Basic night sky image processing using darktable
The moonless night sky is is rarely or never black or blue. Actually it is really colorful! The colors in the night sky have a physical reason, but most night sky pictures you see in the web have an artistic color grading, mostly blue. The reason is often that people don’t know how to fix light pollution. The easiest way is to make the image blue!
The reason why our sky is blue during day time is Rayleigh Scattering. Rayleigh scattering increases in efficiency as the wavelength of light becomes shorter. Blue light has a shorter wavelength than red, green or yellow light. So our daytime is blue due to the scattered light, the moonless night still has a tiny amount of blue scattered light, but it is much fainter than the starlight. There is also Airglow and in the very south or north aurora light, which is brighter than the Rayleigh scattered light from stars.
The colors we find in the night sky are from:
- Planets (Mars, Saturn, Venus, …)
- Stars (most stars are red, orange, yellow, and white. Less than 1% of stars are blue!)
- Light pollution
- Aerosol light from stars and the milky way
- Aurora borealis
To capture natural color of the moonless night sky, set your camera to daylight white balance! A lot of people use artistic values of 3800K but then the stars do not have their natural color nor the night sky. Well if you want to have an artistic night sky then you can do that, but why should I change the beautiful colors of the night sky and turn them into blue?
For fixing colors you can use the stellar classification of well known stars you see in your picture and convert that value to Lab or RGB. Then check it in your image and adjust it accordingly. The details how do do that is something for another article 😉
Image Processing with darktable
First apply the following modules to you RAW file.
- White Balance: Daylight white balance if you did not already do it in the field.
- Demosaic: Set method to VNG (gives normally better results for starts, AmaZE creates mostly hard edges)
- Chromatic abberations
- Lens corrections
- Hot pixels: Only if really needed!
- Exposure: Blacks -0.20
- Base curve: Reset the base curve and make sure we get nice mid tones. Figure 2 shows the points I approximately use. First I set the point which is from the top left in the second box of the first row. This pushes the mid tones. The one in the first row on the right is to reduce the brightness so we don’t loose color in the stars. The others is to get the blacks a bit down and fine tuning.
After we applied those changes we end up with the following picture:
Fixing color cast
In night sky you often have light pollution and sometime some green or magenta color cast. We want to make sure we do not push those colors so we fight them first. I did that differently before but then I learned something new from Frédéric Dux. You can find his video here. I will do a summary here.
- Contrast brightness saturation: Set saturation to maximum value
- Level: Use the highlights and move them to the 1/4 to the left
What you get is a very colorful picture but it will boost all colors which are actually looking strange. Now you can use the tone curve to fix vignetting (L-channel) and light pollution using the a-Channel and b-Channel.
I do an additional step and use the color balance module to fix red and green airglow. Be careful with this module! Once you’re done, deactivate the level and saturation module.
Contrast and brightness
Now we want go get some contrast back and increase the brightness of the sky objects.
Next is to darken the low end and increase the mid tones so details of nebula come out better.
Now you have a picture night photography pictures with the basic fixes.
Next comes the fine tuning to get out the details from the milky way. However this is something for another article …