Telescope Equations

The following equations are frequently used in amateur astronomy. I have composed this page of the most common equations, so they can be easily obtained and used. For a more detailed description and example of each equation please visit their respective pages.

Focal Ratio = ( ƒ_scope ) ÷ ( D_scope )
Magnification = ( ƒ_scope ) ÷ ( ƒ_ep )
True Field of View = ( AFOV ) ÷ ( Magnification )
Resolution:
♦ Rayleigh’s Limit: Resolution [ arcseconds ] = 138 ÷ ( D_scope [ mm ] )
♦ Dawes’ Limit: Resolution [ arcseconds ]  = 116 ÷ ( D_scope [ mm ] )
Limiting Magnitude = 2 + 5log( D_scope )
Max Useful Magnification = ( 60 ) × ( D_scope[inches] )
Lowest Useful Magnification = ( 4 ) × ( 5 ) = 20x
Exit Pupil = ( ƒ_ep ) ÷ ( f-ratio_scope )
Light Gathering Power = [ ( D_scope ) ÷ ( D_pupil ) ]²
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