Comparing Ca-H and Ca-K SHG images

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thesmiths
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Comparing Ca-H and Ca-K SHG images

Post by thesmiths »

This is a bit of an academic topic, but I've been curious for some time just how different are images from the Ca-H and Ca-K lines. Let's start by looking at the lines themselves. The image below is a photo taken using FireCapture while our spectroheliograph (SHG) was focused on the Sun. Quite a bit of structure is visible within the lines.

The Ca-K line (top) and the Ca-H line (bottom) in the solar spectrum.
The Ca-K line (top) and the Ca-H line (bottom) in the solar spectrum.
100204-Ca-K-and-H.jpg (131.06 KiB) Viewed 1603 times

Using the programs RSpec and Excel, we plotted the intensities of the lines in the chart below. The K-line (on the left) is slightly broader and also darker than the H-line (on the right).

Chart of the solar spectrum near the K and H absorption lines (left and right, respectively). The horizontal axis is in pixels.
Chart of the solar spectrum near the K and H absorption lines (left and right, respectively). The horizontal axis is in pixels.
Calcium K and H.png (227.66 KiB) Viewed 1603 times

Normally when taking SHG data, I would narrow the ROI to just focus on a single important spectral line. This allows faster scanning, smaller file sizes, and also avoids confusing the reconstruction software. In the video below, I deliberately included both lines so that it's possible to visually track the structures in the lines (from which the surfaces features are created). As the slit is scanned across the Sun's surface, the two lines behave quite similarly.



We tried to do a very controlled test by imaging the H-line and then the K-line using all the same settings. The focus was the same, as was the exposure. Only the ROI was moved from the lower H-line to the upper K-line. We then stacked the same number of frames (8 out of 20) and used the same IMPPG settings. A very small amount of Photoshop was done, but identical in both cases. Click on the image to see at higher resolution. Or open in a new tab to see at full resolution.

Ca-H (left) and Ca-K (right) taken with the same settings on Aug 8. H-line from 06:55 to 07:03 UTC. K-line from 07:11 to 07:20 UTC. 720mm focal length, 72mm aperture mask.
Ca-H (left) and Ca-K (right) taken with the same settings on Aug 8. H-line from 06:55 to 07:03 UTC. K-line from 07:11 to 07:20 UTC. 720mm focal length, 72mm aperture mask.
080822-CaH-CaK-720mm-72mm-2400g-stack-8.jpg (1.95 MiB) Viewed 1603 times

Although the two images are extremely similar, on close examination of the fine details, I would say the image taken in the K-line is very slightly better, particularly in the definition of the filaments. Maybe others have a different opinion.

We then took a close look at just the prominences. For this comparison, we used just single images (un-stacked) and did no additional sharpening, filtering or contrast enhancement.

Calcium prominences: Ca-H (left) and Ca-K (right). Single frames taken 17 minutes apart.
Calcium prominences: Ca-H (left) and Ca-K (right). Single frames taken 17 minutes apart.
080206_CaH_081904_CaK_protus.jpg (367.97 KiB) Viewed 1603 times

Again, although they are very similar, the K-line seems to show more details in the prominences; they also appear to be significantly brighter against the sky background.
Last edited by thesmiths on Mon Aug 08, 2022 6:28 pm, edited 4 times in total.


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Re: Comparing Ca-H and Ca-K SHG images

Post by hopskipson »

Nice images and comparison of the 2 spectral lines, Douglas. I’m sorry to ask so many questions. I’m guessing you are using a special reconstruction program? Do you use a pre-filter to help show the proms?


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Re: Comparing Ca-H and Ca-K SHG images

Post by thesmiths »

hopskipson wrote: Mon Aug 08, 2022 3:38 pm I’m guessing you are using a special reconstruction program? Do you use a pre-filter to help show the proms?
In this case, there is no pre-filter at all. It's quite difficult to find an appropriate filter for this wavelength. It's easier for H-alpha and H-beta. Those filters can help darken the sky to make the prominences show up better.

As regards to reconstruction software, we use our own version that we heavily modified. It is available on our GitHub. See the various posts in the Solar Software section. The most recent post you can find here: viewtopic.php?t=37066.


