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We introduce a Fourier method (Fm) for the determination of best focus fortelescopes with stars. Our method fits a power function, that we will derive inthis paper, to a set of images taken as a function of focuser position. Thebest focus position is where the power is maximum. Fm was first tested withsmall refractor and Schmidt-Cassegrain (SCT) telescopes. After the successfulsmall 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 popularhalf-flux diameter method.

A Fourier method for the determination of focus for telescopes with stars