What is the purpose of a telescope?
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 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.
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.
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.