H-alpha SHG imaging from Sept 17
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H-alpha SHG imaging from Sept 17
I haven't attempted any SHG imaging since the morning of the day I got on a plane LHR-YUL on Aug 11. The Sun is much lower now and the air much cooler. In the meantime, I upgraded my laptop (now 16GB RAM and a much faster CPU). I didn't actually notice any difference in the acquisition (the fps wasn't any higher -- seems to be camera limited). Our SHG reconstruction software seems to work fine on Windows 11; the faster CPU in the laptop finishes processing as quickly as my desktop now.
I added a 2-inch Astronomik L1 UV/IR filter around 15cm in front of the slit to act as a bit of an ERF. I saw no noticeable attenuation at H-alpha but possibly there is some at Ca-K. However, with the low Sun, I now have very little time to image in the morning before obstacles get in the way, so I could only take some H-alpha scans.
The colder air I think added to image stability; SHG is quite sensitive to any disturbances. However, there was some wind, which may have caused some slight shaking of the mount. The quality is a little lower than my attempts at the beginning of August, which had a string of very nice imaging days.
Below is a stack on the best 10 frames from a total of 20 scans (3 were discarded manually and I then let AS!3 do the rest). Some sharpening with ImPPG and levels adjustment with Photoshop Elements. The second image is a crop from the first image, inverted. The first one is best viewed by opening in a new tab. The second one is best viewed just by clicking on the image.
I added a 2-inch Astronomik L1 UV/IR filter around 15cm in front of the slit to act as a bit of an ERF. I saw no noticeable attenuation at H-alpha but possibly there is some at Ca-K. However, with the low Sun, I now have very little time to image in the morning before obstacles get in the way, so I could only take some H-alpha scans.
The colder air I think added to image stability; SHG is quite sensitive to any disturbances. However, there was some wind, which may have caused some slight shaking of the mount. The quality is a little lower than my attempts at the beginning of August, which had a string of very nice imaging days.
Below is a stack on the best 10 frames from a total of 20 scans (3 were discarded manually and I then let AS!3 do the rest). Some sharpening with ImPPG and levels adjustment with Photoshop Elements. The second image is a crop from the first image, inverted. The first one is best viewed by opening in a new tab. The second one is best viewed just by clicking on the image.
Last edited by thesmiths on Sun Sep 18, 2022 2:33 pm, edited 1 time in total.
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Re: H-alpha SHG imaging from Sept 17
Here is some images created from an earlier set of scans, 20 scans from 09.23 UTC to 09:29 UTC. I had thought they were not as good as the ones above taken a little later. But after stacking (AS!3 chose 6 out of 20), the results were also quite good. I present the results here as an inverted full disk and a non-inverted cropped image.
Last edited by thesmiths on Mon Sep 19, 2022 2:27 pm, edited 1 time in total.
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Re: H-alpha SHG imaging from Sept 17
WoW, these are really good. I would think they are through an etalon equipped scope, not an SHG!!
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Re: H-alpha SHG imaging from Sept 17
I was looking through the posts, Douglas, but could not find the current setup...
Could you provide it a bit?
Could you provide it a bit?
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Re: H-alpha SHG imaging from Sept 17
Excellent Douglas !
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Re: H-alpha SHG imaging from Sept 17
I looked through the posts and had trouble finding them myself! Here are the two posts that show the main outlines of the equipment:Bastelhannes wrote: ↑Sun Sep 18, 2022 10:37 am I was looking through the posts but could not find the current setup...
Overview: viewtopic.php?t=37552
Under the covers: viewtopic.php?p=347782#p347782
There were two small changes made since the photos in the "under the covers" photos. One is I put a tilt corrector between the focuser extension tube and the slit. The issue with a long slit and a (fairly high) focal length is that if there is any tilt in the telescope optics, it is difficult to get focus across the slit from one end to the other. In any case, the focal plane is not entirely flat but has slight spherical curvature. But if there is also some tilt, it is even harder to get the focus right. It seems to be a not uncommon SHG problem for one side of the scan to be in focus but the other side to be slightly blurred.
The other minor change is I put an Asahi Pentax M42 helical focuser in front of the ZWO camera. I had previously used the bellows to focus the camera but I found the control was not fine and smooth enough. Now both the collimator and the camera can be very finely focused using helical type focusers. The adjustments needed for good focus are very minute (and of course change a bit with wavelength).
