First time H-alpha SHG imaging

I LOVE finding out about different ways to appreciate the Sun and light in general. Use this forum to post your info or questions about various outside the mainstream ways to appreciate our life giving star!
Post Reply
thesmiths
Almost There...
Almost There...
Posts: 1053
Joined: Mon Jun 04, 2012 8:13 pm
Location: London, England
Has thanked: 42 times
Been thanked: 1485 times

First time H-alpha SHG imaging

Post by thesmiths »

After some promising Calcium imaging as a warm up viewtopic.php?f=8&t=32189 of the new spectroheliograph (SHG), the time had come to try H-alpha. This is a more difficult task since a much narrower bandwidth is required. To attempt H-alpha, we needed to replace our initial slit which had a nice length of 5mm but was 50 microns wide. We had a 3mm x 10 micron optical slit from Thorlabs used in prior spectroscopy projects so decided to use this. Using the same 480mm f6 telescope meant that the full disk image of the Sun would be around 4.4mm in diameter. The 3mm slit would then crop the Sun, requiring two scans for a full disk. Since the slit was 3/5 smaller, we could add a 1.6x image magnifier (AE Magni-Max, AC555) in front of the ASI 178MM camera (sensor width 7.4mm), which improved the spectral resolution (angstrom/pixel).

We spent the late afternoon of June 12 gathering some big SER files (around 57 GB of useful data). After a lot of fiddling around, I think the last file was probably the best (clouds then became an issue and the time was already just past 5pm (16:04 UTC). Here is a quick processing of the last file, which turned out better than I had expected:

SHG, H-alpha, 8x SR, 120 fps
SHG, H-alpha, 8x SR, 120 fps
SHG_disk2.jpg (390.98 KiB) Viewed 747 times

Most of the data was taken at 16x sidereal rate (SR) scan with a frame rate of around 260 fps. This one was done slower, at 8x SR and 120 fps. I have not yet processed the 16x data so I can't say yet if it's better than the 8x. The advantage of the slower scan is I could use a longer exposure and therefore a lower gain (the lower gain should improve image quality).



Above is an interesting video of how the hydrogen alpha spectral line looks as the slit is scanned across the Sun. What is happening is that the telescope is focusing an image of the Sun on the metal disk in which there is a 3mm x 50 micron rectangular slit (cut by a laser, I believe). Using the HEQ5 hand controller, the telescope moves the image up and down (in DEC) across the slit, creating frame by frame the image represented in the thumbnail.

I have inverted the grayscale in the video since it's easier on the eye to watch a white line on a black background. In real life, the continuum light is almost totally washed out and the hydrogen alpha line is exposed in shades of grey. The spatial and time variations in the line contain all the data that makes up the H-alpha image. At 8x SR, it takes about 120/8 = 15 sec to scan across the Sun; the video has been slowed down by about a factor of 2. Comparing this video to our previous ones on the Calcium line viewtopic.php?p=295375#p295375 clearly shows how much narrower the H-alpha line is and why it is much more challenging.

The thumbnail of the video shows computer processing of the SER file done with the SolEx program called "Inti" https://inti-solex.herokuapp.com/. The idea is you can upload your SER file to this website and it will do some automatic processing. I decided to download and run the program on my PC (it uses the "Docker" method of running a program image in an container -- rather cool). This means the upload process is much faster (although the total processing time is still fairly long).

Below is another version of the same data processed to bring out the prominences. The first version at the top was done to enhance the surface detail. By chance, I was able to catch a fairly large prominence at the top left and a smaller one directly opposite. The bottom third of the image looks slightly out of focus; I'll have to check if this a systematic (geometrical) problem or it was just a transient issue.

SHG, H-alpha, 12 June 2021, 16:04 UTC
SHG, H-alpha, 12 June 2021, 16:04 UTC
solar-SHG-d.jpg (478.79 KiB) Viewed 747 times

In theory, the SER file contains all the Doppler shifted spectral image information, and the data completely off the H-alpha line contains the "normal" solar continuum image information. By my estimation, the foam board and hot glue SHG has already surpassed the performance of my H-alpha PST and is starting to get close to the quality of my Lunt 60.
Last edited by thesmiths on Sun Jun 13, 2021 3:02 am, edited 2 times in total.


thesmiths
Almost There...
Almost There...
Posts: 1053
Joined: Mon Jun 04, 2012 8:13 pm
Location: London, England
Has thanked: 42 times
Been thanked: 1485 times

Re: First time H-alpha SHG imaging

Post by thesmiths »

Here is another SER dataset taken at 8x SR with 157 fps. Size of file is, as before, 2504 x 200 pixels. Time 16:01 UTC. Slightly different image processing than above but resolution (and lower third fuzziness) looks the same. I suspect there is a slight tilt to some optics somewhere.
H-alpha SHG with ImPPG, 8x SR, 157 fps
H-alpha SHG with ImPPG, 8x SR, 157 fps
test-SHG-imppg_c.jpg (492.66 KiB) Viewed 736 times


User avatar
PDB
Almost There...
Almost There...
Posts: 702
Joined: Mon Aug 15, 2016 4:23 pm
Location: Belgium
Has thanked: 2 times
Been thanked: 149 times

Re: First time H-alpha SHG imaging

Post by PDB »

Doug,
.
these Ha images look really good! I see you used Inti (FYI there is now a wInti version which runs as a normal program on Windows. Startup is slow, but processing is faster (less file copies))
Only problem with Inti is that it does not seem to calculate the circle correctly (and the x, y scaling) when there are black borders (solar image larger than slit) in the image.

Rgrds,

Paul


None of my posts or images may be shared on Facebook, Twitter or any other social media other than SolarChat without permission.
thesmiths
Almost There...
Almost There...
Posts: 1053
Joined: Mon Jun 04, 2012 8:13 pm
Location: London, England
Has thanked: 42 times
Been thanked: 1485 times

Re: First time H-alpha SHG imaging

Post by thesmiths »

You're right about inti not doing its resizing correctly. There should actually be a toggle to turn that feature off. I spent quite some time during acquisition to make sure the scanning direction was close to orthogonal and calculated the frame rate to give just a bit of overscan. When I ran the Wah! program that just extracts the spectral line info, the shape looked as predicted: just a slight overscan, which can easily be adjusted in Photoshop using Transform --> Distort. But with inti, it totally distorted the shape and made it look like it was actually underscanned. On the other hand, inti by default also does some really nice filtering so the rest of post processing is quite easy. See comparison below. I may try ISIS next time (a rather ominous name, I just realised).

SHG raw output Wah!
SHG raw output Wah!
SHG_170116_wah.jpg (171.95 KiB) Viewed 713 times
SHG raw output inti
SHG raw output inti
170116_F2819-5734_inti.jpg (155.81 KiB) Viewed 713 times


User avatar
Montana
Librarian
Librarian
Posts: 34526
Joined: Mon Oct 17, 2011 5:25 pm
Location: Cheshire, UK
Has thanked: 17521 times
Been thanked: 8763 times

Re: First time H-alpha SHG imaging

Post by Montana »

These images are absolutely terrific, well done :bow :bow :bow loved the video too :movie

Alexandra


Post Reply