In my experience, Astrophotographers are for the most part a friendly and welcoming bunch. However, there are certain topics that generate a certain amount of discord and animosity. The importance of focal ratio is one of those topics.
Stan Moore published a well known and famous/infamous webpage:
Stan Moore published a well known and famous/infamous webpage:
This tends to get people bent who know from personal experience that F-ratio does seem to make a good bit of difference and in the way they expect, based on terrestrial photography. So what is the truth?
My take is that the page, while perhaps not as clear and well written as it could be does present an important truth. Aperture in some cases does matter. However it doesn't necessarily matter in a way you care about.
What is correct depends a great deal upon your goals.
Goal # 1: Expose your sensor to some signal to noise ratio in as little time as possible.
Goal # 2: Gather as much information about an object as possible in a given amount of time.
For goal #1, f-ratio absolutely matters. Here, the aim is to get good signal to noise over the entire sensor, creating a hopefully pretty picture in as little time as possible. Reducing your f-ratio is the key to achieving this goal. One way of thinking about this is by reducing your f-ratio, you are increasing your field of view if you hold the aperture constant. Thus many more photons are hitting the sensor with a fast f-ratio telescope.
For goal #2, aperture absolutely matters. Here the aim is to get as much information as possible about a given object in a set amount of time. Assuming the object can still fit in the field of view, the way to do this is by increasing aperture.
Fast telescopes tend to be more expensive for a given aperture, and there is a good reason why astrophotographers are willing to pay for them. At the same time, very large telescopes are also very expensive, and there is a good reason why sometimes billions of dollars are spent on them as well.
Understanding your goals will help point you in the direction you need to go.