Telescopes 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.”
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.”
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.”
Chromatic Aberration • Different wavelengths focus at different distances • World’s Largest Refractor - Yerkes Observatory
Why Mirrors? • Light gathering power - size matters! • lgp increases with the square of the radius • More lgp, more resolution! • Magnification is third!
Comparing lgp • lgp depends upon the area of the lens or mirror; A = r2 • lgp = (Dbig/Dsmall)2
Calculating Magnification fobj feye M = Mmax = 20 x Dcm
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. 20X ____ Mmax = x D cm
Resolving Power • The ability of a telescope to distinguish two objects. • Measured in arcseconds ( 1 arcsecond is 1/3600) 11.58 _______ 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
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.
Beyond Visible Light Radio Telescopes
UV and X-Ray Telescopes SOHO Chandra