Telescope: Aperture, Focal length and Resolving power

Telescope aperture

The telescope aperture is largely responsible for how much light the telescope can collect. The larger the aperture, the:

  • larger is the resolution → more particulars / detail
  • more brightly faint objects such as galaxies and nebulae are imaged at the same focal length
  • But: the larger the aperture, the more sensitive the telescope is to air turbulence (seeing)


Since the telescope aperture is a circular area, increasing the aperture has a quadratic effect. If the aperture is doubled, the light gathering capacity is quadrupled.


Telescope focal length

The telescope focal length determines the magnification (visual in conjunction with an eyepiece) or the angle of view (when using a camera). The higher the focal length, the larger the objects appear. However, this also reduces the section that can be seen, which makes it more difficult to find objects and prevents the observation of large-area objects.

With long focal lengths, aberrations are less significant because the light does not have to be refracted as much on the way from the telescope entrance to the focal point.


Resolving power of the telescope

The resolving power of a telescope is its ability to display two objects closely neighboring at an angular distance α still separately. The resolving power depends on the wavelength of the light and the telescope aperture.
It can be calculated according to the Rayleigh criterion as follows (

In this case Lambda is the wavelength of the light. Depending on the used filters, the appropriate wavelength can be used here for photography. When viewing with an eyepiece, the one of green light (535nm) is used, since the eye is most sensitive in this range.

The formula also shows the linear relationship between the telescope aperture and the resolving power. If the telescope aperture doubles, the value of the resolving power halves. The resolution is therefore better by a factor of 2 than before.