Perfect optic would produce a perfect image, but no optic (is there anything?) is perfect.
Defects aberrations (像差)
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Monochromatic aberration
1.
Blurring of an image
Spherical aberration—(affecting
whole image):sharpness
astigmatism—off-axis
object in focus
→asymmetry→elliptical images
coma—(affecting
edges and corners)
2.
degrading the shape of an image
(geometric) distortion : pincushion, barrel
3.
Field curvature—when
center in focus edges are not, vice versa.
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Chromatic aberration
Color
fringes in images →
sharpness↓
Apoch
chromatic “APO” lenses use special glasses to minimize chromatic aberration.
All the above (aberrations) can be corrected in theory, but there is a
natural limit to the sharpness of an image.
Þdiffraction—spreading
of light when it passes through an aperture
—the
smaller the aperture, the more the diffraction
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Other aberrations
Flare
(internal reflection within lens…image “washed out”)
Vignetting
(cutting off light in the corners of an image)
Particular
example —
Schmidt telescope
Instead of a paraboloid (hyperboloid) mirror, a Schmidt telescope uses a
spherical mirror, but has a stop at the center of curvature.
—All rays are “on-axis”
Þ
no off-axis aberrations
Focal length
f=R/2
R: radius of curvature
—To correct for spherical aberration
→a
thin corrector lens
→only
little off-axis aberrations introduced
Þcurved
focal surface
∴need
to bend the photographic plates or CCD chip!
ÞA
Schmidt telescope offers a very wide field-of-view (FOV) at small focal ratio.
Ex. 120/90 cm Schmidt
telescope
90/60cm
Schmidt telescope