DIMENSIONS FOCAL RATIOS
Having the focal length and the aperture in millimeters, a different equation can be done.
Focal Length 900mm
900/114 = 7.9 The focal ratio of the telescope is 7.9.
In specifications it is written F/7.9
|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
Focal Ratio = F/7.9
Aperture = 114mm
|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.
Characteristics of a short focal ratio are:
Wide field of view
Lower magnification capabilities
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
More suited to planetary or the Moon
Less suited to Astrophotography - Longer exposures
Easy to collimate accurately - Reflectors
|have ratios between F/4 & F/8.
|Schmidt and Maksutov-Cassegrains Reflectors
|are between F/10 & F/15.
|Achromatic and Apochromatic Refractors
|are between F/8 & F/20.
|The example telescope's focal ratio is right in the middle of the scale and is
|regarded as an all-rounder. Using a
|will change the focal ratio.
|END OF DIMENSIONS
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