AST2210 - Lab exercise: aberrations
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Introduction
This lab exercise will take you through the most common types of aberrations.
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Chromatic aberration
Chromatic aberration causes lens to have dierent foci for dierent wavelengths.
Exercise 1
Make a simple drawing illustrating chromatic aberration.
We will use the set-up with the white light lamp, the color camera with
microscope objective and a thin singlet lens. We have used this set-up in the
rst lab exercise (CCD) this time you have to make the set-up and align the
optics. You can use the ber laser the align the optics (note to use the cover on
the camera!).
Exercise 2
Measure the distance between the red, green, and blue foci. Take an image of
each focus for your report.
Exercise 3
Do you see an Airy pattern? Explain. Put a color lter in the white light lamp
and explain what you see.
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Seidel aberrations
The ve Seidel aberrations are named after the German Philip Ludwig von Seidel
(18211896) who decomposed the rst order monochromatic aberrations into
ve constituent aberrations. They are: spherical aberration, coma, astigmatism,
eld curvature, and distortion.
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3.1
Spherical aberration
Spherical aberration is caused by the eect of annuli of a lens that are of dierent
radii have dierent focal lengths. See gure 1. Spherical aberration occurs in
every spherical lens (and mirror), and is the only monochromatic aberration
that is still present when the light beam is parallel to the optical axis.
We use the same set-up as the previous exercise with the same thin singlet
lens and color camera, but now with the laser connected to the collimator tube
(with dampening lter). The reason for using a singlet lens is that it has more
spherical aberration than doublet lenses. Doublet lenses are designed to reduce
spherical aberration almost to zero.
Exercise 4
Take images of the Airy pattern 1) in focus, 2) inside focus, and 3) outside
focus. Explain the dierences.
3.2
Coma
Coma is an aberration which causes rays from an o-axis point of light in the
object plane to create a trailing "comet-like" blur directed away from the optical
axis. See gure 2. The further o-axis, the worse this eect is.
Coma is a serious aberration that one should try to avoid in a optical system.
Nowadays it is hard to nd lenses that have signicant amount of coma.
To
demonstrate coma we have a specially designed lens compound.
Exercise 5
Put the lens compound in the set-up with the laser, collimator tube with dampening lter and mono-chromatic camera. Tilt the lens to get various amount of
coma and record images for your report.
Exercise 6
Use the circular aperture to decrease the aperture. Describe what happens to
the pattern.
3.3
Astigmatism
Astigmatism is an aberration for which in the case of an o-axis object, rays
in dierent planes have dierent foci. The planes to consider are the tangential
plane: the plane containing the object and the optical axis, and the sagittal
plane: the plane perpendicular to the tangential plane.
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See the top panel in
Figure 1: Spherical aberration
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Figure 2: Dierent amounts of coma
gure 3. A point source in a system with astigmatism becomes a cross, see the
bottom panel of gure 3.
To illustrate astigmatism, we use a thick doublet lens in the set-up with the
laser, collimator tube with dampening lter and mono-chromatic camera.
Exercise 7
Tilt the lens to get various amount of astigmatism and record images for your
report.
3.4
Barrel/pincushion distortion
When a lens suers from barrel or pincushion distortion, the magnication
decreases or increases with distance from the optical axis respectively.
At the present time we only have a lens with barrel distortion.
The lens
is directly attached to a camera, using an adapter between the lens and the
camera screw the lens halfway into the adapter and then screw the adapter
into the camera until a desired object in the room is focussed. Note that there
is no end-stop for the lens in the adapter and there is a risk that one damages
the camera sensor if one screws the lens too far into the camera.
Exercise 7
Record images to illustrate barrel distortion for your report.
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Report
The report should contain the recorded images taken throughout the exercises,
background to the exercises, comments on the results (the dierent kind of
aberrations etc.) and explanations of the various optical setups used.
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Figure 3: Astigmatism.
Top: Rays in the tangential plane are marked with
red and have their focus in T1 , rays in the sagittal plane are blue and have
their focus in S1 .
Bottom: point source images from an optical system with
astigmatism.
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