1 / 30

Optics and Telescopes

Optics and Telescopes. Lecture 11. Why do we use telescopes?. Human eyes are lenses! Using larger lenses…  collect more light  magnification. Larger lens can make brighter and magnified images. Change in direction of travel. Refraction. light travels at the fastest

sabin
Download Presentation

Optics and Telescopes

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Optics and Telescopes Lecture 11

  2. Why do we use telescopes? • Human eyes are lenses! • Using larger lenses…  collect more light  magnification

  3. Larger lens can make brighter and magnified images.

  4. Change in direction of travel

  5. Refraction light travels at the fastest speed (e.g., speed of light) in vacuum.

  6. Refraction of light by lens

  7. Parallel light rays from distant objects • If a lens is located very far from the light source, only a few of the light rays are entering the lens. • These rays are essentially parallel.

  8. Extended object Extended image • A lens creates an extended image of an extended object. • each point on an extended object passes through a lens and produces an image of that point. • collection of point images = image of an extended object.

  9. Refractive Telescope Objective lens (light-gathering) + eyepiece (making image) Light-gathering power = area of the objective lens magnification = focal length of objective lens focal length of eyepiece lens

  10. Refractive Telescope

  11. Disadvantages of refractive telescope • Hard to make defect free lenses (especially larger one) • Glass is opaque to certain wavelengths (UV is 100% blocked!) • Very difficult to make larger lens • Large lenses are heavy  gravitational distortion

  12. Angle of reflection requals angle of incidence i Law of Reflection Perpendicular to mirror surface incidence angle = reflection angle i r Reflected light ray Incident light ray Mirror

  13. Reflection : Mirror acts as a lens

  14. Newtonian Telescope

  15. Different designs of Reflecting telescopes • Prime focus is good but inconvenient.

  16. All Modern telescopes are Reflecting telescopes Gemini Telescope (8m) Primary mirror secondary mirror Cassegrain focus Large mirrors (nearly defect free : error is less than 8.5 nanometers) are much easier to make. Hollowed mirror base (honeycomb)

  17. Secondary mirror making a hole in the image? Secondary mirror (or Cassegrain focus hole) does not make a hole in the focused image. However, support structure creates a diffraction spike from a point source.

  18. Spherical Aberration Different parts of a spherically concave mirror reflect light to slightly different points  image bluring A solution parabolic mirror (harder to make) correcting lens

  19. Angular resolution Because of diffraction of light (light waves spread out from a point), there is a limit in angular resolution Diffraction-limited angular resolution θ = angular resolution in arcseconds λ = wavelength of light, in meters D = diameter of telescope, in meters (example) Keck telescope, red light. …

  20. Hawaii, Mauna Kea

  21. Light Pollution

  22. Effect of Earth Atmosphere • Light = wave • Perfect waveform got deformed due to turbulence in atmosphere… breeze turbulence in atmosphere

  23. Adaptive Optics Using a nearby star (e.g., point source), reshape the mirror so that it can become a perfect (diffraction limited) point source. Active Optics wind shakes tip/tilt correction

  24. Eliminate the effect of Atmosphere (Adaptive Optics)

  25. Power of Adaptive Optics

  26. Power of Adaptive Optics

  27. Laser-guided Adaptive Optics

  28. In summary… Important Concepts Important Terms Refraction/reflection Focal length light-gatheringpower light pollution Aberration (chromatic, spherical) • Refractive telescope • disadvantages • Reflective telescope • various designs • Angular resolution • Active Optics • Adaptive Optics • Chapter/sections covered in this lecture : sections 6-1 through 6-3

More Related