网络给人获得信息提供了变利，但是很多错误的信息也得以流通，这和以往的由peer reviewed, 有编纪校正的出版大大不同， 如Wikipenia 就是一例。

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


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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Build your own telescope
for schools

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