1. Refracting Telescopes

2. Faint Light Astronomical objects are distant and faint. –Effectively at infinity Light collection is more important than magnification. –Refraction –Reflection The Andromeda Galaxy (M31) is 3° wide. –6 times the moon –Only visible to the unaided eye in very dark conditions

3. Refraction Light is bent at the surface between two media. This bending is called refraction. Lenses use refraction to bend and focus light. –focal length and a focal point –Parallel rays from a distant source focal length focal point

4. Concentrator A refracting telescope uses lenses to concentrate light from a distant object. –Object light rays nearly parallel –Final image rays also parallel objectivefocal pointeyepiece

5. Aperture Lenses collect and concentrate light by refraction. The diameter (D) of the objective lens is the aperture. –Measured in m or mm –Larger apertures for fainter objects The light gathering power (LGP) is related to the area of the lens. –Circular lens: A = (  D 2 )/4 –Relative to eye aperture 5 mm: LGP = D 2 /(5 mm) 2

6. F-Stop The brightness of an image is measured by the focal ratio of the focal length to the aperture. –F-number or f-stop = f/D –Dimensionless quantity –Denoted by f/ Lower f-numbers are “faster” and need shorter exposure times. Primary focal length = Aperture diameter x f-number

7. Magnification Refracting telescopes magnify images with two lenses. Magnification = Primary focal length Eyepiece focal length objectivefocal pointeyepiece

8. Chromatic Aberration Real material refract colors differently. –Blue light bends more than red Compound lenses can compensate for chromatic aberration. Tony and Marilyn Karp Wikipedia

9. Yerkes Refractor World’s largest refractor is in Wisconsin. 40 inch aperture, f/19. 63 foot tube. Yerkes 40 inch