If you’ve spent any time around telescopes, you’ve heard someone say something like “I picked up an f/6 refractor” or “this f/10 SCT is incredible for planetary.”
The Simple Version
F-ratio = focal length divided by aperture. A telescope with a 1000mm focal length and a 100mm aperture is an f/10 scope. A telescope with a 600mm focal length and a 150mm aperture is an f/4 scope.
What the Number Means
A low f-number (f/4, f/5) means a fast telescope — wide field of view, better for deep-sky astrophotography. A high f-number (f/10, f/12) means a slow telescope — narrower field, excellent for planetary detail.
Why Astronomers Have Opinions
F-ratio is treated with the same intensity that audiophiles treat cables. The deep-sky crowd will tell you anything above f/6 is a liability. The planetary imagers will tell you anything below f/10 is a toy. Everyone has a position and reasons for it.
Why Is This On a T-Shirt?
“What the F-Ratio?” is one of the great astronomy double-entendres. It is a legitimate question and also an expression of exasperation. Clouds rolled in right before the peak of a meteor shower? What the f-ratio. You drove an hour to a dark sky site and forgot the eyepiece case? What the f-ratio.