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In case you weren't aware of it, the focal ratio of a scope is the distance
from the primary mirror or objective lens that an object comes to focus,
divided by the diameter of said mirror or lens. For example a scope with
a 200mm diam lens or mirror, and the object comes to focus at a
distance of 1000mm, is said to have a focal ratio of f/5.
The longer the focal length of a scope, the longer it takes to pass a given
amount of light to the camera's sensor, hence long focal length scopes
are called slow, and short focal length ones are called fast. That's it put simply.The main advantage of long focal length scopes is that they
display fewer optical aberrations such as coma and chromatic
aberration.
raymo
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