The 2018 effort was calibrated & processed in Siril, then 'tortured' (too many curve stretches) in GIMP. The 2021 effort was calibrated and processed in Astro Pixel Processor, Siril was used for green noise removal and processing completed with a 'light touch' in GIMP. The additional 2 years of experience in imaging has given me a better idea of what end result to aim for.
For the 2018 version:
- background was too black
- star colours washed out
- brighter stars had a blue effect at the left hand side
- Bubble Nebula was orange
- evidence of a 'green caste'
- background now richer and less black
- stars have richer colours
- bright stars are 'tighter'
- brighter stars not over-saturated
- Bubble Nebula red, which is correct for an Ha region
- green caste removed
- the 'strained' look of the image has gone
The Digital Development Process pre-sets in Astro Pixel Processor have contributed to the image improvement as the uncontrolled curve stretching by me has been removed. The DDP available in dedicated astro-image processing software is of great benefit.
If I was to re-process using Siril I think I could improve over my original due to better understanding of the functionality available in the software.
Capture was with an Altair 66ED refractor and Altair 183C CMOS camera.
The original data: lights 60x60s, darks 50, flats 100, bias 50.
With experience some of the capture settings could be improved. Gain was 150 and black_level set at 100. If I revisit these objects, I will try gain=400 and black_level=50 plus increase (at least double) the number of light frames.
The exercise demonstrates to me that it is worth revisiting old data to measure how skills are evolving.