Optics and Telescopes

Transcription

1 Ohio University - Lancaster Campus slide 1 of 71 Optics and Telescopes Credit: Credit:

2 Ohio University - Lancaster Campus slide 2 of 71 This evening we will investigate: how lenses and mirrors can be used to focus light and form an image. the 3 basic telescope designs and the advantages and disadvantages of each. some numbers that characterize a telescope: f- ratio, light gathering power, resolution, magnification

3 Ohio University - Lancaster Campus slide 3 of 71 This evening we will investigate: recording the images produced by a telescope. telescopes that use the other wavelengths of light.

4 Ohio University - Lancaster Campus slide 4 of 71 Optics The science of reflecting and/or refracting (bending) light so as to produce an image of an object. The image is usually recorded so that it can be studied more extensively.

5 Ohio University - Lancaster Campus slide 5 of 71 Regarding Mirrors The Law of Reflection: When a ray of light strikes a shiny or specular surface, the ray reflects away at the same angle at which it struck the surface. The angle of incidence equals the angle of reflection, as measured from a normal to the surface.

6 Ohio University - Lancaster Campus slide 6 of 71 i = r a shiny or reflective surface

7 Ohio University - Lancaster Campus slide 7 of 71 If the reflecting surface is curved correctly, the light can be focused to a point, called the focal point. An image forms near the focal point. Credit:

8 Ohio University - Lancaster Campus slide 8 of 71 Regarding Lenses The Law of Refraction: When light moves from a less dense medium (empty space or air) to a denser medium (glass), the light slows down and bends INTO the denser medium.

9 Ohio University - Lancaster Campus slide 9 of 71 speed of light in air = 3 x 10 8 m/s speed of light in glass = 2 x 10 8 m/s

10 Ohio University - Lancaster Campus slide 10 of 71 Glass can be formed into a convex lens which will also focus light. An image forms near the focal point. The focal length is the distance from the centerline of the lens to the focal point. focal length

11 Ohio University - Lancaster Campus slide 11 of 71 The f-ratio is a way to compare or rate convex (converging) lenses. The f-ratio is the focal length of the lens divided by the lens diameter.

12 Ohio University - Lancaster Campus slide 12 of 71 Thicker lenses tend to focus closer to the lens and give brighter images. These are fast lenses. Do these lenses have low or high f-ratios? But these lenses have other problems.

13 Ohio University - Lancaster Campus slide 13 of 71 Thinner lenses focus farther from the lens, give less-bright images, and are described as slow lenses.

14 Ohio University - Lancaster Campus slide 14 of 71 When taking photographs of space objects, using a fast lens with a low f-ratio means less time is needed for the photograph. This results in less blurring due to vibration of the telescope and the motion of the stars. Credit: Gemini Observatory/AURA

15 Ohio University - Lancaster Campus slide 15 of 71 Chromatic Aberration a problem with lenses The edges of lenses act like prisms. They split white light into all the colors of the rainbow. Problem: the different colors focus at different focal points. This means that if you focus the blue color of an object, the red is fuzzy, and vice versa.

16 Ohio University - Lancaster Campus slide 16 of 71 Chromatic Aberration

17 Ohio University - Lancaster Campus slide 17 of 71 There s always a trade-off in optics. The problem of chromatic aberration is worst with fast or low f-ratio lenses. These are the lenses we d like to use most! The problem is fixed by making compound lenses out of 2 or more different kinds of glass. Mirror-based telescopes don t have this problem a definite advantage!

18 Ohio University - Lancaster Campus slide 18 of 71 3 Types of Telescopes Refractors (gathers light with a lens) Reflectors (gathers light with a mirror) Mixed (uses a combination of lenses and mirrors) Schmidt-Cassegrain Telescopes Maksutov-Cassegrain Telescopes

19 Ohio University - Lancaster Campus slide 19 of 71 Refracting Telescopes The original type, invented in the 1500 s and first used by Galileo to explore space. Sharpest, brightest images. Lenses are heavy and expensive! Prone to chromatic aberration. Give an inverted (upside-down) image. Can only be made up to about 40 inches in diameter.

