  We introduce a Fourier method (Fm) for the determination of best focus for
telescopes with stars. Our method fits a power function, that we will derive in
this paper, to a set of images taken as a function of focuser position. The
best focus position is where the power is maximum. Fm was first tested with
small refractor and Schmidt-Cassegrain (SCT) telescopes. After the successful
small telescope tests, we then tested Fm with a 2 m Ritchey-Chr\'etien-Coud\'e
(RCC). Our tests show that Fm is immune to the problems inherent in the popular
half-flux diameter method.
