TELESCOPES

12

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DIMENSIONS FOCAL RATIOS

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Focal Ratio
Having the focal length and the aperture in millimeters, a different equation can be done.

Focal Length 900mm
Aperture 114mm

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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 capabilities
Longer telescope
More suited 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.

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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.

INDEX

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.

END OF DIMENSIONS

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