NGC6188 Reprocessed
If you’re a long-term reader of this site you’ll be aware of my standout image from 2023 - NGC6188 (“The Fighting Dragons of Ara”), which can be seen here. I have this image printed at A1 and framed for display.
It’s been a very cloudy year do far, and good nights for imaging have been few and far between. So I though I’d go back and try my hand at reprocessing some of my old data and see what eventuated.
In the last 12 months, the tools I use have evolved, as have my processing workflows. So it made sense to get my best image data from last year and see the results from my current toolset and workflow.
At the very bottom-right corner of each image you can just see part of what’s called “The Dragon’s Egg” - a nebula wrapped around a pair of stars, one of which is appears to be a super-giant Wolf-Rayet star.
The “egg” has been catalogued as NGC6164, and a very good image of it can be seen at “https://www.rc-astro.com/photo/id1255_big.html". If you’re interested, there is a very good write-up about the nebula here “https://phys.org/news/2024-04-beautiful-nebula-violent-history-clash.html".
Given the small size of “The Egg”, it will make a good target for the Megalens. Hopefully I’ll be able to image it soon.
Image data:
- Gain: 100
- Offset: 10
- Temperature: 0 degrees C
- Exposure: 120 seconds
- Frames: 120 Lights, 25 Darks, 50 each of Flats and DarkFlats (4 hours integration in total)
- Filter: L-Enhance (Ha + Oiii dual-narrowband)
Equipment: SW72ED @ 357mm / HEQ5-Pro / ASI183MC-Pro / SV165+SV305 / Kstars/Ekos
This is the final image from last year. I’m still very happy with how it came out, and I still love the amount of detail present in the dust clouds. I kept the stars muted and restrained, as I wanted to keep the viewer’s attention on the detail in the dust clouds.
Image 1 - The original image
When I posted my original image on the IIS forum, one piece of feedback I received was that the image would look better if there was more contrast between the bright and dark areas of the nebula. I didn’t actually set out to replicate that feedback, it’s just how the image turned out this time around.
One important difference between last year’s image and the reprocessed work was the way in which I stretched the linear data. Originally, I used the default automatic Hyperbolic Stretch function in Siril. It’s a good, if somewhat limited, way of getting your data ready for further processing. Unfortunately, it’s a “this is what you get” result. You can’t really control how your data is stretched.
One of the best features of Siril 1.2.X is the GHS - Generalised Hyperbolic Stretch - tool. It allows you to control every aspect of how your data gets stretched. Most importantly, it allows you to make a large number of small stretches, concentrating on how a particular part of the image looks. If a small stretch doesn’t work out, you can reset your work back to the last stretch and try again.
For this image, I used Affinity Photo’s layer blending to recombine the stars and the starless layers, applying manipulations to each layer individually to get the best result. There were also some tweaks applied to the whole image, to finalise how I wanted it to appear.
The astrophotography macros by James Ritson (https://www.jamesritson.co.uk/resources.html) are incredible tools that make processing astro images so much easier. I’ve barely scratched the surface of what can be achieved with these macros, and I cannot recommend them highly enough.
Again, I kept the stars relatively restrained, as I wanted them to enhance the colours and patterns in the dust clouds.
Image 2 - Star Recomposition in Affinity Photo
The final image here was processed solely within Siril. I separated the data into stars and starless layers, used the GHS function to arrive at an image I was for each layer before using the ‘Star Recombination’ tool to put both layers back together.
The recombined image was then tweaked in Affinity Photo, but all changes were applied to the image as a whole, not to the individual layers.
