Understanding Optics: The Eye and Telescopes

1. ISNS 3371 - Phenomena of Nature The Eye The eye consists of pupil that allows light into the eye - it controls the amount of light allowed in through the lens - acts like a simple glass lens which focuses the light on the retina - which consists of light sensitive cells that send signals to the brain via the optic nerve. An eye with perfect vision has its focus on the retina when the muscles controlling the shape of the lens are completely relaxed - when viewing an object far away - essentially at infinity.

2. ISNS 3371 - Phenomena of Nature When viewing an object not at infinity, the eye muscles contract and change the shape of the lens so that the focal plane is at the retina (in an eye with perfect vision). The image is inverted as with a single lens - the brain interprets the image and rights it.

3. ISNS 3371 - Phenomena of Nature Types of Optical Telescopes

4. ISNS 3371 - Phenomena of Nature Magnification Using Two Lenses - Refracting Telescope f1 = 0.5 m f2 = 0.1 m f1 = 0.5 m f2 = 0.3 m Refracting telescope - consists of two lenses - the objective and the eyepiece (ocular). Incident light rays (from the left) are refracted by the objective and the eyepiece and reach the eye of the person looking through the telescope (to the right of the eyepiece). If the focal length of the objective (f1) is bigger than the focal length of the eyepiece (f1), the refracting astronomical telescope produces an enlarged, inverted image: magnification = f1 /f2

5. ISNS 3371 - Phenomena of Nature Refracting Telescope Uses lens to focus light from distant object - the eyepiece contains a small lens that brings the collected light to a focus and magnifies it for an observer looking through it.

6. ISNS 3371 - Phenomena of Nature The largest refracting telescope in the world is the at the University of Chicago’s Yerkes Observatory - it is 40 inches in diameter and 63 feet long.

7. ISNS 3371 - Phenomena of Nature Reflecting Telescope The primary mirror focuses light at the prime focus. A camera or another mirror that reflects the light into an eyepiece is placed at the prime focus.

8. ISNS 3371 - Phenomena of Nature Types of Reflecting Telescopes Each design incorporates a small mirror just in front of the prime focus to reflect the light to a convenient location for viewing.

9. ISNS 3371 - Phenomena of Nature The Keck Telescopes Largest in the world - on Mauna Kea in Hawaii. 36 hexagonal mirrors function as single 10-meter mirror.

10. ISNS 3371 - Phenomena of Nature The Hubble Space Telescope The Hubble Space Telescope is 43.5 ft long and weighs 24,500 lbs. Its primary mirror is 2.4 m (7 ft 10.5 in) in diameter.

11. ISNS 3371 - Phenomena of Nature Refracting vs Reflecting Telescopes Reflecting telescopes are primary astronomical tools used for research: Lens of refracting telescope very heavy - must be placed at end of telescope - difficult to stabilize and prevent from deforming Light losses from passing through thick glass of refracting lens - must be very high quality and perfectly shaped on both sides Refracting lenses subject to chromatic aberration

12. ISNS 3371 - Phenomena of Nature Lens and Mirror Aberrations SPHERICAL (lens and mirror) Light passing through different parts of a lens or reflected from different parts of a mirror comes to focus at different distances from the lens. Result: fuzzy image CHROMATIC (lens only) Objective lens acts like a prism. Light of different wavelengths (colors) comes to focus at different distances from the lens. Result: fuzzy image

13. ISNS 3371 - Phenomena of Nature Chromatic Aberration in Lenses Focal point for blue light Simple lenses suffer from the fact that different colors of light have slightly different focal lengths. This defect is corrected by adding a second lens Focal point for red light The problem Focal point for all light The solution

14. ISNS 3371 - Phenomena of Nature Spherical Aberration in Lenses Simple lenses suffer from the fact that light rays entering different parts of the lens have slightly difference focal lengths. As with chromatic aberration, this defect is corrected with the addition of a second lens. The problem One focal point for all light rays The solution

15. ISNS 3371 - Phenomena of Nature Spherical Aberration in Mirrors The Problem Simple concave mirrors suffer from the fact that light rays reflected from different locations on the mirror have slightly different locations on the mirror have slightly different focal lengths. This defect is corrected by making sure the concave surface of the mirror is parabolic The Solution All light rays converge at a single point

