Asked by Gerry White
The speed of a telescope refers to how much light it can gather and is a term that comes from photography. A lens with a high speed gathers much more light, meaning that the shutter speed can be faster in dim light improving those images. The speed of a lens is often referred to as an f-number and is calculated using a combination of the aperture (the lens diameter) and the focal length. It is worth noting, however, that these numbers and differences only really apply in photography. If the telescope is just being used for viewing with an eyepiece, with no astrophotography, the differences won’t present themselves.
Answered by Josh Barker from the National Space Centre
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