To calibrate an image we have to take dark and flat frames. Dark
frames are frames taken with the scope covered so that only
contain information noise. As the noise is random it is
necessary to take many dark frames and average them to obtain a
master dark frame. Dark frame calibration process subtracts from
each pixel of the image the value of the same pixel in the
master dark master to get only the signal for that pixel. This
eliminates from the image (in fact greatly reduces) noise,
background brightness and hot pixels. Flat frames are frames
obtained by taking a uniformly illuminated field with the same
optics used to obtain the image to be calibrated so that, once
dark frame calibrated indicate how each pixel of the sensor
reacts to the same signal because the light does not reach
evenly to each sensor pixel or because each sensor pixel
responds differently to the light. As the flat frames also
contain noise it is also necessary to take many flat frames and
average them to obtain a master flat frame. The