A technique in which the normal performance of an instrument is deliberately degraded in such a way that the instrument’s performance is actually improved for one specific application. It may be applied to optical and radio telescopes (where it is called tapering the antenna), and is particularly important in Fourier transform spectrometers where it leads to the elimination of spurious spectral features at the expense of a slight reduction in spectral resolution. A common application is the use of an apodizing screen in the observation of a close visual binary where the image of a faint companion star falls on one of the diffraction rings surrounding the Airy disk. A circular screen which gradually becomes opaque towards the edge reduces the brightness of the rings; a screen with a square aperture produces a cross-shaped diffraction pattern which can be rotated clear of the companion star.