Nebula Public Library

The time-honored Rayleigh criterion specifies the mini- mum separation between two incoherent optical sources using a linear imaging system. As a matter of fact, it is the size of the point spread function that determines the resolution: two points closer than the PSF width will be difficult to resolve due to the substantial overlap of their images.
Until recently, this Rayleigh criterion has been consid- ered as a fundamental limit. Resolution can only be improved either by reducing the wavelength or by building higher numerical-aperture optics, thereby making the PSF