J2

-^electrons

£>adus _ 32.84 . 60 60

= 0.55 [e~ / pixel / sec]

levels available

For example, for a commercial-grade chip with a full-well capacity of 80,000 electrons and a 16-bit analog-to-digital converter, most camera designers would set the conversion factor at some value close to 80,000/65536 = 1.2 electrons per ADU.

Readout noise is primarily determined by the CCD's on-chip amplifier. Science-grade chips have levels of 3 electrons r.m.s. or better; top-notch commercial-grade models have readout noise around 6 electrons r.m.s.; and CCDs made for video camera applications and digital SLRs typically have a level between 8 and 30 electrons r.m.s. If your camera has a CMOS sensor, you can expect the conversion factor and readout noise to be larger than you might find in a comparable CCD, but nonetheless in the same range you would expect for a high-quality video camera.

Dark current depends critically on the intrinsic properties of the sensor and the temperature of the sensor. A CCD that has a dark current of 1 electron per pixel per second when it's cooled to -30 C will have between 100 and 1000 times that amount when operating at room temperature. Low-dark-current technologies can push the dark current down to roughly 10 electrons per pixel per second at room temperature; and these devices, when cooled to -10 C, turn in quite remarkable dark current figures. CCDs made by Sony employ a proprietary process that produces very low dark currents at room temperature, and extraordinarily low levels—on the order of 0.002 electrons per pixel per second—when cooled to -10 C. With such low dark currents, dark subtraction may not immediately appear to be necessary, but remember that dark-frame subtraction also removes the bias value, which is necessary for proper flat-fielding.

Variable

Variable

Light CCD

Baffle Bulkhead

Figure 8.3 This low-level light source (L3S) provides a stable, faint source of light for advanced CCD testing. A circuit-stabilized LED emits light that is diffused by an opal-glass or milk plastic diffuser. The variable aperture controls how much light goes to the second diffuser and reaches the CCD.

Light CCD

Baffle Bulkhead

Figure 8.3 This low-level light source (L3S) provides a stable, faint source of light for advanced CCD testing. A circuit-stabilized LED emits light that is diffused by an opal-glass or milk plastic diffuser. The variable aperture controls how much light goes to the second diffuser and reaches the CCD.

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