This is a very short tutorial. We're working now with one of our latest images acquired at CAHA. The photographed object is PK164+31.1, a planetary nebula in Lynx. The original RGB image looks like this:
While thinking about how to calibrate the color of this image, I saw the tiny spiral galaxies at right. So I decided to try to use these galaxies as the white reference of the image. Take into account that these galaxies are significantly redshifted, as well as all the other background objects. So, to me, this would have sense from a documentary point of view. The reference galaxies are these ones:
After applying the color calibration routine, we get this result:
And this is the image after raising the color saturation:
Of course, the H-alpha emission is somewhat muted because we are giving less weight to the red channel. But this can be solved because we have H-alpha data as well. This is a close comparison of the reference galaxies before and after the color calibration process, and after the color saturation curve:
Now these galaxies have blue-cyan spiral arms and orange bulges.
See also the we can now appreciate much better the color of the spiral arms of the tiny galaxy inside the PN.