A dark frame (or dark image) is an image of sensor noise from a covered sensor. To take a dark frame, you need to use an impenetrable light objective lid. Alternatively, one can use a cover for the camera body. Following that you take an image of the dark noise.
It is important that the same settings (exposure time and operating temperature) are used – ideally you take the normal exposed image directly after the dark frame image, so that the temperature of your camera doesn’t change too much.
On the left hand side we see the original image that has been disturbed by the dark noise from the sensor.
In the middle there is the dark frame i.e. the image of the dark noise.
It is clearly recognisable that there is a strong disturbance of the bright red pixel that one can even see in the original image. This disturbance is mostly caused by a microdefect on the sensor.
On the right hand side of the image we see a corrected (calibrated) image that was created with the help of the dark frame.
There are four parameters available for the adjustments of the dark frame corrector:
The iterations specifies the number of cycles that the dark frame corrector algorithm goes through. This correction procedure is very precise. A high iteration number therefore improves the accuracy and consequently might create a softer result.
b) The threshold value
The threshold value determines the minimal brightness of a pixel in a dark frame, before the pixel can be corrected.
c) Radius correction
With the radius correction you will be able to determine, in which pixel environment the correction calculation will take place. A radius of 1 uses the environment of one pixel, so into total 3×3 pixel. The radius creates a corrector matrix of 7×7 pixel.
d) Colour correction
The colour correction has the capabilities to reduce the colour staining. Colour staining is a very soft and large colour noise.
Tip: In practise there are a lot of dark frame images taken in astro-photography that are then eventually calculated together into a so-called master dark. This way 50 or more, dark frame images are calculated into a master dark.
The rule of thumb is to take more dark frames in normal exposure, so that the noise in the master dark is smaller than that in master light.
You can sort this out directly in DENOISE projects 2 professional with the function stacking image sequences.