F/Number
The f/number of a telescope is the
focal ratio.
You find the focal ratio or
f/number by dividing its focal length by its aperture.
A telescope’s size is equal to
its aperture. Aperture is the diameter of the mirror or objective
lens.
Focal length is simply the
distance between the mirror or objective lens and where the image
focuses.
For example:
If your telescope has a focal
length of 1800mm and the aperture is 406mm, the focal ratio would be
f/4.
1800 divided by 406 = 4
Magnification
The magnification of a telescope
is calculated by dividing the focal length of the eyepiece into the
focal length of the telescope.
For Example:
If your telescope has a focal
length of 1800mm and you‘re using a 25mm eyepiece, the
magnification would be 72X.
1800 divided by 25 = 72
