Why does the red channel look softer than green and blue in my stacked astrophotos?
Asked 10/24/2025
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2 answers
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I’m reprocessing stacked FITS images of the Veil Nebula in Siril and noticed that the red channel shows softer, less detailed stars than the green channel. The blue channel looks similar to green.
Setup:
- Canon EOS 1100D (not astro-modified)
- UHC filter
- 160mm at f/5
- 35 × 15s exposures, ISO 1600
- Tracked with a Vixen Polarie
I didn’t shoot separate RGB exposures, so this is all from the same DSLR data. What would cause only the red channel to appear less sharp?
Originally by Neppomuk. Source · Licensed CC BY-SA 4.0
Neppomuk
7mo ago
2 Answers
3
The most likely reason is longitudinal chromatic aberration caused by the lens.
When the light is bent (focused) by the lens elements it causes the three wavelengths to focus at slightly different distances if not perfectly corrected for. Blue wavelengths focus short, and red wavelengths focus long.
Most camera lenses are achromatically corrected which shifts the focus of blue wavelengths towards the red side to bring the red and blue into focus together.
But corrections are seldom ever perfect, so it is still possible for the actual focus to fall between the green and blue wavelengths with similar sharpness/detail, while leaving red farther out.
It's also possible that there is some near IR contamination in the red channel due to some wavelengths that were not blocked by the camera sensor's IR filter. This image shows the focus offset for IR on a cannon EF lens (the red mark).
Originally by Steven Kersting. Source · Licensed CC BY-SA 4.0
Steven Kersting
7mo ago
0
Generated from our catalog & community — verify before relying on it.
The most likely cause is a combination of lens chromatic aberration and DSLR sensor sampling.
First, lenses often focus different wavelengths at slightly different distances (longitudinal chromatic aberration). If your focus landed closer to green/blue, the red light can be a bit out of focus, making red stars look softer.
Second, your Canon uses a Bayer sensor, which has fewer red pixels than green. Green typically shows the most apparent detail because it is sampled more densely, while red and blue require more interpolation during demosaicing. That alone can make green look sharper.
If blue looks nearly as good as green, the lens focus behavior is probably the bigger factor here. There may also be a little extra long-wavelength/near-IR leakage affecting the red channel, though the main explanation is still focus differences by color.
So: this is normal DSLR/lens behavior, not evidence that you captured separate red data incorrectly.
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