CCD cameras are based on strange electronic chips, called CCD sensors. These components, are providing a higher sensitivity to the light than common films, and allow to store the pictures in computers.
That's why astronomer, are quite interested in this technology.
A CCD chip, is an array of light-sensitive elements. Which are ,in fact, some small electronic capacitors. These capacitors are charged by the electrons generated by the light. In fact, each light element(commonly called photon), that reaches the CCD array's atoms, displaces some electrons, which are providing a current source. These current sources, are localised in small delimited areas(the capacitors), called pixels.
Common CCD chips are composed of several pixels, for instance, 192*165,512*512,1024*1024, or much more for rich people !
It's easy to understand that it is physically impossible to access individually each pixel. In fact, a CCD sensor provide only one serial output, through which each capacitor can be discharged (each pixel can be read).
This section will explain how a CCD array can be read through only one output.
The capacitors, are disposed in lines, and there are some control gates, that allow the transfer of one pixel line into the next one. The last line of the array, can be transferred into a horizontal shift register. This Shift register, allows the transfer of one pixel to the next one, and the last pixel of this horizontal register, is connected to the output gate.
This drawing, shows the organisation of the array, and the horizontal shift register. The red arrows indicate the transfer possibilities.
In fact, the CCD chip has one clock entry for the vertical transfer, one for the transfer gate, an one for the shift register. So to readout the image, you have first to transfer the last line into the shift register by providing a clock pulse on the transfer gate control pin. Then, you have to provide one clock pulse on the shift register control pin, for the reading of each point of this line. And finally, you have to provide one clock pulse on the vertical transfer control pin, to shift down the picture of one line before repeating all this procedure for each line of the image.
This is the principle of the CCD technology.
To take a picture, you have to :
The output gate of the CCD array, can either be connected to an analogue to digital converter, in order to digitise the picture, or can either provide a standard video signal, if the clocks timing are accorded to the video norms.
If the image is digitised, it will be easy to store it into a computer memory, so that its processing will be easy to perform.
If a CCD array is used in a warn environement(20 C), the photosensitive area atoms produce a current, wich produce noise on the picture.
To reduce this noise, the CCD array atoms should be cooled, so that their thermal excitation is reduced.
In astronomical use, the CCDs are cooled to temperatures like -50C or -70C.
They are cooled by two main processes :
Cooling a CCD by use of liqid gaz, is most efficient, but is still reserved for profesional purposes. This is mainly due to the equipment needed to store and use such liquids.
So, the most commonly used process, is the use of thermoelectric cooler, also known as Peltier cells. They are devices, working as a heat pump. The heat contained in one face of the cells, is transferd to the other one. These cells, should be powered by direct current.
Efficency improvement of such cells can be performed by cooling their hot face. In order to provide a great cooling to the CCD, it is possible to use 2 peltier cells, plus a water or glycol cooling system. The first cell, cools down the CCD, the second cools the first one, and the liquid circuit evacuates the heat provided by the second peltier cell.