M83 with the Megalens

I had another clear night and the opportunity to tweak the Megalens and it’s focus settings.

If you’re struggling with getting focus working with KStars/Ekos, then I recommend the series of videos on YouTube by John Evans. (https://www.youtube.com/@johneevans1) He’s the lead(?) programmer for the KStars/Ekos focus module and he’s been working on it for quite some time to sort out bugs, improve easy-of-use and implement new features.

The target for this image was M83, the Southern Pinwheel Galaxy. It’s quite high in the sky at the moment, with an early meridian of around 11pm. So a bright target that’s out of the light pollution dome. (For the most part - the LP here is not good, and the entire sky is usually a medium grey colour.)

Unlike my last outing with the Megalens, everything worked first time. Because I don’t need to retract the focuser when I put the lens away (unlike my SW72ED) I can leave it at whatever position it achieved focus at. This time I did drop the focus step size to 15 (down from 25) to see if that made any effect. It might have, but I need to do more testing to verify. The critical focus range on this thing is so small, it would be near impossible to do it manually. I remembered to enable guiding, and the mount averaged under 1.0 arc-seconds error all night.

Looking at the finished image, the focus looks a bit sort, which is probably an issue with the lens - optics have come a very long way since this thing was made.

The stacked image wasn’t too bad, although I’d really love to get out to a dark sky and see how this lens performs, as the signal was pretty well swamped under all the light pollution. I suspect that more imaging time under darker skies would really bring out the colour in the galaxy - this one is rather beige…

I followed my usual stacking and processing workflow in Siril. I did try the ‘Star Resynthesis’ option in the ‘Star Processing’ menu after I had separated the stars from the galaxy/background. This option is brilliant - the odd-shaped stars are changed to completely round. I have no idea how this works, but I’ll definitely remember to use it when I have odd-shaped stars again. The only issue is that further processing of the star layer requires a gentle hand, as several of the larger stars had a visible ‘box’ around them if I stretched the layer too much, or pushed too hard with the brightness / contrast in Affinity.

Image details:

• Gain: 100
• Offset: 10
• Temperature: 0 degrees C
• Exposure: 30 seconds
• Frames: 240 Lights, 50 Darks (2 hours integration in total)
• Filter: UV/IR Cut
• Stacked and pre-proc’d in SiriL, finished in Affinity Photo

Equipment: Megalens @ 1000mm / HEQ5-Pro / ASI183MC-Pro / SV165+SV305 / Kstars/Ekos

---

The first image shows the stars combines with the galaxy and background in Siril.

For some reason, the resynthesised stars didn’t work at all well, so I was left with the odd-shaped ones. Compared to the Affinity Photo processed images below, the image has a warmer, yellow tone to it.

---

For the next two images, I used the same (processed) galaxy background layer. In the second, I have swapped out the star layer to use the resynthesised stars, and you can clearly see the difference in shape and colour. The only issue with resynthesis is that some of the smaller stars disappear from the image, and some of them can be a little ‘hard’ looking. (Yeah, it’s all a bit subjective, when you really get into it.)

---

As above, but with the resynthesised stars. They are a lot more circular, but at the cost of hard edges and a slightly ’not quite right’ look.

UPDATE - 2024-05-26

I finally had another chance to get the rig outside and capture more data of M83. This time I managed a reasonably respectable 4 hours worth - 480 exposures of 30 seconds each. This gives me a total of 6 hours, which should be more than enough for a normal telescope. However, this is the Megalens we’re talking about and it’s relative ‘slowness’ means that it needs a lot more time to capture the same number of photons.

The first image is the result of all that data - stacked and preprocessed in Siril, then split with starnet++ before recombining with Affinity Photo. Again, I used Siril’s Star Resynthesis ability to get the stars back to round and not the strange shape the lens gives them.

You can see that the extra data has brought out the detail in the stellar arms of M83, as well as a more blue/grey tinge.

---

Recently, Cuiv the Lazy Geek released a new video on his YouTube channel (https://www.youtube.com/watch?v=b9ZJPI3IkWU) where he only used free software to process an image. One of those pieces of software was ‘Graxpert’ - an AI-using gradient and noise removal tool for astro images.

Once I had all the relevant support libraries downloaded, I threw my stacked (but unprocessed) image into Graxpert to see what would come out. I have to say that the results are impressive. Yeah, I need to test it with more images but for now I’m happy with what it can do to a noise image with a massive gradient across it.

After processing with Graxpert, I fed the image back into my usual processing workflow (starnet++ separation, GHS stretching in Siril, then recombination and further work in Affinity Photo) to obtain the image below.

Graxpert has managed to pull a lot more faint detail out of the data, although the palette has shifted back to the beige shades, as seen in the original image back up the page. I don’t know why this is, and the image resisted all my attempts at bringing the blue/grey data back.

Right now I am uncertain as to where to go with the Megalens. Do I keep trying to work around it’s terrible vignetting and non-round stars or do I spend money on a new telescope of similar focal length? I can use flat frames to get around the vignetting (and various dust bunnies) but that still leaves me with strange shaped stars. I’ll have a think about it…

For now, enjoy the output of Graxpert and the additional detail in this image of M83.

• Astro