Chargecoupled devices

A CCD chip is divided into pixels. Each photon that falls on a chip shakes an electron loose; this electron is captured in a well (one well for each pixel). When the chip is read out, the number of electrons in each well is recorded, along with the position of the well. The more light that falls onto a pixel, the more electrons are shaken loose; when displayed on a computer, the brightness of any particular spot in the image is related to the number of electrons that were collected in the well corresponding to the position of that spot. Today's biggest chips have many millions of pixels each.

Astronomers call pictures "images." Some telescopes are used mainly for making images, while others are used mainly for spectroscopy. The spectrum of an object also is recorded on a CCD chip, so that it can be analyzed using computers.

Each element and compound has a unique fingerprint that it leaves on light as observed through spectral lines. If a star is moving its speed and direction (toward us or away from us) can be determined because the spectral lines will be shifted away from their "normal" positions. The amount that the lines are shifted is a direct measure of the object's speed along the line of sight from us to it.

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