AST2210 - Lab exercise: aberrations 1 Introduction This lab exercise will take you through the most common types of aberrations. 2 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. 3 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. 1
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. See the top panel in 2
Figure 1: Spherical aberration 3
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. 4 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. 4
Figure 3: Astigmatism. Top: Rays in the tangential plane are marked with red and have their focus in T 1, rays in the sagittal plane are blue and have their focus in S 1. Bottom: point source images from an optical system with astigmatism. 5