F

Faculae

Extended bright regions seen near the sun's limb.

 

Filar micrometer

An instrument attached to the eyepiece of a telescope to measure the angle (separation) between two stars and their relative position.

 

Fireball

An unusually bright meteor.

 

Flare

A very bright flash of light over a small area of the sun's surface, especially near an active sunspot, which is due to an intense outburst of energy.

 

Flash spectrum

The spectrum of the sun's limb that is the bright line spectrum of the chromosphere visible for an instant just before totality in a solar eclipse.

 

Flocculi (plages)

Bright regions in the chromosphere in the magnetic fields around sunspots visible in spectroheliograms in monochromatis light.

 

Flourescence

The absorption of radiation of one wavelength (especially ultrviolet) and its re-emission in an another wavelength (visible light).

 

Focal length

The distance from the center of a lens or mirror to the focal point.

 

Focal ratio

The ratio of the focal length of a lens or mirror to its diameter.

 

Forbidden lines

Spectral lines not usually obtainable under laboratory conditions because their emissions result from atomic transitions that are most improbable.

 

Fraunhofer line

An absorption (dark) line in the spectrum of the sun or a star.

 

Fullmoon

The phase of the moon when it is in opposition (opposite side of the earth from the sun) and its entire visible hemisphere is illuminated.