TELESCOPES Palomar 200-in
Functions of Telescopes • Collect more light --- depends on (diam)2 • Resolve sources better (see more detail) • Magnify images
Telescope “Objectives”: Specially Shaped Main Optical Element Purpose: form an accurate representation of original scene at a “focus” • Lens ---> “refracting” telescope • Mirror ---> “reflecting” telescope
Refracting Telescope Minimum 2 lenses needed for visual use Place detector here
Telescope Performance Characteristics • Focal Ratio (f/ number) • Magnification ("power") • Field of view • Light Gathering Power • Resolution
Focal Ratio (or f/number) • f/ number = Obj FL / Obj Diam • Smaller numbers give more concentrated light in focal plane (better for faint extended objects); allow shorter exposures with film/electronic detectors • Higher numbers have better resolution; better for high magnification (e.g. for planets)
Magnification • Defined to be ratio of apparent angular size of image to original angular size (without telescope) • Mag = FL (telescope) / FL (eyepiece) • For Celestrons, Mag = 2034 mm/FLE (mm) • Moderate magnifications (<150) best
Field of View • FOV = True angular diameter -- i.e. as viewed without telescope -- of field visible in eyepiece. • Usually quoted in degrees or minutes of arc • Depends on eyepiece used • Is smaller for higher magnification with given telescope
Light Gathering Power • Most important attribute of telescope • Light collected is proportional to the area of the objective, or to Dobj2 • If the pupil diameter of your eye is 5mm, an 8" telescope collects (203/5)2 = 1600x more light
Table by D. Haworth
Image Quality (Resolution) • Design optics to reduce "aberrations" -- e.g. chromatic, spherical, etc. • Optical figuring to intended shape: must be better than “1/4 wavelength” • Larger telescopes better because of “diffraction” of light waves • Turbulence in air strongly affects image blur. “Seeing” = size of blur.
Longer focal lengths reduce chromatic & spherical aberration (Hevelius, ca. 1650)
“Schmidt-Cassegrain” design uses a thin refractive corrector to eliminate spherical aberration from a spherically-shaped primary
“Diffraction” of Light Waves Ideal case Real waves
6" 20" Effects of diffraction on size of image of a double star in a telescope 200" 94"
Video of enlarged image of bright star in a large telescope. Image size/motion caused by Earth’s atmosphere.
Telescope Designs: A Multitude • Optical design • Mounting design • Equatorial • Altitude-Azimuth
Equatorial Mount Alt-Az Mount
200-in Dedication (1948) (Largest equatorial mount for optical telescope; "horseshoe")