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

This contamination source is a significant problem. Sky background light can be strongly colored thus distorting the colors of the celestial objects you are trying to image. Furthermore, light-polluted skies are usually brighter near the horizon, so the sky might change brightness across the image. This is intensity gradient, and in color imaging, intensity gradients cause color gradients.

20.3.2 Step 2: Correct the Filtered Images

In the previous section, we saw that the color images, SR, SG, and SB, are contaminated by an additive signal, the sky background light, and scaled by three multiplicative factors: filter transmittances, detector quantum efficiency, and atmospheric attenuation. To achieve accurate color, we need to subtract the additive factor and to divide out the multiplicative factors. Subtract the Skylight Background

Subtracting sky background appears simple, but it is often complex. There are several difficulties, including:

Table 20.4 Bright* Sun-Like Stars for White Balance





Sp. Type



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