Color Balance, Stretch | Orion 52080 specs

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.

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.