Optical Telescope. Faint Light. Astronomical objects are distant and faint. Effectively at infinity Light collection is more important than magnification. Refraction Reflection. The Andromeda Galaxy (M31) subtends 3 ° . 6 times the moon
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Lenses shaped like parts of spheres are easy to make.
Easy to calculate rays
Use Snell’s Law on a small part of a sphere.
Radius of curvature R
Focal length f
Index for air is 1
A single narrow slit creates diffraction.
No minimum for m = 0
Fraunhofer patterns are symmetric around the opening.
A circular hole produces rings around a central maximum.
84% of energy in center
The limit of resolution is set by the aperture.
The Rayleigh criterion is calculated from the first minimum of the Airy disk.
Aperture radius a
Bessel function J1
Yerkes 40 inch
The index of refraction depends on the wavelength.
Longer wavelengths - lower indexes
Blue light bends more than red
Compound lenses can compensate for chromatic aberration.
A spherical surface does not focus all parallel lines to the same point.
Aspheric lenses can be used to correct the aberration .
Light that begins at one focus of an elliptical mirror converges at the other focus.
A parabola for a focus at infinity
secondary diagonal mirror
Schmidt corrector lens
Good for detail
Good for dim objects
Uniform treatment of colors
Combines best optical qualities
Good for photography
Infrared is absorbed by water vapor.
Observe at high altitude
Satellite telescopes avoid the atmosphere.
IRAS (1983) - first evidence of planets around other stars
Spitzer Space Telescope (2003-9).
Ultraviolet is largely absorbed by the atmosphere.
M81 from GALEX