Illustration 13: The raw image
downloaded from the camera
The Dark frame is taken with the same exposition time at the same CCD chip
temperature. Because hot pixels are less stable than normal pixels, it is always
better to take more dark frames (at last 5) and to create resulting dark frame as
their average or better median.
Illustration 14: The dark frame
corresponding to the above raw image
Illustration 15: The raw image with
subtracted dark frame
Subtraction of the dark frame eliminated majority of thermal noise, but
unevenly illuminated field is still obvious. Image center background is much
brighter than the border parts.
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