To learn about Focal Ratio, this is a good teacher:
https://cloudbreakoptics.com/blogs/news/astrophotography-pixel-by-pixel-part-2
And the image below from the site is a good illustration of F-ratio vs. exposure. Basically, to take good quality image in astrophotography, you want to stay away from long exposure if possible, thus, you want lower F-ratio = focal length (obj. lens or primary mirror) / aperture.
