What Focal Length is Best?
Now I can’t tell you what focal ratio is best- it depends on use. Planetary viewing has very different requirements from Deep Space objects (like Nebula, star clusters, galaxies), and unfortunately, they are somewhat mutually exclusive. I can say f/5 is about as fast as you might want to try for a first scope (to go faster – lower #) is much more difficult to figure. Fast scopes are good for deep space views, becuase of wide field of view and their light cone. A f/10 is good for planetary use, but starts to get difficult to read the shadows in testing. A good compromise is f/7. But f/6 or f/8s are common and no more work.
