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Sa lens side winter 2008 zero sa pos sa 41 winter

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Unformatted text preview: lo • Positive spherical lenses have positive S.A., where exterior rays focus closer to lens • Negative lenses have negative S.A., as do plates of glass in a converging beam • “Overcorrecting” a positive Overcorrecting” lens (going too far in making asphere) results in neg. S.A. asphere) neg. Coma • Off-axis rays meet at different places depending on ray height • Leads to asymmetric image, looking something like a comet (with nucleus and flared tail) – thus the name coma • As with all aberrations, gets worse with “faster” lenses faster” • Exists in parabolic reflectors, even if no spherical aberration neg. S.A. lens side Winter 2008 zero S.A. pos. S.A. 41 Winter 2008 42 UCSD: Physics 121; 2008 UCSD: Physics 121; 2008 Chromatic Aberration Optical Alignment Techniques • The performance of an optical system often depends vitally on careful positioning of the optical elements • A step-wise approach is best, if possible: aligning as the system is built up • Glass has slightly different refractive index as a function of wavelength – so not all colors will come to focus at the same place – leads to colored blur – why a prism works – if using a laser, first make sure the beam is level on the table, and going straight along the table – install each element in sequence, first centering the incident beam on the element • Fixed by pairing glasses with different dispersions (dn/d ) dispersions – typically a positive lens of one flavor paired with a negative lens of the other – can get cancellation of aberration – also helps spherical aberration to have multiple surfaces (more design freedom) Winter 2008 Lecture 6 • often reflections from optical faces can be used to judge orientation (usually should roughly go back toward source) – a lens converts position to direction, so careful translation cross-wise to beam is important • orientation is a second-order concern • Whenever possible, use a little telescope to look through system: the eye is an excellent judge 43 Winter 2008 44 11 Geometrical Optics 01/31/2008 UCSD: Physics 121; 2008 UCSD: Physics 121; 2008 Zemax Examples Lab 5: Raytracing • While it may not be Zemax, I’ve cobbled together a Zemax, I’ C-program to do raytracing of any number of lenses – restricted to the following conditions: • • • • ray path is sequential: hitting surfaces in order defined ray path is left-to-right only: no backing up elements are flat or have conic surfaces refractive index is constant, and ignorant of dispersion • We will use this package to: – – – – – Winter 2008 45 analyze simple lens configurations look at aberrations build lens systems (beam expanders, telescopes) learn how to compile and run C programs (and modify?) in conjunction with some geometrical design Winter 2008 46 UCSD: Physics 121; 2008 UCSD: Physics 121; 2008 Raytracing Algorithm References and Assignments • Detailed math available on website under Lab Info • Basically, compute intersection of ray with surface, then apply Snell’s Law Snell’ • Optics, by Eugene Hecht – a most excellent book: great pictures, clear, complete • Text reading: – Section 4.2.1 – Section 4.2.2: • Ray Tracing; Paraxial Ray Tracing; other topics of interest – Section 4.2.3: • Apertures, Stops, Pupils; Vignetting • Geometrical Aberrations & skim 5 types thereof – Section 4.3.3: • Can have as many surfaces as you want! • Must only take care in defining physical systems • Simple and Gal. Telescopes; Laser beam expanders & spatial filters; Lens aberrations – Flip through rest of chapter 4 to learn what’s there – e.g., make sure lens is thick enough for the diameter you need Winter 2008 Lecture 6 47 Winter 2008 48 12...
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