Here it is, my 500th post and it's just for you.
I'll keep my fingers crossed I've got my facts straight.
You are right. For stars
, providing lens quality and diffraction aren't limiting, aperture is the most important factor as it determines light gathering power. As usual, even with a perfect lens if you operate at too high an f-number diffraction will become an issue.
For extended objects like nebulae (or stars so close together they can't be individually resolved) f-number is important. For a given aperture the smaller the f-number the brighter the nebula is on the sensor. If sensors counted every photon and had no noise that wouldn't matter but in practice a small f-number is needed to get the signal from faint nebulae above the sensor noise.
Obviously there are trade-off's involved. There is no point in using a hypothetical 10mm f/1.2 lens as the small aperture restricts the stellar limiting magnitude
and the short focal length means that any nebulae in the field of view will be too small on the sensor to be of much use.
An optic capable of f/1.2 (the Canon EF 85mm f/1.2 for example) would be brilliant at showing nebulae and the Milky Way etc. but, quite apart from the inevitable optical aberrations from such an extreme lens when wide open, f/1.2 might sometimes be too fast as stray background light from the sky (from the Moon or as a result of many local authorities trying to light up the universe rather than the streets) may manifest itself so quickly that the stellar limiting magnitude is quite poor. At least with camera lenses it is easy to stop the lens down.
This isn't just a matter of theoretical nicety either. Take the Canon EF 85mm f/1.2 and the EF 135mm f/2 and ignore considerations like field of view. The 135mm lens has nearly the same light gathering power(10% less in fact) but at f/2 is two stops slower than the 85mm lens. However, the 85mm lens costs nearly twice as much. Assuming I had pockets deep enough to afford the 85mm I would still have to be convinced that I would be able to use it wide open for more than a few nights each year. I am still trying to work that issue out.
P.S. Nice avatar.