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

2: Linear and nonlinear

In order to effectively process a image requires that it is linear and calibrated. What is linear can easily be expressed as:

A star that is 10 times bright as another star also shall provide 10 times the signal. Not so obvious that it is so! More mathematically, a linear function satisfy:

• f (x + y) = f (x) + f (y) and f (a * x) = a * f (x)

When a camera is set to save images in RAW format is it usually in its linear format. However digital cameras even in raw mode, they have a certain compression in the higher range and then not exactly linear. Normal film is strong nonlinear and also a digital image in jpg format. A nonlinear reproduced image is very difficult to by math "count" back to linear format.