I'll mention a few more things while I'm at it. This is a somewhat scaled up version of the Solex design. The camera lens and collimator we use are both 135mm f3.5 lenses, so a diameter of 38.6mm. The Solex design is essentially 125mm f5, a diameter of 25.4mm. The grating in the Solex is 25mm square while I am using 50mm square. The Solex slit is only 4.5mm long but it's not clear whether the 1-inch optics of the Solex design can really achieve good quality across a slit that is much longer than that. The Solex is quite carefully designed so the standard components all work together. An analogy would be the difference in lenses required to image 35mm film vs 60mm film. I am able to image fairly successfully with a solar diameter of approximately 6.6mm in one go (using a slit that is 12mm long) but Solex users typically limit themselves to a solar diameter of 4.0mm or stitch together two (or more) such images to achieve a full disk at longer focal lengths (and greater apertures). If you want to stack SHG scans, the multiplying effect of having to do partial scans is quite an increase in imaging time (and storage space and processing time).
Another point I'll make about our setup is that it can only scan in DEC. But generally, I think most mounts will scan better in DEC than RA -- the moment of inertia is smaller (no counterweight) and the mount electronics is not busy also trying to track as well as to scan. Our setup is by default almost perfectly aligned in DEC so there is very little "tilt" in the scans and no adjustment is necessary to align the spectrometer perpendicular to the scan direction (a slightly finicky adjustment typically).
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Re: H-alpha SHG imaging from Sept 17
You know, it is kind of amazing how far SHG imaging has come recently. As Christian Viladrich pointed out recently, it now sort of looks like triple stack etalon imaging. I contributed to Ken Harrison's book "Imaging Sunlight Using a Digital Spectroheliograph" which was published in May 2016 and the quality of the SHG imaging has improved so much since then.DeepSolar64 wrote: ↑Sun Sep 18, 2022 3:20 am I would think they are through an etalon equipped scope, not an SHG!!
If I were to try to identify the drivers for the recent improvement, I would point to the following technological advances:
1. Better cameras. Larger sensor size; smaller pixels; faster interfaces, leading to high capture rates of high-resolution, high-bit-depth video files.
2. Improved slits. Lithographically formed slits are much more uniform, greatly eliminating imaging defects.
3. Much improved software. The INTI software originally written by Valerie Desnoux and added to by others has dramatically improved the quality and throughput of the image reconstruction process. Improvements in capture software (FireCapture and SharpCap) I think have also been important because of the increase in acquisition rates.
4. Better computers. More DRAM, faster CPUs, NVMe SSDs, USB 3.0 and USB-C has allowed fast acquisition and storage of large amount of data. The processing time for an image reconstruction has dramatically decreased as well.
5. The ability to stack multiple SHG scans. Back in 2016 the idea of SHG stacking seemed out of reach, almost absurd, I would say. Now it is relatively common due to all the advances listed above. Where would etalon imaging be if not for the ability to stack frames and therefore digitally sharpen?
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Re: H-alpha SHG imaging from Sept 17
Great, thank you Douglas. And now I am thinking about building a bigger Sol'ex. Grump. It is even more addictive than cocaine...thesmiths wrote: ↑Sun Sep 18, 2022 2:27 pm
Another point I'll make about our setup is that it can only scan in DEC. But generally, I think most mounts will scan better in DEC than RA -- the moment of inertia is smaller (no counterweight) and the mount electronics is not busy also trying to track as well as to scan. Our setup is by default almost perfectly aligned in DEC so there is very little "tilt" in the scans and no adjustment is necessary to align the spectrometer perpendicular to the scan direction (a slightly finicky adjustment typically).
One question arrived some time ago and I asked it already, but never got an answer:
How should we scan? You scan in DEC, some in RA. I am the lazy guy: I just turn off the tracking and let the sun move itself. What are the advantages and disadvantages?
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Re: H-alpha SHG imaging from Sept 17
In my opinion, "drift scanning" is good in the sense that the scan speed will be highly regular, but the problem is all the data is pushed down in frequency. It is a good assumption that most sorts of noise in nature have a 1/f frequency distribution. Thus noise will be greater at lower frequencies so it is always a good idea to push the data to higher frequencies. This means scanning at as fast a rate as practical will reduce ambient noise in the images.Bastelhannes wrote: ↑Sun Sep 18, 2022 4:46 pm How should we scan? You scan in DEC, some in RA. I am the lazy guy: I just turn off the tracking and let the sun move itself. What are the advantages and disadvantages?
