H-alpha Solar Imaging with Coronado Double-Stacked PST - My Story

[RoryG]

Here's a story about my recent foray into H-alpha solar imaging with a Coronado PST, it may be helpful for others who want to try imaging with their own PST. The sun is particularly active at the moment as we're not far off the solar maximum, so it's a great time to get into imaging our nearest star!

Where I live at 60ºN we don't get dark nights in the summer, in fact in late May, the whole of June and most of July it doesn't really get dark at all, but the upside is we have very long days! So I have turned to solar imaging to get my astro fix (initially with my Seestar and Baader solar filter), and after some trial and error I think I'm producing reasonable results with the Coronado double-stacked PST which I bought in FLO's summer sale. It's not the ideal scope for imaging, however the price was right and it was in stock, most of the other scopes were either completely out my budget, or had 40 to 60 day lead times which would have been autumn / winter before it arrived.

One of the challenges imaging with the PST is achieving focus. ZWO's mini series of cameras (primarily designed for guiding) slot right into the eyepiece receptacle so focus easily, but as they are not really intended for lucky imaging they are USB2 with correspondingly low capture rates. It is possible to use the standard versions which are USB3, however they need barlows or very short nose pieces to focus and even then it can prove challenging. The ZWO mini cameras that focus with the PST are the ASI120MM Mini, ASI220MM Mini and the ASI174MM Mini, the latter producing quite a small full disk so not recommended without a barlow.

I already had a EQM-35 Pro mount and an ASI120MM Mini so I was ready to image the day the PST arrived, and amazingly for this summer it was mainly sunny! I set up the mount roughly polar aligned and pointed at the sun (without the second etalon to start with) and was initially quite surprised how small the sun was in the eyepiece. Once you get accustomed to it, however, you start to see more detail, and there was some great prominences on view, as well as some surface detail. With the second etalon fitted and tuned I could still see the proms but the surface detail exploded into a 3D like view, absolutely stunning!

   

My solar imaging rig

Time to try some imaging so I replaced the eyepiece with the ASI120MM Mini hooked up to my MacBook Pro using ASICap. Once focused and the exposure set I was impressed with the detail in the preview image, although I could clearly see the dreaded Newton's rings (NRs) I had read about. Also, the sensor doesn't quite cover the full disk so it's necessary to take two captures. With the ASI120MM Mini having a USB2 interface the capture frame rates are quite low, especially in 16bit mode (more on that later), and "mainly sunny" meant there were still some passing clouds, so the most I could capture was around 750 to 1000 frames each shot. The very first image I shot is below, quickly stacked at 30% in ASIVideoStack, the two jpgs stitched in Photoshop and some minor levels adjustment whilst sitting outside. Not bad for the first go, but the NRs are clearly visible.

  

My very first H-alpha solar image, 03 August 2024

24/08/03 ASI120MM two panel composite processed with ImPPG and a contrast mask added in Photoshop. NRs still visible but faint

I tried several methods to reduce / eliminate the NRs, from trying to tilt the camera in the eyepiece to a FFT filter in Photoshop with little success; the best results I had was using a contrast mask (basically an inverted overlay layer with gaussian blur) which evens the exposure at the expense of some contrast. I had planned to get a new guide camera for my deep sky rig so bought an ASI220MM Mini. This camera captures the full disk, but I was disappointed to see the NRs still there, although not as bad as the 120. I had also learned a lot more about processing solar images and found the amazing ImPPG image processing software. I am mainly Mac based, however I do have a powerful Windows workstation for my work so I could run the Windows executable. Since then I have built a Mac version of ImPPG which keeps all my capture and processing apps on my MacBook. I'm still stacking in ASIVideoStack as I'm not seeing any great difference using AutoStakkert! (a Windows only app), and I can't get Lynkeos to work reliably on my Mac, it tends to crash as soon as I click align. ASIVideoStack is also very fast on my MacBook Pro, taking literally around 15 seconds for 2,000 frames at 20%.

I did get some reasonable images with the ASI220MM Mini, however the detail wasn't as good as the ASI120MM Mini (the solar disk was about 90% of the size of the stitched images from the 120). In addition, the 220's frame rate was even lower, so it was taking nearly two minutes for 1,500 frames.