20 Ohio University - Lancaster Campus slide 20 of 71 Credit: library.thinkquest.org

21 Ohio University - Lancaster Campus slide 21 of 71 Reflecting Telescopes Advantages Mirrors are much cheaper to make than lenses, and are very light-weight, easy to carry. Mirrors can be VERY large. Multiple mirrors can be combined to simulate a single gigantic mirror. No chromatic aberration.

22 Ohio University - Lancaster Campus slide 22 of 71 Reflecting Telescopes Disadvantages Not quite as sharp or bright an image as the same size refractor. Large scopes get currents of different temperature air inside their tubes. This can make images blurry. Mirrors will oxidize (corrode) over time.

23 Ohio University - Lancaster Campus slide 23 of 71

24 Ohio University - Lancaster Campus slide 24 of 71 Combination scopes the Cassegrains Very short tube length, because the light gets folded back on itself twice. This makes the scope easy to handle & transport. Moderately expensive. Best choice for amateur astrophotography, because the tube doesn t vibrate or shake very much.

25 Ohio University - Lancaster Campus slide 25 of 71 The corrector plate is a type of lens. A secondary mirror is glued to its inner surface.

26 Ohio University - Lancaster Campus slide 26 of 71 The telescope mount is as important as the optics! There are two types Altitude-Azimuth. Like aiming a tank. Point it in the compass direction (azimuth) you want, then point it up to the angle (altitude) you want. Easy to use, but image rotates over time.

27 Ohio University - Lancaster Campus slide 27 of 71 Equatorial. Part of the mount is aimed at the north celestial pole. The mount then swivels east-west to follow an object through the sky. Disadvantage: a real bear to use! Advantage: the picture in the telescope doesn t appear to rotate over time.

28 Ohio University - Lancaster Campus slide 28 of 71 What is the function of a telescope? It s not just to make the image bigger! Gathering light Resolving details Magnifying the image

29 Ohio University - Lancaster Campus slide 29 of 71 A Telescope is a Light Funnel Gathering light from dim objects is the MOST important function of a telescope. Which would you rather see, a large but very dim image or a smaller, but very bright image?

30 Ohio University - Lancaster Campus slide 30 of 71 Light-gathering power (LGP) How much light can the human eye gather? A typical human eye has a pupil that is about 0.5 cm in diameter when fully dilated at night. Area of the pupil = r 2 = (0.25 cm) 2 = about 0.2 cm 2. The main purpose of the telescope is to take light from a much larger area and funnel it into your pupil.

31 Ohio University - Lancaster Campus slide 31 of 71 How much light can a telescope gather? A 10 inch diameter scope (25 cm diameter) gathers (12.5cm) 2 = 490 cm 2. This is 490 cm 2 / 0.2cm 2 = almost 2500 times more light than the naked eye.

32 Ohio University - Lancaster Campus slide 32 of 71 To compare a telescope s LGP to that of a typical eye, use the formula LGP = 4D 2 where D is the telescope s lens/mirror diameter in centimeters. (2.54 cm/inch) What is the LGP of a 6 inch telescope?

33 Ohio University - Lancaster Campus slide 33 of 71 Seeing Small Details Resolution Resolution is defined as the minimum angle between 2 objects, that will allow you to see them as 2 separate objects and not one big blob. Units are arcseconds (1/3600 th of a degree) The smaller the theoretical resolution number is, the smaller the details you can see.

34 Ohio University - Lancaster Campus slide 34 of 71 Theoretical Resolution ( )= (2.1x10 5 )(wavelength in m) (diameter of objective mirror or lens) The diameter is in meters, not inches! What is the resolution of a 10 inch scope for blue light (450 nm or 4.5 x 10-7 meters)? Calculate the resolution again for red light (7.0 x 10-7 meters)

35 Ohio University - Lancaster Campus slide 35 of 71 Resolution not the same for all light! What color of visible light would have the poorest resolution? The best? What color of all the types of light would have the poorest resolution? How is this limitation overcome?

36 Ohio University - Lancaster Campus slide 36 of 71 There s a practical limit to resolution for a ground-based telescope the Atmosphere! Air currents in the atmosphere will make the image blurry. Think twinkling stars! The best time for viewing is in the hours before dawn, since the air currents are least. Are there any other accommodations that could be made?

