Light and Telescopes Chapter 5

In the early chapters of this book, you looked at the sky the way ancient astronomers did, with the unaided eye. In this chapter, you will see how modern astronomers use telescopes and other instruments to gather and focus light and its related forms of radiation. That will lead you to answer five essential questions about the work of astronomers: What is light? How do telescopes work, and how are they limited? How do astronomers record and analyze light? Why must some telescopes go into space? Guidepost

Astronomy is almost entirely an observational science, so astronomers must think carefully about the limitations of their instruments. That will introduce you to an important question about scientific data: How do we know? What limits the detail you can see in an image? Guidepost (continued)

I. Radiation: Information from Space A. Light as a Wave and a Particle B. The Electromagnetic Spectrum II. Optical Telescopes A. Two Kinds of Telescopes B. The Powers of a Telescope C. New-Generation Telescopes D. Interferometry III. Astronomy from Space A. The Ends of the Visual Spectrum B. Telescopes in Space Outline

I. Radiation: Information from Space Common Misconception: For some people, radiation means threat. So what is radiation? Anything that radiates from a source.

A. Light and Other Forms of Radiation In astronomy, we cannot perform experiments with our objects (stars, galaxies, …). The only way to investigate them, is by analyzing the light (and other radiation) which we observe from them.

What is light as a whole?

Light as a Wave (1) Light waves are characterized by a wavelength and a frequency f. f = c/ c = 300,000 km/s = 3*10 8 m/s f and are related through

Light as a Wave (2) Wavelengths of light are measured in units of nanometers (nm) or Ångström (Å): 1 nm = m 1 Å = m = 0.1 nm Visible light has wavelengths between 4000 Å and 7000 Å (= 400 – 700 nm).

Wavelengths and Colors Different colors of visible light correspond to different wavelengths.

Light as Particles Light can also appear as particles, called photons (explains, e.g., photoelectric effect).photoelectric effect E = h*f h = 6.626x J*s is the Planck constant. A photon has a specific energy E, proportional to the frequency f:

B. The Electromagnetic Spectrum Need satellites to observe Wavelength Frequency High flying air planes or satellites

Another Misconception Radio Waves are related to sound. No. Radio waves are a type of light

Scientific Argument What would you see if your eyes were sensitive only to X-rays? Would be in the dark if your eyes were sensitive only to radio wavelengths?

Just to make sure….. Most waves require a material medium in which to be transmitted: * water waves travel along the surface of water * sound waves move through air * earthquake waves propagate through the solid earth * e.m waves may propagate through a pure vacuum at the speed of light c.

Summary (1) Light is a form of electromagnetic wave. Complete electromagnetic spectrum includes: gamma rays, X-rays, ultraviolet (UV), visible light, infrared (IR), microwaves, and radio waves. Earth’s atmosphere is transparent in only two atmospheric windows: visible light and radio.

II. Optical Telescopes

Video Trailer: Eyes on the Sky – 400 Years of Telescopic Discoveries

II. Optical Telescopes Astronomers use telescopes to gather more light from astronomical objects. The larger the telescope, the more light it gathers.

A. Two kinds of Optical Telescopes: Refracting/ReflectingRefracting/Reflecting Refracting Telescope: Lens focuses light onto the focal plane Reflecting Telescope: Concave Mirror focuses light onto the focal plane Almost all modern telescopes are reflecting telescopes. Focal length

Secondary Optics In reflecting telescopes: Secondary mirror, to re- direct the light path towards the back or side of the incoming light path. Eyepiece: To view and enlarge the small image produced in the focal plane of the primary optics.

