Telescopes Refractors and Reflectors Various Designs Important Properties Modern Advances Beyond Visible Light Refractors and Reflectors Various Designs Important Properties Modern Advances Beyond Visible Light

Basic Optics - Refraction Light is bent or refracted when it changes speed going through a lens or prism. Where the refracted rays meet is the “focal point.” Distance from the lens to the focal point is the “focal length.” Light is bent or refracted when it changes speed going through a lens or prism. Where the refracted rays meet is the “focal point.” Distance from the lens to the focal point is the “focal length.”

Basic Optics - Reflection When light strikes a mirror it is reflected at an angle equal to its incident angle. Rays of light striking a curved mirror will meet at a “focal point.” The distance from the vertex of the mirror to the focal point is the “focal length.” When light strikes a mirror it is reflected at an angle equal to its incident angle. Rays of light striking a curved mirror will meet at a “focal point.” The distance from the vertex of the mirror to the focal point is the “focal length.”

Reflectors and Refractors A reflector telescope has a mirror as its objective. A refractor telescope has a lens as its objective. The main function of a telescope is to “gather light.” A reflector telescope has a mirror as its objective. A refractor telescope has a lens as its objective. The main function of a telescope is to “gather light.”

Chromatic Aberration Different wavelengths focus at different distances World’s Largest Refractor - Yerkes Observatory Different wavelengths focus at different distances World’s Largest Refractor - Yerkes Observatory

Reflectors

Why Mirrors? Light gathering power - size matters! lgp increases with the square of the radius More lgp, more resolution! Magnification is third! Light gathering power - size matters! lgp increases with the square of the radius More lgp, more resolution! Magnification is third!

200’ Hale Telescope Mt. Palomar

Comparing lgp lgp depends upon the area of the lens or mirror; A =  r 2 lgp = lgp depends upon the area of the lens or mirror; A =  r 2 lgp = (D big /D small ) 2

Calculating Magnification M = f obj f eye f obj f eye M max = 20 x D cm

Practical Limit of Magnification Smaller focal length eyepieces give greater magnification. They also limit light & increase diffraction. At some point the image is too poor to be useful. Smaller focal length eyepieces give greater magnification. They also limit light & increase diffraction. At some point the image is too poor to be useful. M max = 20X ____ cm x D

Resolving Power The ability of a telescope to distinguish two objects. Measured in arcseconds ( 1 arcsecond is 1/3600  ) The ability of a telescope to distinguish two objects. Measured in arcseconds ( 1 arcsecond is 1/3600  ) a = _______ D (in cm)

Modern Advances Binocular Telescopes LBT - two 8.4 m mirrors with adaptive optics LGP of an 11.8 m mirror Detail of a 22.4 m mirror Surpasses HST in clarity Binocular Telescopes LBT - two 8.4 m mirrors with adaptive optics LGP of an 11.8 m mirror Detail of a 22.4 m mirror Surpasses HST in clarity

Adaptive Optics Correct for atmospheric turbulence Use the light from a guide star or laser to determine what the atmosphere is doing to the light passing through it. Distortable mirror will change shape to correct the image. Correct for atmospheric turbulence Use the light from a guide star or laser to determine what the atmosphere is doing to the light passing through it. Distortable mirror will change shape to correct the image.

Beyond Visible Light Radio Telescopes

VLA Telescope

UV and X-Ray Telescopes SOHO Chandra

HST and James Webb