37 Ohio University - Lancaster Campus slide 37 of 71 Magnification the least important function of a telescope M = focal length of the objective lens or mirror focal length of eyepiece lens What is the magnification factor (power) of a telescope with a 1000 mm focal length, using an eyepiece with a 2.5 cm focal length?

38 Ohio University - Lancaster Campus slide 38 of 71 My 10 inch (25 cm) Schmidt-Cassegrain telescope has a 250 cm focal length. If I use an eyepiece with a 1.25 cm focal length, what is the magnification? If I want to increase the magnification, should I use a 2.5 cm focal length eyepiece, or a 0.75 cm focal length eyepiece?

39 Ohio University - Lancaster Campus slide 39 of 71 A bit of review: If you doubled the size of a telescope s objective mirror without making any other changes, how would the telescope s properties change?

40 Ohio University - Lancaster Campus slide 40 of 71 Why do astronomers no longer use film in their cameras? Film has been replaced by CCD chips (Charge-Coupled Device).

41 Ohio University - Lancaster Campus slide 41 of 71 Credit: rst.gsfc.nasa.gov/intro/ccd.jpg

42 Ohio University - Lancaster Campus slide 42 of 71 The surface of a CCD chip is divided up into rows of rectangular light-sensitive pixels (picture elements). Films have irregularly shaped and distributed grains of light-sensitive chemicals. The pixels are usually much more sensitive than the chemical grains. Advantage???

43 Ohio University - Lancaster Campus slide 43 of 71 Film Emulsions Credit:

44 Ohio University - Lancaster Campus slide 44 of 71 Individual pixel Light-sensitive layer (gives off electrons when struck by light) This stack of 3 layers is one pixel. Semi-conductor layer (acts as an electron filter) Collector layer (holds the electrons until counted)

45 Ohio University - Lancaster Campus slide 45 of 71 Why use CCD s instead of film? CCD Detector 70% efficient Shorter exposures Resolution can be higher (8 Mpixels or higher) Film 5% to 10% efficient 7 to 14 times longer exposures Resolution is limited by grain size

46 Ohio University - Lancaster Campus slide 46 of 71 Pictures are available in seconds. Pictures can be digitally added together. Initial cost is similar to film but operating costs are much lower. Pictures must be developed (hours to days) Digital techniques are possible, but more difficult. Operating costs higher.

47 Ohio University - Lancaster Campus slide 47 of 71 A typical, high-res image produced by a CCD. Credit: solarsystem.nasa.gov/multimedia/gallery/pia02888.jpg

48 Ohio University - Lancaster Campus slide 48 of 71 All astrophotographs are black & white. Photographs can be taken in color, but you lose resolution. 4 pixels must be binned or clustered for color photographs (1 B&W, 1 red, 1 green, 1 blue) This makes the overall pixel size 4 times bigger = lower resolution.

49 Ohio University - Lancaster Campus slide 49 of 71 1 big pixel if the photo is taken in color 4 smaller pixels if the photo is taken in B/W. Better resolution.

50 Ohio University - Lancaster Campus slide 50 of 71 So how can we see all those beautiful color photographs? NGC 2393 The Eskimo Nebula Credit: Andrew Fruchter (STScI) et al., WFPC2, HST, NASA

51 Ohio University - Lancaster Campus slide 51 of 71 We take 4 pictures in succession then combine them into a single image: one photo through a red filter. one photo through a green filter. one photo through a blue filter. one photo in B/W (often called a Luminance filter) for overall brightness levels.

52 Ohio University - Lancaster Campus slide 52 of 71 M57 Ring Nebula taken through red, green, and blue filters. Notice the different details which come out. Credit: Chris Brown, University of Manitoba The composite color photo.

