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Lenses and telescopes range from photographically "slow" to "fast", literally.
The lower the f number the faster the scope/lens. Put simply, the lower the f number the more light that will get to the camera sensor in any given time.
In short, an f/3.5 scope/lens will provide the same result as an f/5 scope/lens
in about half the exposure time, allowing the user to get twice as many subs
for stacking from any given length of imaging session.
raymo
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