For image brightness at the sensor, f-number is still the key measure of “speed.” A larger physical aperture gathers more total light, but if focal length also increases, that light is spread over a larger image, so each part of the sensor can end up receiving the same brightness at the same f-number.

A simple way to see it: doubling focal length narrows the field of view and spreads a given patch of sky over about 4× the sensor area. To keep the same image brightness, the entrance pupil must also scale accordingly — which is exactly what the f-number describes.

Why the confusion in astronomy? Telescope users often talk about aperture because larger apertures improve total light-gathering and resolution, especially for visual use and for magnification. But for photographic exposure of extended subjects (like the sky, nebulae, or a uniformly lit scene), image surface brightness is governed primarily by f-number, not aperture alone.

The “sky fog limit” discussion likely comes from stray or atmospheric light. Since that unwanted light may not behave like parallel light from distant stars, the physical aperture and optical setup can influence how much background haze reaches the sensor. But that is about background contamination, not a replacement for f-number as the exposure metric.