TELESCOPES

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focal
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DIMENSIONS FOCAL RATIOS
ratio
CONTENTS
Focal Ratio
Having the focal length and the aperture in millimters, a different equation can be done.
magnifications dimensions
Focal Length 900mm
Aperture 114mm
TELESCOPE LINKS:
Basics
900/114 = 7.9 The focal ratio of the telescope is 7.9.
In specifications it is written F/7.9
Mounts
Light Gathering Power
Closer Look At Refractors
The equation can of course be turned around if the focal length isn't known and the focal ratio is.
Closer Look At Reflectors
Balancing
Eyepieces
Focal Ratio = F/7.9
Aperture = 114mm
Cleaning Optics
Tips
Astrophotography
7.9 x 114mm = 900mm Focal Length
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The focal ratio describes how long and wide the telescope is. It could be short and fat (short focal ratio) or long and skinny (long focal ratio) or anywhere in between. Focal ratios can range from F/3 to F/20 and beyond these ranges in some extreme cases.
Binoculars
Characteristics of a short focal ratio are:
Wide field of view
Lower magnification capabilities
Shorter telescope
More suited to deep sky objects
More suited to Astrophotography - Shorter exposures
Hard to collimate accurately - Reflectors

Characteristics of a long focal ratio are:
Narrow field of view
High magnification capabilites
Longer telescope
More sutied to planetary or the Moon
Less suited to Astrophotography - Longer exposures
Easy to collimate accurately - Reflectors
Generally the Newtonian Reflectors have ratios between F/4 & F/8.
focal
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Schmidt and Maksutov-Cassegrains Reflectors are between F/10 & F/15.
Achromatic and Apochromatic Refractors are between F/8 & F/20.
ratio
CONTENTS
The example telescope's focal ratio is right in the middle of the scale and is
regarded as an all-rounder. Using a Barlow will change the focal ratio.
magnifications focal
END OF DIMENSIONS
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