CCD chips capture patterns of energy and converts the information into data streams.
A charged-coupled device (CCD) is a semiconductor device that collects a pattern of charges---like a photographic image---and then shifts this array of charges off the chip into a device that turns the array of signals into a stream of data. This process can be continued to produce a sequence of images (like a movie). This technology is rapidly becoming the image-processing standard for cameras, movies and astronomy.
Frame Rate
The frame rate is the speed at which images are transferred off the CCD chip. The image on a CCD is quickly shifted off the semiconductor collector and then converted to a data stream while the next image is collected, so the frame rate represents the exposure time. Functionally, this makes the frame rate the equivalent of shutter speed in film cameras. Longer frame rates collect more light, so when the Hubble takes a CCD image the frame rate is large. For ordinary cameras (and movie cameras) a long frame rate will cause blurred images. The frame rate is a specification that is relative to the scene being photographed. If everything in the scene is stationary, the longer the frame rate, the better. The faster things are moving in the scene, the faster the frame rate needs to be.
S/N Ratio
The signal-to-noise (S/N) ratio is a measure of how much noise the CCD device adds to the data stream. It is impossible to build a CCD with no noise at all, but the better constructed the CCD device is, the better the S/N ratio will be. This is primarily a quality control issue. The two major sources of noise are read noise and shot noise. Read noise is dependent on the manufacturing process. If the CCD can only register electrons in groups of 10, then 2 electrons or 8 electrons might register the same. Shot noise is caused by the physics of light, so there is nothing that can be done about it. The noises added and the read noise stay the same, while the shot noise is dependent on how long the exposure is. In other words, for long exposures the read noise becomes a small component of the noise---the longer the exposure, the less does the quality of the CCD matter.
CCD Size
The CCD size---the number of elements---is the specification that is advertised for cameras. Each element in the CCD represents 1 pixel (picture element) in the finished product. The higher the CCD size the greater the resolution of the picture. It would seem that the greater the CCD size, the better, but this is not true. Past the point of resolution you need---the eye has its limits---all the money is wasted. Worse than that, the CCD elements have a small amount of interference with each other, which shows up as noise. Very high CCD sizes have a high S/N ratio.
Tags: frame rate, frame rate, data stream, device that, noise dependent
