F Ratio or Speed want could this mean?
In astrophotography we are usually pushing the camera and lens to the limit, running high ISO settings for long periods of time. We very rarely stop a lens down; in the diagram below all Lenses are the same Diameter with different F ratios. The bigger the patch of sky the more light it contains and the brighter the picture will be.
I hear you say what is an F ratio? It is how many times the lens diameter, can fit into the focal length of a lens. See scale on left. The boxes at the bottom of the page are a comparison of the amount of sky that can be seen with a curtain lens F ratio.