Quote:
Originally Posted by acarleton
key terms to remember are focal ratio or F value is the focal length/the aperture and is a measure of how fast or bright objects appear, smaller f means a brighter object.
focal length, larger focal length means more magnification, having said that, many objects in the sky do not require large magnification and larger focal lengths tend to have higher f values
then there are a variety of scopes and mounts
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Sorry to butt in but the above statement needs some clarification. Brightness of an image when a scope is used visually, not for imaging, is not affected by the focal length. If you get an 8" f5 and an 8" f10 and compare the image will be the same at the same magnification. Using the same eyepiece will produce a image with a lower surface brightness in the f10, but that simply because it is a bigger image.
Focal length can be important especially with dobs or refractors as it determines the length of the tube which can impact on the weight and maneuvourability of a scope as well as ease of setup and portability.
The whole "faster scopes makes brighter images" is certainly true for imaging where a sensor or film is capable of cumulative collection of photons, but the human eye is not.
Malcolm