Just 15 years ago, astronomers primarily used photographic plates to observe the heavens. These plates are panes of glass coated with photographic emulsion, and are basically no different in technology from film bought at the drug store.
But plates suffer several problems.
First, they are not very efficient. For each photon (particle of light) that hits the plate, there's only a one per cent (roughly) chance that it will be recorded.
Second, they are not linear detectors. This means that the density of grains is not proportional to the amount of light. Instead, the density is roughly logarithmic. So, if 100 grains corresponds to 10000 photons, then 200 grains corresponds to 100 million photons.
Third, they do not have a great deal of dynamic range. This means that you can't reliably photograph a bright star and a faint star on the same frame. The bright star will saturate (produce maximum grain density) long before the faint star has been recorded reliably.
The last 15 years have brought us leap after bound in silicon technology, as anyone using the Internet must surely know. One important type microchip is called a charge-couple device or CCD. These chips are arrays of silicon pixels, each of which reacts very sensitively to light, and as such are very efficient photon detectors. For every photon that strikes a CCD pixel, there's more than a 50 per cent chance it will be detected, a vast improvement over the one per cent chance with photographic plates. These detectors, by their nature, are linear, because they simply count photons. This means each pixel contains a count of the photons that have struck it, far simpler than measuring the density of grains on a photographic emulsions that is roughly logarithmic to number of photons.
In the last several years, CCD's have also come to have great dynamic range, due to very low and very stable readout noise (this electronic noise establishes the reliability of the faintest detections). Perhaps most significantly, CCD's, like the rest of computer chips, have grown in size, quality, and speed, but fallen in price. A typical astronomical CCD these days has 2048 x 2048 pixels, a dynamic range of 65000 per pixel, and a read noise of two per pixel. Less impressive CCD's occur in cam-corders and night-vision goggles, where smaller formats, and poorer noise characteristics will do. Those green night-vision images from the Gulf War came from so-so CCD's, for instance.
Currently underway are projects to make extremely large arrays of CCD's that can grab vast amounts of data at a time. The Sloan Digital Sky Survey will use
Go To Page: 1 2
