Custom white balance
My DSLR is a Canon 350D that has its standard IR filter replaced by a neutral filter. This way the camera is more sensitive for Red and IR wavelengths. For instance, this camera is 5x more sensitive than a standard camera for the 656 nm wavelength. This is the so-called Hα-light that is abundant in reflection nebulae.
The side-effect of this camera modification is that daylight images have an overall red glow. This can be easily corrected afterwards in software like Canon's Digital Photo Professional (DPP) but it is a burden to correct all daylight images this way. The camera is no easy snapshot camera anymore.
There is an easy solution: I can change the white balance setting in my camera. This way, the RGB colours of an example image are analyzed in the camera and the correction factors to obtain a nice colour balanced image are added to the RAW data. When viewing the RAW images in DPP or other RAW viewer these correction factors are applied automatically. This is called Custom White Balance (CWB)
I wanted to know if this CWB correction affects my astro images when processing the RAW Canon data in Nebulosity V2. I took two pictures of a white laptop screen, one with automatic white balance and one with custom white balance. When viewing the AWB frame in a RAW viewer, the picture looks reddish. This is caused by the higher red sensitivity. When viewing the CWB in a RAW viewer, the picture looks grey, as it should be. The CWB info is applied by the RAW viewer.
Now the processing in Nebulosity. First I need to demosaic the raw data. Then I can save the Red, Green and Blue signals in separate files. Then I can determine the average (mean) signal strength of the three signals. This way I found out that the Red signal is about 25% higher than Green and Blue. Most important: Nebulosity does not show any difference between AWB and CWB images. The CWB info is ignored by Nebulosity.
I found out about another feature of Nebulosity: In "Edit→preferences" you can setup what DSLR camera is used. I tested the colour balance in Nebulosity when setting "Standard EOS 350D" and "Modded EOS 350D" using my modded 350D.
It follows that setting "Modded EOS 350D" corrects for the higher Red signal of the modded camera, but it does an overcorrection. Blue is 25% higher than Red and Green is 15% higher than Red. The picture looks blue.
In both cases (not corrected or overcorrected Red) the skewed white balance can be corrected in Nebulosity using "Image →adjust colour background offset". This equals the average signals for R, G, and B.
Conclusion:
- I need to try both Camera preference settings in Nebulosity using real astro frames to find out if there is any difference in results after using "Image →adjust colour background offset".
- Nebulosity ignores CWB info set up in the camera. I can use CWB settings in my camera in order to obtain proper white balance for daylight photography without changing my Astrophoto results.