Tutorial:
Flat calibration and how to shot flat images for color cameras

Content:

1. Flat image for color camera
2. Pixel gain differences
3. Hardware setup
4. Exposure
5. Master flat 3D analyze
6. Alternative light source, RGB LED bulb
7. To be continued

1: Flat image for color camera

When I make flat calibration I often use the computer monitor to be able to control the color temperature. For a "white" in raw mode on the camera that will provide the same signal strength on all three color channels require anything else than a white screen to shoot. The advantage of to have the color channels to have the same level is that no one over exposure (saturating) before someone else or one is to low and make noise.

Here is a normal raw histogram, gain equal to 1 for all channels. As you see the levels are very different between the channels, not good. To the right is a line graph diagonal over the sensor, very heavy vignetting because of the lens aperture set to wide open, a Pentax 500mm f4.5 lens.

If we instead of a white image have a colored one we can handle this problem. I make them in the simple photo program Paint.

Chosen like this it will give the same level on the r, g, b channel and it will look something like this. The color will be different with other combination of cameras and lenses/telescopes.

And here is how the histogram will look with this image:

Much better, now the level are equal between the channels. Here I have set the mean level to 50% in the peak center of image, 2000 to 2400 (image to the right). My camera is a Canon 5D that has 12 bit raw files, 4096 levels, but this camera is a little bit special, only use the first 3700 levels. Important, do not oversaturate the histogram to the right (left image), clip the high level and the calibration will go wrong!