Concerning astrophotography and the recording of stars, there seems to be some confusion about f-ratios, aperture, focal lengths, etc. For point sources such as stars, it is the focal length, not the physical aperture, that determines the limits of what will be recorded on film. This is because the amount of background sky included in the picture varies with focal length and thus the amount of magnitude-limiting sky fog goes up as the focal length decreases. Longer lenses include less of the sky and therefore less of the sky fog. Since the stars are points, their light is not spread out as focal length (magnification) increases. This effect results in an increase in the ratio of starlight (point source) to skylight (non-point source) as focal length increases, and fainter stars are recorded before being limited by the sky fog. This light-source ratio is not affected by the f-ratio or physical aperture of the lens. For example, a 50mm lens at a dark site has a limiting photographic magnitude of about 11.5. A 500mm lens has a limiting magnitude of about 16. The magnitude scale is a way of estimating the brightness of an object, with each successive magnitude number being about 2.5 times brighter than the next one (magnitude 1 is 2.5X brighter than magnitude 2). The f-ratio does determine how fast the sky fog limit is reached. Exposures longer than that needed to reach the sky fog limit will not record fainter stars.
For star trails, you should pick an f-ratio that will give you a decent star exposure for the faintest stars you want to record. For a given exposure time, too low a ratio will cause a fast sky fog build up with little contrast between stars and sky. Too high an f-ratio will result in fewer stars against a darker background. The f-ratio you choose will depend on the local sky conditions and the focal length of the lens for the reasons stated above.
