Equipment

Focal Ratio (f/number)

Telescope focal length divided by aperture diameter

Definition

The focal ratio (f/number) is calculated by dividing focal length by aperture. Lower f/numbers (f/5) mean faster optics that collect more light per pixel, but with smaller image scale. Higher f/numbers (f/20-f/30) are common in planetary imaging to increase image scale. Barlow lenses increase effective f/ratio. For planetary work, f/20-f/30 is typical.

Related Terms

Barlow Lens

Optical element that increases telescope magnification

Planetary Camera

High-speed camera designed for planetary imaging

Arcsecond

Angular measurement unit (1/3600 of a degree)

See It In Action

Learn how Focal Ratio (f/number) is used in planetary imaging with our step-by-step guides:

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