Download 471/Lectures/notes/lecture 29 Abberations

Imaging problems
Depth of field in imaging
But not ideal! Maps (and
blurs) a very narrow angle
range from one object
point Dq  r/L to one image
point.
Depth of field in imaging
Sometimes a nice effect, sometimes a problem
Increasing depth of field by closing lens aperture (f-stop)
Pinhole camera: simplest imaging system to make
How does it work?
But not ideal! Maps (and
blurs) a very narrow angle
range from one object
point Dq  r/L to one image
point.
What is the limit to the resolution?
20 minute exposure taken with a pinhole camera
Solar eclipse viewing
Holes in the leaf canopy create
images of the eclipse on the
ground.
Abberations
Latin aberrare
"go astray"
Chromatic Aberration
Problem: n = n(l), so f = f(l)
Chromatic Aberration
Chromatic Aberration
Correction: Use a lens pair, with the strongest lens (smallest f) of low
dispersion coupled with a weaker one (long f) of high dispersion
calculated to match the focal lengths for two chosen wavelengths.
Cemented doublets (“achromats”) of this type are a mainstay of lens
design.
Spherical aberration in lenses, mirrors
Problem: Even for light rays parallel to the optical axis, at
large distances (h) from the axis, small angle approximation
breaks down: f = f(h)
Spherical aberration
Expensive correction: Use aspherical lenses or mirrors for
fixed imaging/object distances…new optics for each
distance. Used if need more light.
Spherical aberration
Cheapher correction : Choose best spherical lens radii
If you want to best focus a laser beam with a planoconvex lens, which
way should you face the flat side: _______ the laser
a) toward b) away)
Spherical aberration
Correction : Choose best spherical lens radii
You get a different
curve for each do,
di pair (new
position factor)
Spherical aberration
Cheapest, most flexible correction: Restrict h:
Aperture or “field stop”.
f #
f
D
Effect on light-gathering ability or “speed” of optical system:
Spherical aberration and Hubble
They paid a lot for a primary hyperbolic mirror
Spherical aberration and Hubble
hyperbolic design e= 1.00115
Problem: e = 1.00660.
Test optics: assembled wrong
Spherical aberration and Hubble
Test optics were assembled incorrectly.
Corrective optics installed 3 yrs later
Corrected with two more mirrors
“Pincushion”
“Barrel”