New SHG setup and images in H-alpha and Ca-H

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thesmiths
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New SHG setup and images in H-alpha and Ca-H

Post by thesmiths »

Ken Harrison recently posed the question: "Why not just use a ND1.8 sub-diameter filter rather than the expense of a Herschel wedge?" Ken is usually right about things so I took his question seriously. I came across a vintage 77mm Hoya ND x4 neutral density filter. This reduces the light by a factor of 4 (i.e. by 75%) so lets through quite a bit more light than a Herschel wedge, which is an advantage. I also used a 2" GSO IR blocking filter. Between, these two filters, enough energy was reduced where I could put my hand at the focal point of the solar beam and feel no heat. This test is apparently sufficient to prevent damage to the spectroscopic slit (according to Christian Buil).

The 77mm ND filter was positioned at the first baffle and perfectly fit the light cone there. The IR blocking filter was put at the front of the SHG. Here is how the setup looks, attached to my 106mm f6.6 APO. With the huge Baader Herschel wedge, this setup did not have enough back focus. Without it, I needed to add an extra 50mm extension to come to focus. The balance was also much better without the wedge.

SHG with ND sub-aperture filter
SHG with ND sub-aperture filter
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In a way, this setup bears some resemblance to a PST mod. The SHG diffraction grating takes the place of the PST etalon. Both serve as a kind of very narrow bandpass filter. With the extra light from using the ND filter, I could use a 2ms exposure in order to get to 500 FPS at 656nm (H-alpha) and 3ms and 330 FPS at 397nm (Ca-H). This allowed scanning at 16x sidereal rate with H-alpha and 8x with Ca-H. I was using a 10 micron wide side that was only 3mm long so even with two scans, it would not cover the full disk at the 700mm focal length of the telescope. I was scanning in DEC so the images taken were of East and West hemispheres, from which I could create a kind of mosaic (with a blank strip in the middle).

We used our Solex_ser_recon_EN software, which pretty much automates the image reconstruction, geometry correction, bad line removal and basic levels adjustment. Here is a quasi full disk image in H-alpha:

H-alpha SHG mosaic
H-alpha SHG mosaic
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Here are the automatically produced protuberance images (taken simultaneously with the images above). The H-alpha measurements were done on Aug 10 so can be compared with Full disc h-alpha, 10th of August.

H-alpha SHG protuberances
H-alpha SHG protuberances
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The power of the SHG is to do other wavelengths so by rotating the diffraction gratings and refocusing the optics, we could do the Calcium-H line (not Ca-K!). These were taken the next day, on Aug 11.

Ca-H SHG mosaic
Ca-H SHG mosaic
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There were some slight traces of protuberance visible in Ca-H:

Ca-H SHG protuberances
Ca-H SHG protuberances
143656__prom-r.jpg (45.15 KiB) Viewed 928 times

The advantage of Ca-H over Ca-K is the slightly longer wavelength is actually considerably brighter so leads to shorter exposures. This allows lower camera gain and faster scanning rates. Ca-H seems to be preferred in SHG imaging and the images look very similar to Ca-K. The Ca-H line appears to be narrower than Ca-K so that's probably the reason why filters tend to focus on Ca-K (it's easier).

I'm guessing someone might want to see a close up of the snaky looking area in the H-alpha image so here is a 100% crop of that part on the left (the JPEGs above are at 25%):

H-alpha close up
H-alpha close up
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Last edited by thesmiths on Thu Aug 12, 2021 3:29 am, edited 3 times in total.


thesmiths
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Re: New SHG setup and images in H-alpha and Ca-H

Post by thesmiths »

Just one more thing: here's a interesting observational tool also mentioned by Christian Buil. If in the middle of acquiring an image you turn off the scan, the slit is stationary over a position on the Sun and the noise you see is a representation of the atmospheric turbulence (and perhaps also vibrations). It's a kind of cute graphical representation of how good the "seeing" is.

SHG H-alpha noise scan
SHG H-alpha noise scan
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Re: New SHG setup and images in H-alpha and Ca-H

Post by Montana »

Wow!! well done! the progress in imaging is coming on in leaps and bounds. Fabulous images :bow :bow :bow

I look forward to many more wavelengths :hamster:

Alexandra


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Re: New SHG setup and images in H-alpha and Ca-H

Post by marktownley »

These look good Douglas!


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Re: New SHG setup and images in H-alpha and Ca-H

Post by Merlin66 »

Doug,
Glad to see your successes.
Christian Buil and Valerie have turned “conventional “ SHG performance on its head!!!
The current work that’s being done on Valerie’s INTI by yourself and Matt D as well as other French guys, has brought the processing of SHG videos down to almost “live time” reconstruction of the image. This is a remarkable breakthrough.
We are now getting close to the SHG being a highly competitive alternative to the current etalon filters, with the added benefit of multiwavelength coverage.
Watch this space……


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Re: New SHG setup and images in H-alpha and Ca-H

Post by thesmiths »

To Ken: yes, the software development is active and keeps on getting better. At the time you wrote your book "Imaging Sunlight", the state of the software was, I believe, the limiting factor in enjoying the activity of imaging. One French fellow is incorporating our version of the software that runs from the command line into his software to control SharpCap and EQ Mod. So it will start the capture, start a scan, stop those and begin processing the image. So real automation. He thinks he could even put in an automatic mosaic acquisition feature at some point.

The Solex design, in my opinion, could use a bit of improving. But they apparently have a v2 Solex coming out soon with an ED lens for the collimator, which should help things. My adaptation allows focusing of both the camera and collimator while the Solex has no facility for refocusing the collimator (which is why, I think, their calcium images all look a bit fuzzy).


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Re: New SHG setup and images in H-alpha and Ca-H

Post by Merlin66 »

Pascal Berteau, did this wonderful animation of stepping through the Ha line at 0.1A increments.
You would be hard pressed to replicate this with conventional etalon filter.
This was based on one video of the Ha wavelength processed by Valerie's INTI.
image.gif
image.gif (2.53 MiB) Viewed 859 times


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Re: New SHG setup and images in H-alpha and Ca-H

Post by Montana »

Fabulous to see this progress so quickly :hamster: isn't it wonderful how people working together from all over the world can achieve so much :hamster:

Alexandra


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