Article Index


Here are a few important values you may want to know how to find for the telescope that you are working with!

Focal Ratio:This is also called f-number, and it is written as a ratio:  f/4.5

Focal Ratio= (focal length)/(diameter of primary mirror)

So if I have a telescope with a 10in primary mirror, and a 45in focal length, its f-number is f /4.5!  

4.5 = 45 / 10

Magnification:This is the the power of magnification possible for a specific telescope when using different eyepieces.  This value is found by dividing the telescope‚Äôs focal length by the focal length of the eyepiece.

Magnification = (focal length of primary mirror) / (focal length of eyepiece)

**hint: the focal length of the eyepiece is always printed on the eyepiece in mm.  Remember to convert to like terms before dividing!!

Go to top