Hello all,
the method of integrating the star light of a single photo can be valid as a
relative color calibration. IMO, this cannot be an
absolute color calibration method for your optical system, as it will give you a different RGB scale factors in each photo.
OTOH, sometimes can be preferable to have a relative color calibration. In my experience in professional telescopes, when photographing far far away galaxies, it's better to state as white reference the closest galaxies in the photo. If you take as white reference a near galaxy, all your photo of distant galaxies will appear yellowish, as
all the objects in the photo have a significant redshift. So making a relative color calibration can enhance the redshift effects
of the objects of the photo: you will see the reddening of the farther galaxies respect to the closer ones.
I have given a try to George's idea with a M42 photo, but with some significant variations...
To measure only the star intensities, I have removed the wavelet layers of the image down to the 16 pixel one. This acts as a kind of
local background substraction.
Also, to remove saturated pixels, I made a mask that is a binarization of the image. This mask is white only for the saturated pixels. With this mask, you make the pixels of the flattened image to zero. This will cancel these pixels for statistic calculations.
Last, to measeure only the illumination of the 0.5% brigther pixels, we will do a small trick. In the histograms tool, we will set the shadow clipping parameter to 99.5%.
Applying this clipping to the flattened image we will set to zero 99.5% of the pixels, and will remain only the brighter ones.
At the end, we will have the original pixel values of the 0.5% brighter pixels that are not completely saturated.
In this screen image you can see the the complete process:
"m42_Original" is the original image and "M42_ColorStars" is the processed one. "satmask" is the mask of the saturated pixels. "color_Stars" is the
linear flattened image. "bg" is the background reference image, used to preserve background level in the RGB channels.
You can see also in the image two instances:
- The Statistics instance shows the average illumination of the flattened image in each RGB channel.
These values are our white reference and are used to scale the RGB channels.
- The PixelMath instance shows the formuae used to scale the RGB channels. As the G channel has the higher average illumination, we use this as the unitary channel. So we will scale the R and B respect to G. The formula acts as below:
- We multiply the channel by a factor after substracting the background level (
(m42[0]-Med(bg[0])) ). The factor is calculated as the average illumination of the reference color channel divided by the average illumination of the target color channel (
*(Avg(color[1])/Avg(color[0])) ).
- We add the background level, multiplied by the same factor we have multiplied the image without the background level (
+(Med(bg[0])*(Avg(color[1])/Avg(color[0])) ).
As you can see, the color is slightly redder. The original image has a completely manual color calibration, adjusted simply by eye. The resulting color balance must by judged by your taste, I think... What do you think?
Regarding color calibration with spiral galaxies. I'm starting a work on this problem. My idea is to stablish a spiral galaxy as a standard candle for astrophotography color calibration. The fact is that spiral galaxies act as a good white reference as they contain the broader type of objects in a single object. My idea is to make a survey of spiral galaxies to calculate an "average spiral galaxy", and to stablish a correction factor for each survey galaxy. In this way, we will have a limited number of "standard galaxies" (around 70 - 80) where to stablish the RGB scaling factors for our imaging system. The galaxies in the survey must have the properties below (to be reviewed):
- Must be closer than 50 mpc.
- Hubble classification of Sa, Sb, Sc, Scd, SBa, SBb, SBc or SBcd.
- Inclination of <60º.
- Integrated intrinsic intergalactic and galactic reddening of <0.5 mag in Johnson B.
I think I will have the results at the end of this year.
Hope this helps... Best regards,
Vicent.