24/08/18 ASI220MM Mini processed with ImPPG with the curve adjusted for prominences. NRs still visible but not as bad as the ASI120MM Mini

My quest for a USB3 camera that would focus without additional hardware led me to the Altair GP-CAM 3 290M which has a 2.07MP 1920 x 1080 Sony Exmor IMX290 backlit sensor with Starvis technology. The pixel size of 2.9um gives a resolution of 1.5", perfect for normal seeing conditions, although a bit undersampled if conditions are very good. Up to now I had been using ASICap for capture, but have now moved to Firecapture for Mac to use the Altair camera, with the added advantage of being able to use flats. The sensor only covers about 85% of the full disk so two captures are required, like the ASI120MM Mini, adjusting the etalons can be challenging to try and keep the sweet spot in the same place for both captures. The two big plusses for this camera are fast USB3 speeds, I get around 63fps in 8bit mode, and NO MORE NRs!

  

24/08/2024 Altair GP-CAM3 290M false colour and monochrome images from 24/08/2024. Composite image merged in Photoshop from ASIVideoStack jpg files. No other processing apart from false colour

Now I have ImPPG running on my MacBook I can spend more time playing around with this amazing editor. It's possible to pull out the prominences whilst still preserving the surface detail using the tone curve editor, and the Lucy-Richardson deconvolution and unsharp masking gives very fine control over the sharpening.

ImPPG built and running on a M3 Max MacBook Pro

 

24/08/2024 Altair GP-CAM3 290M two panel composite. Stacked FITS file processed in ImPPG using a tone curve to bring out the proms whilst retaining surface detail

24/08/2024 Altair GP-CAM3 290M false colour three panel composite stitched in PTGui Pro. Individual stacked FITS files processed in ImPPG using a tone curve to bring out the proms whilst retaining surface detail

Going forward I'll be doing the bulk of my solar imaging with the GP-CAM 3. I've had a few issues with FireCapture locking up during preview, although this was down to the 2m braided "super fast" USB cable I was using, it's only happened once with the supplied cable, but that may be the USB-A to C adapter I'm using. I'm mainly imaging in 8bit mode, given the relatively low resolution of the imaging train I really don't see much visual difference between 8bit and 16bit modes (actually 12bit with the GP-CAM 3). The 8bit files are large enough anyway at around 3.1GB for a 2,000 frame SER file.

My workflow is as follows:

• Setup: focus, then adjust the built-in etalon first then the double stack filter to get the required view and sweet spot;
• Capture: FireCapture, 8bit mode, histogram at 85% (no gamma), 2,000 frames (optional flat calibration frames which I have only done once with a plastic bag);
• Stacking: ASIVideoStack, normally stack at 20%, FITS and JPG output;
• Curves, deconvolution and  sharpening: ImPPG using FITS from stacking, normalise brightness levels, L-R deconvolution, unsharp mask and tone curve, saved as TIFF;
• Merging: Photoshop, photomerge (I have also used PTGui Pro which produces better results if there's more than two panels);
• Post processing: Photoshop, camera raw filter, levels layer, optional contrast mask. For false colour I use quadtone mode with curves for black, yellow, orange and red.

FireCapture for Mac

Photoshop Duotone options for false colour

So that's my story! I'm overall really pleased with what's possible with the Coronado PST, it's about the cheapest option there is for H-alpha viewing and imaging. It's by no means a perfect scope, the focusing is horrible compared to R&P focusers, and it takes a bit of trial and error tuning the etalons and keeping the sweet spot over the correct part of the sensor. The slightest turn will change the image completely, and it's difficult to get it back to exactly where you were before you touched it!

My next project is to create some time lapse videos over the course of a week or so, however the chances of getting seven sunny days in a row is pretty much zilch!

23/08/2024 Altair GP-CAM3 290M three panel composite. Stacked FITS file processed in ImPPG using a tone curve to bring out the proms whilst retaining surface detail

[Altocumulus]

That's a long story well told. Thanks for sharing.

[RoryG]

1 hour ago, Altocumulus said:

That's a long story well told. Thanks for sharing.

Thanks Geoff, it's the sort of info I was looking for when I first started my H-alpha journey. Still lots to learn, and I may look at a Lunt scope or Daystar filter for next summer, I'm hooked now!

[Hals]

Excellent write up with ups and downs which most of us can identify with. 👍

Pete.

Edited August 26 by Hals

[Planetary Observer]

Nice images, and good story. I too have the Coronado PST Double Stack, and use a Altair Astro GPCAM3 178c USB3 ara, which requires no barlows or anything else. I just use the 5mm extension piece and the 1.25" nosepiece that comes with it, and it focuses correctly.