Orion 52080 - Manual (Page 13)

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Color Balance
The sensitivity of most CCD cameras as a function of wavelength (color) is dif-
ferent from the response of the human eye. The filters used for creating color
composites also have their own characteristics, as do the telescope optics.
Although “perfect” color rendition is an elusive if not impossible goal (all indi-
viduals see colors slightly differently), it is straightforward to get “good” color
balance with simple weightings. This is where the Color Balance command (in
the Process menu, Figure 15) comes in handy to touch up the resultant colors
in your images. Use the Preview Image to see how altering these parameters
will affect your image (or click the Full Screen button to see the changes
applied to your full image).
Color images from CCD cameras typically require a background level adjust-
ment. This is accomplished by bringing the background level (or bias) in each
color plane down to zero. Each of the Background Level values is subtracted
from every image pixel in its color plane. Any pixel values that become nega-
tive are forced to zero. The Auto button automatically determines the settings
necessary to equalize the image background in all three color planes. The
Reset button resets the background level subtraction to zero on all planes.
Scaling adjustment (entered as a percentage) allows you to compensate for
transmittance differences between the filters used to acquire the three color
planes. Values of 100% result in no change. The scaling percentages can be
typed in or adjusted using the “spin” controls (small up and down click arrows
to the right of the scaling numbers). The Preview Image is particularly helpful
in monitoring the results when using the spin controls. The Reset Scaling but-
ton resets to 100% on all three planes.
The Click On White Area to Set Scaling check box enables the operation of
the mouse to set the scaling. Set the Background Level first (you can use
Auto), then click on a white object (e.g. a neutral-colored star) in the image (not
the Preview Image) with the mouse. The Scaling settings will automatically
be adjusted to make the selected point appear white. If an area of the image
is known to be white (or gray), this is an easier way to determine the scaling
factors, and can be used to instantly color balance the image.
Stretch
The Stretch command (located in the Process menu, Figure 16) modifies the
brightness and contrast, and optionally the range (maximum and minimum
pixel values) of an image. Unlike the screen stretch settings in the Screen
Stretch Window, which only affects the how an image appears on the com-
puter screen, the Stretch command actually changes the image data pixel
values in the memory buffer. In reference to the histogram, the Screen Stretch
Window changes how the histogram is displayed on the computer screen, the
Stretch command alters the histogram itself.
There are three elements which must be set: the Permanent Stretch Type, the
Input Range, and the Output Range. You can use the Preview Image to see
how changing these parameters will affect the resultant image (or click the
Full Screen button to see the changes applied to your actual image). You can
also open the Screen Stretch Window to see how changing these parameters
change the histogram.
Figure 14. The Filter functions can help bring out subtle detail
in an image.
Figure 15. The Color Balance command allows weighting of
each color plane to improve the overall color appearance of an
image.