Astronomy
The Good, Bad, and the Ugly Telescopes If you are looking at 8" telescopes, you already know the telescope's aperture is either 203 mm or 200 mm, depending on how the telescope's lens or mirror is mounted in its tube. 203 mm is better, both for brightness and useful magnification.  You should be given either the telescope's Focal Ratio referred to as its F: number, or the telescope's Focal Length, if not, - run away from that vendor.
The rule is simple: Any telescope that has a magnification greater than twice its own aperture (in millimeters) is magnifying beyond its resolving power.
The telescope's focal length (in millimeters) is the product of its F: number and its aperture (in millimeters).
Twice 203 mm is 406, so a good 8" telescope's magnification should not exceed 406X.
The telescope's magnification is its focal length divided by the focal length of the eyepiece in use.  Since the telescope's focal length is the product of its Focal Ratio and its Diameter in millimeters, - an 8", F:11, telescope using a 5.0 mm eyepiece, would have a magnification of 11 X 203 / 5.0 = 446.6, which is greater than 406, and therefore fails the test of a good telescope.