A charged-coupled device (CCD) is basically an electronic image sensor, from which digital images can be downloaded directly into a computer. Astronomical CCD's fit onto a telescope in place of an eyepiece.
A typical astronomical CCD camera has the following features:
- Sensitive as 1600 speed film but no reciprocity failure.
- Much greater brightness range than film
- Linear characteristic curve.
- Easy to subtract sky fog from final image.
- Responds to entire visible spectrum and near infrared.
- Resolves better than 50 lines per millimeter.
- Measures and compensates for some of its own defects.
- Allows images to be seen almost immediately on a computer screen so you can adjust exposure and focus.
The actual CCD chip is tiny measuring only a few mm's squared. This chip is an array of cells called charge wells which are tiny capacitors within a semiconductor. Each charge well has the ability to store electrons and transfer electrons from one cell to the next. To image something, the cells are set to zero volts. When a photon of light falls on a cell it causes electrons to enter the cell. The amount of electrons that will enter the cell is proportional to the amount of light falling onto a cell.
Digital cameras use CCD chips to image and can be used in astrophotography by aiming the camera lens into the eyepiece of a telescope. This is known as afocal photography.
There are some limitations to these cameras, however. During an exposure the charge wells gradually fill up with electrons in an uneven manner, even if there is no light falling on the particuliar cell. This effect is known as dark current and it limits digital cameras to short exposures. Astonomical CCD cameras reduce this effect by cooling the CCD chip with a thermoelectric cooler.
This is an extremely useful feature. Some CCD cameras have a secondary chip within them. This other chip is not for imaging but instead acts as a motion detector. In our observatory we have a second CCD detector feed by a fraction of the image, whose output feeds the autoguiding electronics. This electronics plugs directly into the computer drive system of the telescope or a computer which controls the scope. As the CCD image shows tiny movements from the stars it relays this information to the drive system of the telescope so that we can maintain near perfect tracking of an object during those long exposures.