my technique for capturing Ca, Ha and Hb but I see very limited information on Helium. The video by Christian Buil seems to have a lot of information but google translate doesn’t work well with it.

Perhaps a comprehensive list of links, videos and posts on this subject would be helpful for all.


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Re: Helium imaging -- surface features and prominences (July 16)

Post by fulvio.mete »

I've tested and imaged a lot of lines, but not He line till now.Your interesting discussion leads me to give a try with it.

Fulvio


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Re: Helium imaging -- surface features and prominences (July 16)

Post by hopskipson »

fulvio.mete wrote: Sun Aug 07, 2022 4:41 pm I've tested and imaged a lot of lines, but not He line till now.Your interesting discussion leads me to give a try with it.

Fulvio
I downloaded your book today (grazie) and am halfway through it .
Christian Buil’s video and software manipulation seem to provide a useful technique but I’m loosing key information in the translation. Perhaps someone could dub English over his video?


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Solar Equipment: Solar Spectrum RG-18 0.3A, Coronado Solarmax 90mm etalon Isle of Man SN-001, Tuscon SN-380 and Meade SM2, Lunt LS80 DS, Quark Chromosphere, Lunt 2" wedge, 2-Lunt CaK II 1200, Baader 3.8 and 5.0 solar film in 208mm cells, and 3D printed Sol'Ex SHG, Lunt 40mm
Coming Soon: Solar Spectrum CaK II <1A filter
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