16. ISNS 3371 - Phenomena of Nature The image from an reflecting telescope is inverted. Focus Inversion Animation The focus is adjusted by changing the secondary mirror position. Mirror Position and Focus Animation

17. ISNS 3371 - Phenomena of Nature Uses of Telescopes • Imaging • use a camera to take pictures (images) • Photometry  measure total amount of light from an object • Spectroscopy • use a spectrograph to separate the light into its different • wavelengths (colors) • Timing • measure how the amount of light changes with time (sometimes in a fraction of a second)

18. ISNS 3371 - Phenomena of Nature In astronomy, filters are usually placed in front of a camera to allow only certain colors to be imaged Single color images are superimposed to form true color images. Imaging

19. ISNS 3371 - Phenomena of Nature Nonvisible Light • Most light is invisible to the human eye - gamma rays, x-rays, ultraviolet, infrared, radio waves. • Special detectors/receivers can record such light - each type of light can provide information not available from other types. • Digital images are reconstructed using false-color coding so that we can see this light. Chandra X-ray image of the Center of the Milky Way Galaxy

20. The Crab Nebula ISNS 3371 - Phenomena of Nature Visible Infrared X-rays Radio Waves

21. ISNS 3371 - Phenomena of Nature Atmospheric Effects Earth’s atmosphere causes problems for astronomers on the ground: • Bad weather makes it impossible to observe the night sky. • Man-made light is reflected by the atmosphere, thus making the night sky brighter. • this is called light pollution • The atmosphere absorbs light - dependent on wavelength • Air turbulence in the atmosphere distorts light. • That is why the stars appear to “twinkle”. • Angular resolution is degraded.

22. ISNS 3371 - Phenomena of Nature Atmospheric Absorption of Light • Earth’s atmosphere absorbs most types of light. • good thing it does, or we would be dead! • Only visible, radio, and certain IR and UV light make it through to the ground. To observe the other wavelengths, we must put our telescopes in space!

23. ISNS 3371 - Phenomena of Nature Space Astronomy

24. ISNS 3371 - Phenomena of Nature Space Based Telescopes FUSE (Far UV) Chandra X-ray Obs. Hubble Space Telescope Compton Gamma Ray Obs. Spitzer Space Telescope (IR)

25. ISNS 3371 - Phenomena of Nature Radio Telescopes • The wavelengths of radio waves are long. • So the dishes which reflect them must be very large to achieve any reasonable angular resolution. 305-meter radio telescope at Arecibo, Puerto Rico

26. ISNS 3371 - Phenomena of Nature Spectrum of a Low Density Cloud Animation

27. ISNS 3371 - Phenomena of Nature

28. ISNS 3371 - Phenomena of Nature Kirchhoff’s Laws of Radiation First Law. A luminous solid, liquid or gas, such as a light bulb filament, emits light of all wavelengths thus producing a continuous spectrum of thermal radiation. Second Law. If thermal radiation passes through a thin gas that is cooler than the thermal emitter, dark absorption lines are superimposed on the continuous spectrum. The gas absorbs certain wavelengths. This is called an absorption spectrum or dark line spectrum. Third Law. Viewed against a cold, dark background, the same gas produces an emission line spectrum. It emits light of discrete wavelengths. This is called an emission spectrum or bright line spectrum. .

29. ISNS 3371 - Phenomena of Nature So what astronomical body has this spectrum?

30. ISNS 3371 - Phenomena of Nature The Doppler Effect

31. ISNS 3371 - Phenomena of Nature Doppler Effect for Light Animation

32. ISNS 3371 - Phenomena of Nature The Doppler Effect 1. Light emitted from an object moving towards you will have its wavelength shortened. BLUESHIFT 2. Light emitted from an object moving away from you will have its wavelength lengthened. REDSHIFT 3. Light emitted from an object moving perpendicular to your line-of-sight will not change its wavelength.

33. ISNS 3371 - Phenomena of Nature Doppler Shift of Emission Lines Animation

34. ISNS 3371 - Phenomena of Nature The amount of spectral shift tells us the velocity of the object: v  c =

35. ISNS 3371 - Phenomena of Nature The Doppler shift only tells us part of the object’s full motion - the radial part or the part directed toward or away from us.