Astronomy dictionary – F

Browse through our astronomy dictionary to find definitions for some of the most common terms used in practical astronomy and space science.

Click on one of the letters below to search for a term.


F-number (focal ratio)

The ratio of the focal length divided by the aperture (in the same units).


Faculae are brighter, more prominent patches of the solar photosphere (the ‘surface’ of the Sun). They are sometimes an indication of a forming sunspot and show up particularly well through white-light solar filters.

Field stop

The field stop is the physical size of aperture available to look through on any given eyepiece.


The small, low-power telescope that is attached parallel to the main telescope tube. It is used to initially locate celestial objects as it has a wider field of view than the main tube.


An extremely bright (brighter than Venus appears in the night sky), sometimes disintegrating, meteor that often leaves a trail.

Flat frame

An image that helps you remove unwanted dust specks and reduce vignetting in an astro image. It’s taken by covering the end of the telescope with a white cloth and shining a torch on it.


Debris that floats in the liquid inside the eye. A shadow is cast on the retina by floaters that can be seen in your peripheral vision. They come from detached parts of the retina.

Focal length

The distance between a telescope’s primary lens or mirror and the point at which an image is brought into focus.


The focuser is the small piece of mechanical equipment that holds the eyepiece. It is moveable to vary the distance between the eyepiece and the lens or mirrors to achieve focus. There are many different types of focuser on the market from simple rack-and-pinion devices to more elaborate double speed, feather-touch focusers.

Fossil group

These are the remnants of galaxy clusters that have seen most of their galaxies merge together. They can be identified by their vast haloes of X-rays. Massive elliptical galaxies will form in these groups, which contain the dark matter of entire galaxy clusters.

Full cut-off street light

Street lights that don’t allow light to escape from their lamp assembly above the horizontal. Full cut-off street lights are one way to reduce light pollution, especially sky-glow.