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10.21: Optical Detectors

Astronomers need quantitative information about distant objects. Pictures of astronomical objects are interesting, of course, but much astronomical work requires measuring an object's brightness: the amount of light coming from it at all wavelengths or over a range of wavelengths. This is called photometry. Astronomers use photometry for a precise measure of the amount of light at various wavelengths; this allows them to measure temperature, composition, and other properties of a remote object. Photometry must be calibrated with a target of known brightness, perhaps the Sun, or one of the planets, or a nearby bright star. Photometry is not confined to optical wavelengths; astronomers working across the electromagnetic spectrum from radio waves to X-rays need to measure the precise amount of radiation received by their detectors. Spectroscopy is the technique where radiation in a particular part of the electromagnetic spectrum is dispersed in wavelength and recorded behind a telescope. Spectro photometry is a measure of the brightness at each wavelength.

For the first two hundred years of telescope use, observations were made with the naked eye and subsequently recorded on paper. Naturally, the reliability of these observations could be questioned. When photography was developed in the middle of the 19th century, astronomers were quick to use properly exposed film to make a permanent record of their observations. Many small telescopes today come equipped with photographic attachments for this purpose. Faint objects like nebulae and galaxies are usually better represented in photos, because light can be accumulated in long exposures. By contrast the eye only "stores" light for a fraction of a second. Many photos of star fields and nebulae in textbooks and magazines were taken with small telescopes or ordinary cameras. Consult the picture captions, most of which describe the equipment and exposure used. In many cases, you can duplicate or improve on the results with your own equipment. Neither the eye nor a photographic emulsion are very efficient; each only captures about 1% of the photons they receive.

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The OmegaCAM array of 32 CCDs used on the Very Large Telescope. Click here for original source URL.

In the past 25 years, photographic techniques have been supplanted at most research telescopes by electronic detectors. The most important type of electronic detector is a charge-coupled device, commonly known as a CCD detector. CCDs are extremely light-sensitive detectors, made possible by microelectronic technology (CCDs are found in most camcorders, for example). The device works by converting incoming photons into electrons, which are then stored and accumulated until being read out and converted into an electrical signal. The electrical signal is then converted to intensity units that can be displayed as an image. CCDs were invented in 1969 by Willard Boyle and George Smith, who were working at Bell Labs in New Jersey. Their project was about to be shut down due to lack of progress when, over lunch, they sketched out the idea of a back-illuminated silicon light detector. Their innovation was recognized with the Nobel Price in Physics in 2009.

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LSST camera concept with a person for size scale. Click here for original source URL.

CCDs consist of arrays of typically 4000 × 4000 = 16,000,000 light detectors in a device the size of a postage stamp. Each tiny detector is called a picture element, or pixel. Each pixel has its own measure of brightness, and the many pixels combine to form an image similarly to the way dots of different sizes combine to form a photographic image in a newspaper. The best current devices use a mosaic of many CCDs to view a larger area of sky. Clever electronics is used to read out the information from hundreds of millions of independent pixels in just a few seconds. The state of the art in astronomical imaging now uses cameras with over a billion pixels! Each of the four Pan-STARRS telescopes in Hawaii has a camera with 1.4 billion pixels. One of the largest CCD devices in current operation is on the High Resolution Imaging Science Experiment (HiRISE) camera on the Mars Reconnaissance Orbiter. It has 14 CCD sensors that can take images containing up to 2520 megapixels or 2.5 gigapixels (that's 2.5 billion pixels). The upcoming Large Synoptic Survey Telescope (LSST) will have a 3.2 gigapixel camera for surveying the entire night sky. This will be the largest camera ever constructed. This 3200 megapixel camera will take images approximately 300 times larger than the average consumer digital camera today of resolutions between 10-20 megapixels.

CCDs have revolutionized optical astronomy. A CCD on a large telescope can produce images of light sources that are roughly a billion (109) times fainter than the eye can see! Where does this fantastic gain come from? CCDs are ten times as efficient as the eye in detecting incoming photons, and the diameter of a large reflector is 1000 times the diameter of the pupil of the eye. An additional factor of 100,000 in sensitivity comes from the fact that the brain "reads out" the eye's image every 1/30 of a second (allowing us to see continuous motion), whereas a CCD can collect data on a single target for an hour or more. Astronomers mount CCDs in sealed cameras cooled with liquid nitrogen, which reduces the background signal due to thermal noise. CCDs are nearly perfect detectors, converting almost every incoming photon into an electrical signal. This nearly 100% efficiency is a hundred times better than the eye or a photograph. For this reason, the only way astronomers can see even fainter objects is to build larger telescopes and collect more light. CCDs have also been constructed that work at infrared and ultraviolet, even at X-ray, wavelengths.

Photographs are not quite obsolete; they are still the favored medium for wide-angle imaging by amateur astronomers. The heart of a CCD is a silicon wafer that converts light into electrons, and it is difficult to make this device larger than a couple of centimeters across. By contrast, photographic emulsion can be made up to a meter across. However, as digital cameras become more widespread, photography has shrunk to a small niche in astronomy. All of the major research results you hear about come from digital detectors.