Hobbies And Interests
Factors to Determine F/Ratio
There are two aspects of a telescope used to determine its f/ratio. One is the aperture, which is simply the size of the primary optical lens. The diameter of this critical piece of the telescope can be measured in either inches or millimeters. The second component of the f/ratio is the focal length. This is determined by measuring, also in either inches or millimeters, the area between the lens and the focal point (the spot where the eyepiece is situated).
How to Determine F/Ratio
The formula for determining the f/ratio of a telescope is the focal length divided by the aperture. It is not uncommon to see an aperture measured in millimeters and the focal length in inches, or vice versa. If this is the case, converting one or the other to a common measurement is required before completing the f/ratio formula. Here is an example of arriving at the f/ratio: a telescope with a 1 meter (1,000 millimeters) focal length and 100 millimeter aperture has an f/ratio of 10 (1,000 divided by 100). With this example, the f/ratio is usually indicated as f/10.
Impact of the F/Ratio
As a rule, the lower the f/ratio the faster (or brighter) the telescope. For telescope users, faster equates to a wider field of view (FOV). The wider the fov, the more light the telescope is able to capture and the farther and more clearly an observer is able to see. The same concept applies to photography as well. Also, a smaller f/ratio often equates to a more portable telescope, one that can be moved and situated by one or two people. The larger f/ratio scopes may require more effort to move and position.
Telescope Uses Based on F/Ratio
In general, lower (faster) f/ratio scopes are good for deep space viewing as they are brighter. For planetary observation, longer f/ratio scopes are often used. Telescopes with f/ratios under 6 are considered small, 6 to 10 medium, and scopes with f/ratios beyond 10 are classified as large. Medium-sized telescopes are often used by hobbyists and astronomy aficionados for general purposes as they provide good fields of vision without the need for expensive, special eyepieces.