What is the purpose of a telescope?

  • The main purpose of a telescope is to gather light, i.e. to collect and focus photons. We can think of a telescope then as a "light bucket" - the bigger the bucket, the more photons a telescope can collect.

  • In addition, a telescope can allow a more detailed view of the structure of a celestial source or to infer that there are two sources, not just one. In an astronomical image, the smallest angular separation between sources (or structures within the image of a single extended source) is called the ANGULAR RESOLUTION. The size of the smallest "point" source in an image is called the image's SEEING.

    The minimum angular separation of two sources that can be distinguished by a telescope depends on the wavelength of the light being observed and the diameter of the telescope. This angle is called the DIFFRACTION LIMIT.

  • In this equation, the computed angle is in radians. Remember that there are 206,265 arcseconds in one radian.

  • Click here to see some examples of calculations of diffraction limit.

  • The image obtained by an astronomer on the Earth may also be affected by ATMOSPHERIC SEEING, that is, the blurring of an image by turbulence in the Earth's atmosphere. Space-based telescopes are not affected by atmospheric seeing.

  • The seeing of an image is a measure of its quality or sharpness. The seeing is always bigger than the either (1) the diffraction limit or (2) the atmosphere seeing, whichever is greater.


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