Specifications

effects of unevenly illuminated field, CCD thermal noise etc. significantly
degrade image quality when imaging dim deep-sky objects for many
minutes.
This is why every CCD image should be calibrated. Image calibration
basically consists of two steps:
1. Dark frame subtraction
2. Applying flat field
Image calibration is supported by the “Calibration” tool in SIPS .
The raw image downloaded from the camera contains not only the
information desired (the image of the target field), but also CCD chip
thermal noise and artifacts caused by unevenly illuminated field
(vignettation), shadows of dust particles on camera cover glass and filters
etc.
Illustration 16: 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.
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