
10.5.
Diffraction Integrals and Fourier Theory 271
that are dififraction-limited for visible and near ultraviolet wavelengths. For an
aperture at larger field angles, on the other hand, the residual astigmatism of the
HST must be corrected by the optics following the aperture.
If we approach the angular size of an image fi-om the point of view of
diffraction theory, then an image that is diffraction-limited has an approximate
diameter for the Airy disk oflAAX/D or
1.22>l/a
for a clear aperture. Contrary to
predictions
fi*om
geometric optics, an image cannot be smaller than that given by
diffraction theory. It is instructive to take the top