Using Field Stars for White Balance

Color correction using G2V standard stars is sometimes either not practical or not possible, but fortunately it is usually possible to take a shortcut to good-looking color images. This method assumes that the average color of the field stars in the image is either white or close to white. This method relies on the background population of stars in the image to serve as the color reference standard.

In the shortcut approach to RGB imaging, the observer can make a set of red, green, and blue images without making images of a white standard, and without correcting the images prior to displaying them.

Image capture is exactly the same as the G2V method. The observer takes a filtered set of images, dark frames, and flat frames, and produces a set of calibrated filtered images, SR, SG, and SB. However, except for correcting any sky background gradients, this method skips over the color correction step.

Creating the color image begins by measuring two quantities in each image:

• SR , SG , and SB , the pixel value of the sky background,

SKY SKY SKY

measured well away from any celestial objects in the image in an area free of stars.

# ^rmax> ^bmax ' a va^ue defined by histogram endpoint. Effective endpoints usually have a value between 0.995 and 0.9995 of the pixels in the image; that is, bright stars represent between 0.5% and 0.05% of the total pixel count in each of the color filtered images.

The conversion to 8-bit display values then proceeds as follows:

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