Aperture size !!!
An F5 1000mm scope is 200mm diameter
an F8 1000mm scope is 125mm diameter
The bigger aperture gobbles up more photons,
its like filling a bucket with a hose, 20cm diameter hose with constant flow fills the bucket two and a half times FASTER than a 12.5cm hose.
Much faster !!!
The terminology was particularly relevant in film photography where in order to form an image a certain number of photons are required,
they may be gathered quickly with a large diameter diaphragm (fast lens or aperture) or over a longer period with a smaller opening (slow lens or aperture)
With telescopes its the same thing, the scope is the lens, our eye is the camera and the brain
sometimes does the processing and archival storage.
