Saturation

Since the CCD stores the photoelectrons in a potential well of finite size within each pixel, there must be a maximum cutoff value for the number of electrons stored in each pixel. When this maximum value is reached, we say that the pixel becomes saturated. However, the CCD that we are using only has a digital resolution of 16 bits. This corresponds to a readout of 65535 ( $2^{16}\!\!-\!\! 1$).¹ Thus our image becomes saturated when the pixel provides a readout of 65535 ADU. In most cases for CCD's, this is not actually the point in which the pixels become full (for our CCD the well has a capacity of about 100,000 $\textrm{e}^{-}$).² If this were the case the pixels would overflow into other nearby pixels once they become saturated and cause an effect known as blooming. Therefore it is important to distinguish between the two different types of saturation. One is when the digital data becomes saturated and the computer readout no longer increases as more photoelectrons are collected by the CCD. The other is when the potential wells themselves physically become full with photoelectrons and overflow into other pixels.