the original raw image.
Also please note the settings of the “Bayer X odd” and “Bayer Y odd” check
boxes must be altered when any geometric transformations are applied to the
raw image (e.g. mirroring, rotation, etc.). Some transformations (e.g. soft
binning or resampling) cannot be performed on raw image at all. It is always
better to Debayer images first and process them later.
Also note that stacking of raw color images results in loss of color information.
Stacking algorithms align images regardless if the particular pixel is red, green
or blue. SIPS allows also sub-pixel stacking, which can mix pixels of different
colors. Images must be Debayer processed first and then stacked.
Balancing colors
CCD chip sensitivity to red, green and blue light is different. This means the
exposure of uniformly illuminated white surface does not produce the same
signal in pixels covered with different color filters. Usually blue pixels gather
less light (they have less quantum efficiency) then green and red pixels. This
results into more or less yellowish images (yellow is a combination of red and
green colors).
The effect described above is compensated by so-called “white balancing”.
White balancing is performed by brightening of less intensive colors (or
darkening of more intensive colors) to achieve color-neutral appearance of
white and/or gray colors. Usually is one color considered reference (e.g. green)
and other colors (red and blue) is lightened or darkened to level with the green.
Automatic white balancing can be relatively easy on normal images, where all
colors are represented approximately uniformly. But this is almost impossible
on images of deep-space objects. For instance consider the image of emission
nebula, dominated by deep-red hydrogen alpha lines – any attempts to lighten
green and blue color to create color-neutral image result to totally wrong color
representation. Astronomical images are usually color balanced manually.
As already described in the “Brightness and Contrast – Image Stretching”
chapter, image can be visually brightened by altering its stretch limits. SIPS
“Histogram and Stretch” tool displays and also enables altering of stretching
curve limits and shape for red, green and blue color individually.
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