A telescope work as follow: light is collected through the telescope aperture and is then transmitted toward the focal point (focuser) where it is focused for you to see through the eyepiece.
The focal length is essentially the distance (in millimetres) that light travels inside the telescope from its entry point (the aperture) to the exit point (the focuser where you place your eyepiece or your DSLR camera, also called prime focus imaging).
In a refractor or a reflecting telescope, the focal length start at the aperture (primary mirror or objective lens) and ends at the focuser where the light rays come together. Generally, when looking at a telescope, it’s safe to assume that a long tube means a long focal length and a short tube means a shorter focal length.
However, some telescopes type such as the Schmidt-Cassegrain have a fairly different optics setting and are equipped with secondary mirrors on which the light is bounced off before being sent to the focuser. This means that they have a much longer focal length even tho their tube looks much smaller than a Newtonian telescope.
Hey there, great article, easy read for starters. Worth noting an error in calculating magnification in this article. 8″ = 203mm is the objective lens specs and not the focal length of the telescope that you’ll need for magnification. Telescope’s focal length isn’t specified in the example provided under ‘Focal length and magnification’ header.
Hello Rishi, thank you very much for taking the time to comment. I have fixed the calculation error. All the best and clear skies 🙂