Refracting Telescopes Faint Light Astronomical objects are distant
Refracting Telescopes
Faint Light • Astronomical objects are distant and faint. – Effectively at infinity • Light collection is more important than magnification. – Refraction – Reflection • The Andromeda Galaxy (M 31) is 3° wide. – 6 times the moon – Only visible to the unaided eye in very dark conditions
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 point focal length
Concentrator • A refracting telescope uses lenses to concentrate light from a distant object. – Object light rays nearly parallel – Final image rays also parallel objective focal point eyepiece
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
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
Magnification • Refracting telescopes magnify images with two lenses. objective Magnification = focal point eyepiece Primary focal length Eyepiece focal length
Chromatic Aberration Tony and Marilyn Karp • Real material refract colors differently. – Blue light bends more than red • Compound lenses can compensate for chromatic aberration. Wikipedia
Yerkes Refractor • World’s largest refractor is in Wisconsin. • 40 inch aperture, f/19. • 63 foot tube. Yerkes 40 inch
- Slides: 9