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Re: Comparing Ca-H and Ca-K SHG images

Post by p_zetner »

Hi Douglas.
Are you still using an asi183 camera for these results?
Thanks.
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Re: Comparing Ca-H and Ca-K SHG images

Post by hopskipson »

thesmiths wrote: Mon Aug 08, 2022 4:40 pm
hopskipson wrote: Mon Aug 08, 2022 3:38 pm I’m guessing you are using a special reconstruction program? Do you use a pre-filter to help show the proms?
In this case, there is no pre-filter at all. It's quite difficult to find an appropriate filter for this wavelength. It's easier for H-alpha and H-beta. Those filters can help darken the sky to make the prominences show up better.

As regards to reconstruction software, we use our own version that we heavily modified. It is available on our GitHub. See the various posts in the Solar Software section. The most recent post you can find here: viewtopic.php?t=37066.
Thanks for the response and the link!
I was thinking of using the blocking filter on the nosepiece of a Lunt Ca k filter. My thinking was it’s probably wide enough for both h & k lines and would provide some blocking and decent transmission.


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Re: Comparing Ca-H and Ca-K SHG images

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p_zetner wrote: Mon Aug 08, 2022 5:01 pm Are you still using an asi183 camera for these results?
Yes, I'm exclusively using the ASI 183MM lately. The 178MM is also very good, maybe better in some ways (it doesn't seem to ever drop frames like the 183MM does sometimes). But with the 13.2mm width of the 183MM, I can see the whole length of the 12mm long slit (with 1:1 system magnification). I typically only use about half the width but it's nice to have the room for error and to chose which part of the slit to use. If I can confirm that the whole slit is fairly evenly illuminated and in reasonably good focus, then I know the full solar disk should image well. I had to put a tilt corrector in front of the slit -- one end of slit was around 20 microns closer than the other, which I found made a difference in getting good focus across the solar disk.


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Re: Comparing Ca-H and Ca-K SHG images

Post by marktownley »

Excellent results Douglas! For me the CaK has the edge.


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Re: Comparing Ca-H and Ca-K SHG images

Post by thesmiths »

marktownley wrote: Tue Aug 09, 2022 6:46 am For me the CaK has the edge.
I tend to agree. I was actually surprised that the difference was fairly noticeable. This was a fairly controlled experiment, the only difference really would be the seeing conditions could have changed. But in fact, I think it was very slightly worse seeing for the CaK (it was taken after and things got worse with time).

Most Calcium SHG images seem to be for CaH, rather than CaK. I think the reasons are that CaH is a little brighter, which allows for more favourable camera settings. Also, the CaH line is a little narrower, so you can in principle use a narrower ROI, which also has some acquisition advantages. Depending on the camera sensor and any filters used, the UV cut-off could be even more pronounced than seen here, putting CaK at an even greater acquisition disadvantage.


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Re: Comparing Ca-H and Ca-K SHG images

Post by Montana »

I tested myself and looked at the images first to draw my conclusions before reading your narrative, it was funny as I came to exactly the same conclusions as you did :) CaK is a definite winner in the proms, the disc is very close indeed but I felt there was just a slight more contrast in the filaments and plage areas in CaK. This was an absolute cracker of a test!
:bow :hamster:
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Re: Comparing Ca-H and Ca-K SHG images

Post by thesmiths »

Montana wrote: Tue Aug 09, 2022 12:01 pm CaK is a definite winner in the proms
I realised that I could make the prominence images nicer looking by turning off the "transversalium correction" in the reconstruction software. This filter compensates for the small dark lines due to things like dust on the slit. The problem is it causes artefacts on the sky. I therefore turned it off, and also reduced the size of black disk (using the "protus adjustment" feature). The disks don't always turn out exactly circular so shrinking the black disk gives a little more room for error.

I also rendered this jpeg at full size (the previous one was at 50%) so there are more fine details to see if you want to zoom in (open image in a new tab).

Calcium prominences: Ca-H (left) and Ca-K (right). Single frames taken 17 minutes apart.
Calcium prominences: Ca-H (left) and Ca-K (right). Single frames taken 17 minutes apart.
080206_081904_protus_big.jpg (946.19 KiB) Viewed 1514 times


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