The resolution of an
optical imaging system – a
microscope,
telescope, or
camera – can be limited by
factors such as imperfections in the lenses or misalignment. However, there is a principal limit to the resolution of any optical system, due to the
physics of
diffraction. An optical system with resolution performance at the instrument's theoretical limit is said to be
diffraction-limited.
[1]
The diffraction-limited
angular resolution of a telescopic instrument is proportional to the
wavelength of the light being observed, and inversely proportional to the diameter of its
objective's
entrance aperture. For telescopes with circular apertures, the size of the smallest feature in an image that is diffraction limited is the size of the
Airy disk. As one decreases the size of the aperture of a telescopic
lens, diffraction proportionately increases. At small apertures, such as
f/22, most modern lenses are limited only by diffraction and not by aberrations or other imperfections in the construction.
For microscopic instruments, the diffraction-limited
spatial resolution is proportional to the light wavelength, and to the
numerical aperture of either the objective or the object illumination source, whichever is smaller.
In
astronomy, a
diffraction-limited observation is one that achieves the resolution of a theoretically ideal objective in the size of instrument used. However, most observations from Earth are
seeing-limited due to
atmospheric effects. Optical telescopes on the
Earth work at a much lower resolution than the diffraction limit because of the distortion introduced by the passage of light through several kilometres of
turbulent atmosphere. Advanced observatories have started using
adaptive optics technology, resulting in greater image resolution for faint targets, but it is still difficult to reach the diffraction limit using adaptive optics.
Radio telescopes are frequently diffraction-limited, because the wavelengths they use (from millimeters to meters) are so long that the atmospheric distortion is negligible. Space-based telescopes (such as
Hubble, or a number of non-optical telescopes) always work at their diffraction limit, if their design is free of
optical aberration.
The beam from a
laser with near-ideal beam propagation properties may be described as being diffraction-limited. A diffraction-limited laser beam, passed through diffraction-limited optics, will remain diffraction-limited, and will have a spatial or angular extent essentially equal to the resolution of the optics at the wavelength of the laser.
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layers
Opens to the size of a tennis court
Protects Webb’s optics from the Sun
Right now, we just successfully completed the first step: unfolding the forward sunshield pallet. Think of Webb’s pallets as a cake stand that will hold 5 layers of sunshield — the cake’s layers, if you will. 
