Tutorial:
Dark and flat calibration

And an illustration and explanation why it's important to do dark and flat calibration

Content:

1. The digital DSLR camera features
2. Linear and nonlinear
3. The ideal image
4. The real image and its components
5. Vignetting
6. Making of the calibration images
7. What happens in the stages when we calibrate?
8. Did we came back to the ideal picture?
9. The math behind it

1: The digital DSLR camera features

In the examples, the data is partly simulated by a digital DSLR, Canon EOS 350D set to save images in RAW format. The camera records the images with 12-bit precision (raw), gives a scale of 4096 steps per color. Typically jpg-files have 8 bits only and gives you 256 levels, it is not enough for us astronomers and actually we would like to have it in 16-bit, 65000 levels! This camera has its optimal properties for astronomy photo at ISO800, ISO speed is the camera's gain factor. In principle, the described calibration is possibly to do on analog (film) images too, but much more difficult.