Full-disk H-alpha, beta SHG -- stacking, high resolution

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thesmiths
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Full-disk H-alpha, beta SHG -- stacking, high resolution

Post by thesmiths »

This morning (July 8) had some fairly good conditions. I again used the 106mm aperture, 700m APO triplet refractor. The equipment is described in our previous post viewtopic.php?t=36901. As I mentioned before, our SHG doesn't have quite the optics to handle the full disk at 700mm. We normally scan in DEC but this time we scanned in RA. This put the region of poor optical quality at the north and south poles, which typically has less interesting features than the equator. The images before stacking were 3200 pixels square so the final image has quite a lot of detail. To view at full resolution do [right click --> open image in new tab --> left click to magnify].

H-alpha SHG. 2400 l/mm grating, 9 micron x 12mm lithographic slit. Best 25% of 22 good frames stacked. Imppg, Photoshop.
H-alpha SHG. 2400 l/mm grating, 9 micron x 12mm lithographic slit. Best 25% of 22 good frames stacked. Imppg, Photoshop.
080722-Ha-700mm-stack-22-25.jpg (1.71 MiB) Viewed 830 times

You can compare this full disk with one from Arne Danielsen, taken on the same day and around the same time: see viewtopic.php?t=36971; and https://www.astrobin.com/full/h9ciqe. His was taken with a Coronado 60mm etalon.

The left edge has a lot of interesting details so I cropped and rotated the above image to focus on that region. Clicking on the image gives an almost full-resolution view.

Crop and rotate, H-alpha SHG image.
Crop and rotate, H-alpha SHG image.
080722-Ha-700mm-stack-22-25-crop-rotate.jpg (645.51 KiB) Viewed 830 times

The total amount of video data to create these images was about 117 GB. There is of course a lot more information that could be extracted (for example, wavelengths away from the centre of the H-alpha line).

I've added the H-beta image data that was taken shortly afterwards. The best 50% 0f 11 SHG images were stacked (there were not more frames due to the arrival of some clouds).

H-beta SHG, taken shortly after the H-alpha. The best 50% of 11 good frames stacked. Imppg, Photoshop.
H-beta SHG, taken shortly after the H-alpha. The best 50% of 11 good frames stacked. Imppg, Photoshop.
080722-Hb-700mm-stack-11-50-small.jpg (492.46 KiB) Viewed 823 times
Crop and rotate, H-beta SHG image
Crop and rotate, H-beta SHG image
080722-Hb-700mm-stack-11-50-crop.jpg (489.32 KiB) Viewed 823 times
Last edited by thesmiths on Fri Jul 08, 2022 9:49 pm, edited 4 times in total.


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Re: Full-disk H-alpha, beta SHG -- stacking, high resolution

Post by christian viladrich »

Hello Douglas,
I think this is one of the very best images I have seen taken with a SHG.
The resolution is very good. This is the kind of resolution we usually have with a "classic" Ha telescope. But here, you have a much better spectral selectivity (maybe equivalent to a triple or quadruple stack).
On top of that, processing is really excellent.
Well done !

PS : How long do you need for one scan ?


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Re: Full-disk H-alpha, beta SHG -- stacking, high resolution

Post by thesmiths »

christian viladrich wrote: Fri Jul 08, 2022 7:00 pm How long do you need for one scan ?
Hi Christian, I will give a long answer to your short question (in order to explain how scan time is chosen). At the end of the day, the scan time depends on the ROI and the USB3 interface bandwidth. The ROI for these images were all 3200 x 150 pixels. The width of 150 could potentially by reduced to 100 but for a full disk at 700mm, there is significant "smile" (i.e. curvature in the spectral line).

H-alpha spectral line with cubic polynomial fit.
H-alpha spectral line with cubic polynomial fit.
Test_100817_spectral_line_data.png (276.04 KiB) Viewed 809 times

Note that the axes are not the same so the amount of curvature is exaggerated. Nevertheless, the width cannot be reduced that much. So the number of pixels per frame results in a frame rate of around 290fps. To achieve an approximate 1:1 ratio of scan lines to pixels length (in this case 3200), a scanning rate of 12 times sidereal rate was chosen. 12X would of course give a time of approximately 120/12 = 10 secs to cover just the solar disk. Looking at the camera log files, I see that the acquisition times were actually around 14 to 15 secs. The number of frames was around 4100.

The software calculates that the solar radius is around 1410 pixels, so the time to cover the disk is approximately 2800/290 at 1:1 ratio, or 10 seconds, which agrees with the scan rate chosen.


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Re: Full-disk H-alpha, beta SHG -- stacking, high resolution

Post by Montana »

Oh my :bow :bow :bow

You have set the bar so high, you are a master :hamster: I have never seen so much detail in a hydrogen beta before, I would love to see a Helium D3 at this resolution. I can't express my delight in seeing your progress :bow

Alexandra


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Re: Full-disk H-alpha, beta SHG -- stacking, high resolution

Post by thesmiths »

Montana wrote: Fri Jul 08, 2022 9:00 pm
I would love to see a Helium D3 at this resolution.

Oh, Helium. I never tried and I hear it is quite difficult. But Sunday should be good weather so I may give it a try. I will read up on it first.


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Re: Full-disk H-alpha, beta SHG -- stacking, high resolution

Post by p_zetner »

I agree with Christian. Your H alpha image is comparable to the better full disks produced by conventional double stacked telescopes (today’s SPOD for example). I also echo Alexandra’s comment….looking forward to seeing your HeD3 results.


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Re: Full-disk H-alpha, beta SHG -- stacking, high resolution

Post by Carbon60 »

Outstanding results! Superb work. There’s such a lot of detail in the Ha FD and close view with the images being bang on the central spectral line. Simply wow 😮

Stu.


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Re: Full-disk H-alpha, beta SHG -- stacking, high resolution

Post by fulvio.mete »

Excellent images again,Douglas.
Fulvio


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Re: Full-disk H-alpha, beta SHG -- stacking, high resolution

Post by Simon2940 »

I must be doing something wrong, ive not see the details come out like that. There's always something else to chase, and people say imaging the sun is boring..... pfffffff


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Re: Full-disk H-alpha, beta SHG -- stacking, high resolution

Post by marktownley »

Stunning Douglas!!!!


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Re: Full-disk H-alpha, beta SHG -- stacking, high resolution

Post by thesmiths »

Thanks for all the positive comments. I would say the biggest improvement comes just from practice.

The new lithographic slits we started using are also a significant step forward. They are are not perfect, but there is definitely a step up in quality. I know how I would improve the slits if I ever made a new batch.

The new software has improved results because it makes stacking much more manageable. I think it's stacking of even a modest number of good image files (even as low as 5 or 6) that reduces the noise and allows normal sharpening procedures.

Also, I found it's crucial to be able to process some files during setup to see if the focus (and other settings) are good. With a very clean, high quality slit, it's actually very difficult to figure out if the collimator is perfectly focused. Normally, one can try to focus on the defects on the slit -- but what if there are almost no defects? So it's very important to be able to reconstruct an image or two right away in order to verify the focus.

Once found, I notice the focus stays put for a long time. So then I try to taken at least 20 or so scans (each of which takes around 15 seconds or so). Out of 20, maybe 4 will have an obvious fatal defect (typically camera related, but also atmospheric). Stacking the best 50% means using 8 image files. But I think a better result is stacking the best 25%. Assuming stacking 6 very good frames means starting out with 24, which probably means capturing around 30. With the setup shown here, the video files are averaging 3.7 GB each. So that means planning for over 100 GB of data (I've actually had to stop a few times because of lack of storage space due to bad planning).


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