With regards to RA vs DEC scanning: I mention above why I think mounts might work better for DEC than RA. This is certainly true for lighter weight mounts (e.g. EQ5) but may not be so true of heavier and better mounts (e.g. EQ6). If to do partial scans (by that I mean not full disks), then I think RA scans have some advantages because most solar features tend to occur along equatorial bands. So a partial scan done parallel to the solar equator will capture all the features along the band. This will also make creating a mosaic much easier, I have found. For full disk scans, this is less obviously an advantage, except that typically is not so easy to get both ends of the slit in good focus. It is generally the case that images will look better if the north or south pole is a bit out of focus than if the east or west limb of the Sun is blurred.
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Re: H-alpha SHG imaging from Sept 17
With regards to the 2-inch Astronomik L1 UV/IR filter that I recently installed: I have the impression that the data at the camera was a little cleaner than previously and was therefore easier to focus. This might be because before there was a lot of IR light bouncing around inside and the CMOS cameras are of course quite sensitive in the infrared.
Moreover, the transmission of chrome increases at longer wavelengths so the slit will actually let increasing amounts of stray light through unless there is a cut-off filter.
Moreover, the transmission of chrome increases at longer wavelengths so the slit will actually let increasing amounts of stray light through unless there is a cut-off filter.
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Re: H-alpha SHG imaging from Sept 17
Exceptional images there Douglas!
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Re: H-alpha SHG imaging from Sept 17
Excellent images, Douglas, with an intelligent setup.
About the scans, I'm not sure that the Dec scans are better of AR scans .In my case, that use only the natural flow of sundisk along the slit, AR scans are surely better ,as you stated, in capturing equatorial data.
About the IR cut filter in CMOS Cameras, surely it could make a noticeable difference, for these are much sensitive to IR radiation.I own a ZWO 290 MC, and the use of IR cut make a BIG difference in the output images.
About the scans, I'm not sure that the Dec scans are better of AR scans .In my case, that use only the natural flow of sundisk along the slit, AR scans are surely better ,as you stated, in capturing equatorial data.
About the IR cut filter in CMOS Cameras, surely it could make a noticeable difference, for these are much sensitive to IR radiation.I own a ZWO 290 MC, and the use of IR cut make a BIG difference in the output images.
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Re: H-alpha SHG imaging from Sept 17
Do you mean when using in one of your spectroscopy instruments? Or for another application?fulvio.mete wrote: ↑Tue Sep 20, 2022 3:43 pm I own a ZWO 290 MC, and the use of IR cut make a BIG difference in the output images.
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Re: H-alpha SHG imaging from Sept 17
I am confused:fulvio.mete wrote: ↑Tue Sep 20, 2022 3:43 pm About the IR cut filter in CMOS Cameras, surely it could make a noticeable difference, for these are much sensitive to IR radiation.I own a ZWO 290 MC, and the use of IR cut make a BIG difference in the output images.
1. We are using a grating to select specific wavelengths. The ones not wanted go aside the sensor. An IR cut filter does not make any sense to me due to this physical behavior.
2. I want to take pictures at 850 nm. I am not sure, if an IR cut filter prohibits this (I do not know the edge/threshold wavelength of the filter.
Due to my brain damage I am not sure, if I am correct. I figured out my brain created some weird alternative facts, which are totally wrong but seem real... I can really recommend COVID vacs to create a new impression of life.. sigh...
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Re: H-alpha SHG imaging from Sept 17
1. It turns out there can be a lot of scattered light which does not bounce off the grating so therefore is not selected. Or light of the wrong wavelength from the grating reflects off the walls and into the camera. It is not uncommon to, for example, put an H-alpha filter in front of the camera for this reason. But then you need to remove it or change it to look at a different wavelength, such as H-beta or violet.Bastelhannes wrote: ↑Sat Sep 24, 2022 10:05 am I am confused:
1. We are using a grating to select specific wavelengths. The ones not wanted go aside the sensor. An IR cut filter does not make any sense to me due to this physical behavior.
2. I want to take pictures at 850 nm. I am not sure, if an IR cut filter prohibits this (I do not know the edge/threshold wavelength of the filter.
2. For 850nm, you could put an IR pass filter (common for planetary imaging) onto the camera. This would increase the contrast.
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Re: H-alpha SHG imaging from Sept 17
And I thought using the IMX462, which is very sensitive at 850 nm...
Okay, i am going to try....
Okay, i am going to try....
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