Focal ratio

The focal ratio or f-number is a term borrowed from the photographic world. It's the focal length divided by the aperture. The lower the number the brighter the image plane is. A telescope with a low f-number are sometimes called 'fast' telescopes, referring to the photographic world where a lower f-number meant a faster shutter speed. It's also worth noting that for visual astronomy the brightness of the image is determined by the aperture and the magnification used, not the focal ratio.