Specifications

Section 1 - Introduction to CCD Cameras

Page 4

encoded and stored in a computer to be reconstructed and displayed on a

television monitor."

X=1 X=M

Y=1

Y=N

Output

Readout Register

Amplifier

Figure 1.1 - CCD Structure

1.2. CCDs Applied to Astronomical Imaging

When CCDs are applied to astronomy, with the relatively long exposure times (compared to the

30 frames per second used in video camera), special considerations need to be applied to the

system design to achieve the best performance. This section discusses the cooling and dark

frame requirements of astronomical imaging.

1.2.1. Cooling

Random noise and dark current combine to place a lower limit on the ability of the CCD to

detect faint light sources. If the CCD is producing more electrons from its own internal

processes than is produced by photons from a distant object, the signal from the object is said to

be "lost in the noise", and will be impossible to display without sophisticated image processing

software. Noise here refers to the "gritty" look of short exposure images.

Internally, the CCD generates thermal noise and readout noise caused by the operation

of the electronics on the chip. The goal is to eliminate unwanted sources of electron production

in the chip and thus make the detector more sensitive to the remaining source of electron

production by incoming photons. As you can imagine, the reduction of unwanted noise is

important for the best performance of the CCD.

Dark current is thermally generated electrons in the device itself. All CCDs have dark

current which can cause each pixel to fill with electrons in only a few seconds at room

temperature even in the absence of light. By cooling the CCD, this source of noise is reduced,

the sensitivity increased, and longer exposures are possible. In fact, for every 8°C of additional

cooling, the dark current in the CCD is reduced to half. In your camera for example, cooling the

CCD from room temperature (25°C) down to 0°C results in an eight-fold reduction in dark

current.

The ST-5C uses a thermoelectric (TE) cooler to cool the CCD. The TE cooler is a solid-

state device that acts like a heat pump. By running electrical current through the TE cooler, heat

is pumped out of the CCD through the TE cooler. The camera also has a temperature sensing

thermistor attached to the CCD to monitor the temperature, and the camera electronics control

"History and Advancements of Large Area Array Scientific CCD Imagers", James Janesick, Tom

Elliott. Jet Propulsion Laboratory, California Institute of Technology, CCD Advanced Development

Group.