Telescopes Glossary

Data Reduction The process of turning data from astronomical instruments into measurements useful for doing science with. It is called "reduction" because you are usually taking large quantities of data (such as an entire image) and turning it into one or two useful numbers (such as the brightness of an object in the image).

Exposure Time The time that the "shutter" on the telescope instrument is open. The longer the exposure time, the fainter the things that can be seen. Exposure times on professional telescopes can be anywhere from tiny fractions of a second for studying bright stars to many hours when hunting for very faint objects.

Field of View The size of the piece of sky that a telescope can look at at one time. This is usually given as an angle.

Pixels Digital images are made up of lots of little squares of colour in a grid. These are called pixels.

Resolution The smallest piece of detail that can be seen with a telescope. Anything smaller than the resolution of an image will be "blurred" out. The resolution of a telescope improves as the telescope's aperture gets bigger. However, for the larger telescopes, the blurring by the atmosphere is the main effect. This is called seeing.

Reflecting Telescopes Telescopes that use mainly curved mirrors to collect and focus light are called Reflecting Telescopes or Reflectors. You can see how this works from ray diagrams like this one.

Refracting Telescopes Telescopes that use mainly lenses to collect and focus light are called Refracting Telescopes or Refractors. You can see how this works from ray diagrams like this one.

Seeing The blurring of images of distant stars and galaxies by the atmosphere of the Earth. This limits the resolution of a telescope.

Sensitivity The sensitivity of an instrument is a measure of how well it detects light. More sensitive instruments let astronomers see fainter objects and make better measurements.

Telescopes Glossary

Factfile: Gravity