53 Ohio University - Lancaster Campus slide 53 of 71 Telescopes which see at other wavelengths than visible light. Not all objects are visible at optical wavelengths ( nm). Many hot objects are only visible at shorter wavelengths (UV, X-rays, -rays) Many cool objects are only visible at longer wavelengths (IR, microwaves, radio waves)

54 Ohio University - Lancaster Campus slide 54 of 71 Radio Telescopes Detect cool gases: H + H H 2 Can detect molecules out in space: oxygen O 2, carbon dioxide CO 2 hydrogen cyanide HCN formaldehyde H 2 CO Ethanol CH 3 COOH

55 Ohio University - Lancaster Campus slide 55 of 71 Advantages & Problems Operate night or day Atmosphere doesn t absorb radio waves Poorest resolution of any type of light (doesn t see details well) Solution is to make antennas (dishes) VERY large

56 Ohio University - Lancaster Campus slide 56 of 71 The Arecibo Radio Telescope, Puerto Rico. Credit: National Radio Astronomy Observatory

57 Ohio University - Lancaster Campus slide 57 of 71 The Green Bank Telescope (GBT) in Green Bank, W.Va. The largest steerable dish in the world. As tall as the Statue of Liberty, the dish would hold the building you re in. Credit: National Radio Astronomy Observatory

58 Ohio University - Lancaster Campus slide 58 of 71 The Very Large Array (VLA), Socorro, N.M. Credit: National Radio Astronomy Observatory

59 Ohio University - Lancaster Campus slide 59 of 71 Infrared Telescopes Very similar to visible wavelength telescopes, except for the detector, called a bolometer. IR scopes detect heat from warm gas or warm objects. Warm means not hot enough to glow in visible light. These scopes must be kept very cold or the heat that the scope itself radiates will swamp out what is being observed.

60 Ohio University - Lancaster Campus slide 60 of 71 What kinds of objects do IR telescopes observe? IR telescopes see molecules & dust. In some cases, they can look through cooler dust to see what s inside the dust clouds! Since stars form where there s lots of dust, these scopes are used for for looking inside dusty nebulas where new stars form.

61 Ohio University - Lancaster Campus slide 61 of 71 Star-forming regions around Orion, in visible and IR Credit: Akira Fujii / NASA

62 Ohio University - Lancaster Campus slide 62 of 71 The Spitzer Space Telescope, part of the Great Telescope Series Credit: NASA/JPL-Caltech

63 Ohio University - Lancaster Campus slide 63 of 71 The Sombrero Galaxy (in Leo) in IR and in visible light. Credit: JPL / NASA (top) Credit: NASA/ESA and The Hubble Heritage Team STScI/AURA)

64 Ohio University - Lancaster Campus slide 64 of 71 Ultraviolet Telescopes Look for hot, young stars. These stars help us better define star-forming regions, which contributes to a better understanding of the evolution of our galaxy. They also look for hot, distant galaxies, as they looked in the early universe. What famous scope is also a UV telescope?

65 Ohio University - Lancaster Campus slide 65 of 71 GALEX Telescope (Galaxy Evolution Explorer) Credit: JPL / NASA

66 Ohio University - Lancaster Campus slide 66 of 71 Galaxy NGC 300 in Sculptor Constellation, 7 million light years away Credit: NASA/JPL-Caltech/Las Campanas

67 Ohio University - Lancaster Campus slide 67 of 71 X-ray and Gamma Ray Telescopes See very hot objects: Black Holes Pulsars & Neutron Stars Supernovas VERY good resolution great ability to observe fine details

68 Ohio University - Lancaster Campus slide 68 of 71 Core of the Elliptical Galaxy NGC 4261 (accretion disk of a black hole.) Credit: NASA/ESA and The Hubble Heritage Team STScI/AURA)

69 Ohio University - Lancaster Campus slide 69 of 71 Cassiopeia A - the remnant of a supernova which exploded about 300 years ago. Credit: X-ray: NASA/CXC/SAO; Optical: NASA/STScI; Infrared: NASA/JPL-Caltech

70 Ohio University - Lancaster Campus slide 70 of 71 The Chandra X-ray Telescope, part of the Great Telescopes series. Credit: chandra.nasa.gov (artist s conception)

71 Ohio University - Lancaster Campus slide 71 of 71 A gamma ray burst beginning. Credit: NASA (artist s conception) The GLAST ( Gamma-ray Large Area Space Telescope, renamed FERMI ) Credit: General Dynamics for NASA (artist s conception)

72 Discovered November, 2010