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Scope means f 2 70 mm 35 m scope means m 500 at f10

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Unformatted text preview: just an image of the entrance pupil satisfying 1/s’ + 1/(f1 + f2) = 1/f2 – size is smaller than entrance pupil by magnification factor • M = f1/f 2; in this picture, f1 = 48; f2 = 12; M = 4; s’ = 15 Winter 2008 Lecture 6 35 – dark adapted pupil up to 7 mm diameter (2–3 mm in daylight) – sets limit on minimum magnification (if you want to use the full aperture) • 210 mm aperture telescope must have M > 30 • for f/5 scope, means f2 < 35 mm; f /10 scope means f 2 < 70 mm • 3.5-m scope means M > 500; at f/10, f 2 < 7 0 mm Winter 2008 36 9 Geometrical Optics 01/31/2008 UCSD: Physics 121; 2008 UCSD: Physics 121; 2008 Vignetting Infrared Cold Stop • Rays that don’t make it through an optical system are don’ said to be vignetted (shadowed) – maybe a lens isn’t big enough – maybe your eye’s pupil isn’t big enough, or is improperly placed • An infrared detector is very sensitive to terrestrial heat • Often appears as a gradual darkening as a function of distance from the field center – the farther out you go, the bigger your lenses need to be – every optical system has a limited (unvignetted) field of view – beyond this, throughput goes down – so want to keep off of detector – if detector located at primary focal plane, it is inundated with emission from surroundings and telescope structure • note black lines intersecting primary focal plane • Putting a “ cold” stop at a pupil plane eliminates stray emission cold” – cool to LN2; image of primary objective onto cold stop – only light from the primary passes through; detector focal plane then limits field of view to interesting bit • Also the right place for filters, who prefer collimated light Winter 2008 37 Winter 2008 38 UCSD: Physics 121; 2008 UCSD: Physics 121; 2008 Raytrace Simulations Aberrations: the real world • Lenses are thick, sin • In Google, type in: phet Google, – sin – tan – top link is one to University of Colorado physics education page – on this page, click: go to simulations – on the left-hand bar, go to: light and radiation – then click the geometric optics simulation link (picture) Lecture 6 7/5040 + … + 5/120 + 2 5/15 + 17 7/315 + … – spherical aberration • all spherical lenses possess; parabolic reflector does not – coma • off-axis ailment: even aspheric elements have this real and virtual images lens radius of curvature, diameter, and refractive index see principle rays (ones you’d use to raytrace) see marginal rays use a light source and screen see the effect of two sources Winter 2008 3/3 • Different types of aberration (imperfection) • Can play with lots of parameters – – – – – – 3 /6 + – chromatic aberration • in refractive systems only: refractive index is function of – astigmatism • if on axis, then lens asymmetry; but can arise off-axis in any system – field curvature/distortion • detectors are flat: want to eliminate significant field curvature 39 Winter 2008 40 10 Geometrical Optics 01/31/2008 UCSD: Physics 121; 2008 UCSD: Physics 121; 2008 Spherical Aberration • Rays at different heights focus at different points • Makes for a mushy focus, with a ha...
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