Figure 2.1 In a seemingly identical sequence of images, the number of photons that arrive during a constant exposure time varies randomly. The more photons, the smaller the percentage variation. To make an image, it is necessary to expose long enough to gather enough photons for a good image.

will differ less than one standard deviation from the mean value; and the other one-third of the time, the measured value will differ by more than one standard deviation from the mean value. However, departures greater than three times the standard deviation are rare, and departures greater than five standard deviations are extremely rare.

So—getting back to those ten pictures of the galaxy—what do you see?

Of course, you see the galaxy. In the pixels that comprise its image, you see pixel values scattered about the mean value that defines the galaxy. In the next section, we will dig deeper into the relationships between samples, signals, photons counts, and noise.

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