What Is The Significance Of A Telescope's Focal Length?
All things otherwise being equal, slower focal ratios produce less aberrations, but in many designs, a faster focal ratio produces a more compact telescope, meaning there is always some sort of compromise.
The purpose of the main optical element of a telescope (primary mirror or lens) is to gather light from an object and to concentrate that light into an image. This image is a fixed size but the size depends on the focal length of the optics - the longer this is, the larger the image will be at the focal point. You can think of this like the distance between a slide projector and the screen - move the screen and slide projector further apart, and the image gets larger, but dimmer.
It is the distance from the mirror or lens to where the image is formed. For an optical system, such as a telescope, it is a measure of how strongly the system converges (focuses) or diverges (defocuses) light.
For an optical system in air, it is the distance over which initially collimated rays are brought to a focus. A system with a shorter focal length has greater optical power than one with a long one; that is, it bends the rays more strongly, bringing them to a focus in a shorter distance.
It is often expressed as an f/ratio:
f/ratio = focal length of lens/aperture of lens
Example: 130mm lens with a f length of 650mm
f/ratio = 650/130 = f/5
What is considered a long focal length?
These are considered to be in the f/9 or greater range. A telescope of a given diameter coupled with a fairly short one, say f/5 produces bright images but wide fields.
This is fine for observing large deep-sky objects and star fields, but if you also want to observe planets, you'll want a slightly longer focal length.
Speed
Focal ratio is the ratio between an optical system's focal length and aperture. For example, a 100mm aperture telescope with a focal length of 1000mm would have a focal ratio of 10.
Focal ratio determines photographic speed, so an f/5 telescope requires shorter exposure times than an f/10 telescope. Therefore higher focal ratios are called slower, and lower focal ratios are called faster.
Also, all things otherwise being equal, slower focal ratios produce less aberrations, but in many designs, a faster focal ratio produces a more compact telescope, meaning there is always some sort of compromise.
About The Author:About the Author: John Dixon documents his astronomy exploits in his blog www.MyAstronomyBlog.com. He also maintains a bookkeeping software application called MyBookkeepingManager.
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