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A 'fast' scope is one with a ratio smaller than f/6, such a f/5 or f/4.5.
A 'slow' scope is a ratio is every ratio larger, like f/10 or f/12.
It is a relationship like the diaphram of a camera, a large diameter diaphram allows alot of light in, so in a way, a faster shutter speed. Conversly, a small diaphram diameter, a slower shutter speed is needed to allow the same amount of light in.
The f/ ratio is often overlooked, misunderstood & underestimated when selecting a telescope. A slow scope will deliver a dimmer image than a fast scope, often disappointing beginners. They do, however, disguise the poor quality of eyepiece usually sold with beginner, & even advanced, scopes.
I'll try to explain it with some practical examples.
The schmidt-cassegrain telescope, either by Meade, Celestron or whoever, has a focal ratio of f/10, very slow. This makes it a difficult beast to work with in photography because it needs really long exposure times to get a picture.
These same companies now make lens adaptors which can reduce the f/ratio to f/6.3 & f/3.3, making them 'faster', & reducing exposure times. They work like a negative barlow, where a barlow would increase focal length, these adaptors reduce focal length.
Another way to describe the visual effect of a fast scope vs. a slow one the comparison of an image of the same object with both a fast & slow scope using the same diameter of objective or mirror. The background brightness of the slow scope will be darker than the background in the fast scope. The slow scope has higher contrast in its image, ie, the blacks are blacker, but the fast scope can show fainter detail, though can be more diffiuclt to see because of the lighter background.
Refractors & reflectors deal with light differently, but each, as you may know has its own pros & cons which you need to balance up in choosing a scope, or assessing its performance. These then dictate what EPs & accessories you use & buy.
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