* Stars brighter than mv < 7.1 with color closely matching the Sun.

• Sky background may not be uniform across the image, so it becomes necessary to determine the pattern (gradient) present in it,

• celestial objects may fill a large part of all of the image, making it difficult to find a spot representing the true sky background, and

• Poisson noise and readout noise mean that there is no single pixel value for the sky background, but instead a range of pixel values due to these noise sources.

When the subject of the image and the sky are clearly separate (that is, the subject is not a frame-filling nebula), it is reasonable to use median of the pixel values in star-free regions of the image as the sky background level. After subtraction of the median sky background from the image, however, the sky will have negative as well as positive pixel values. Later color processing stages must be able to deal with these physically unrealistic sky brightness values.

When the subject of the image fills the frame, even if there are no gradients present, determination of the sky background is difficult. It is necessary either to find one or more places where the subject appears to be absent and to measure the sky background in those places, to make a sky background image of an adjacent

Table 20.5 Faint* Sun-Like Stars for White Balance





Sp. Type


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