Correct Exposure

Astronomical CCD cameras produce acceptable images over an extremely wide range of exposure—but there are limits. In an underexposed image, noise dominates and obscures the target; in an overexposed image, the sensor is near saturation, or pixel values have gotten pegged at the maximum value of the camera's output format.

During an exposure, photons rain down on the CCD and accumulate. If the

Table 5.1 Surface Brightness of Celestial Objects

Object

B-Value

Object

B-Value

The Sun

Diffuse Nebulae

Solar disk

10,000,000

M42, core

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