Disadvantages of Refracting Telescopes Chromatic aberration: Different wavelengths are focused at different focal lengths (prism effect).Chromatic aberration: Can be corrected, but not eliminated by second lens out of different material Difficult and expensive to produce: All surfaces must be perfectly shaped; Glass must be flawless; Lens can only be supported at the edges

Review Questions 1. ____________ has (have) wavelengths that are longer than visible light. a. Gamma-rays b. Ultraviolet light c. Infrared radiation d. X-rays 2. Chromatic aberration occurs in a __________telescope when a. reflecting; different colors of light do not focus at the same point. b. refracting; different colors of light do not focus at the same point. c. reflecting; light of different wavelengths get absorbed by the mirror. d. refracting; light of different wavelengths get absorbed by the lens. 3. List two main reasons why a reflecting telescope is better than a refracting telescope.

Chapter Summary (2 ) There are two types of optical telescopes: refracting and reflecting telescopes. Refracting telescopes use a primary lens, and a reflecting telescope use a primary mirror. Refracting telescopes suffer from chromatic aberration and expensive to make. Reflecting telescopes are easier to build and less expensive than refracting telescopes of the same diameter.

B. The Powers of a Telescope 1.Light-gathering power 2.Resolving power 3.Magnifying power

The Powers of a Telescope (1) : Size Does Matter Video 2: Bigger is Better Video 2: Bigger is Better 1.Light-gathering power: Depends on the surface area A of the primary lens / mirror, proportional to radius squared: A =  R 2 D = 2R

Small Telescope Large Telescope

The Powers of a Telescope (2) 2. Resolving power:Resolving power: Wave nature of light => The telescope aperture produces fringe rings that set a limit to the resolution of the telescope.  min = 1.22 ( /D) Resolving power = minimum angular distance  min between two objects that can be separated. For optical wavelengths, this gives  min = 11.6 arcsec / D[cm]  min

10' 1' 5" 1" Andromeda galaxy using telescopes of different resolutions The smaller the resolution, the better is the image.

Seeing Weather conditions and turbulence in the atmosphere set further limits to the quality of astronomical images. Bad seeingGood seeing

Cause of Bad Seeing – Jupiter ComparisonJupiter Comparison

The Powers of a Telescope (3) 3. Magnifying Power = ability of the telescope to make the image appear bigger. The magnification depends on the ratio of focal lengths of the primary mirror/lens (F o ) and the eyepiece (F e ): M = F o /F e A larger magnification does not improve the resolving power of the telescope!

Another Misconception The purpose of a telescope is to magnify images. Magnifying power is the least important of the three powers.

The Best Location for a Telescope Far away from civilization – to avoid light pollution

The Best Location for a Telescope (2) On high mountain-tops – to avoid atmospheric turbulence (  seeing) and other weather effects Paranal Observatory (ESO), Chile

C. New-Generation Telescopes

Traditional Telescopes (1) Traditional primary mirror: sturdy, heavy to avoid distortions Secondary mirror

Traditional Telescopes (2) The 4-m Mayall Telescope at Kitt Peak National Observatory (Arizona)

Advances in Modern Telescope Design (1) 1. Lighter mirrors with lighter support structures, to be controlled dynamically by computers Floppy mirror Segmented mirror Modern computer technology has made significant advances in telescope design possible: Active Optics: Video 3Video 3

Adaptive Optics Computer-controlled mirror support adjusts the mirror surface (many times per second) to compensate for distortions by atmospheric turbulence

Adaptive optics using laser technology. to correct for atmospheric smearing

Advances in Modern Telescope Design (2) 2. Simpler, stronger mountings (“Alt-azimuth mountings”) to be controlled by computers

Examples of Modern Telescope Design (1) Design of the Large Binocular Telescope (LBT)

Examples of Modern Telescope Design (2) 8.1-m mirror of the Gemini Telescopes The Very Large Telescope (VLT)

Chapter Summary (3 ) Light-gathering power refers to the ability of a telescope to produce bright images. Resolving power refers to the ability of a telescope to resolve fine detail. Magnifying power, the ability to make an object look bigger, is a less important telescope power. Adaptive optics techniques involve measuring seeing distortions caused by Earth’s atmosphere and corrected by using laser technologies. Light-Gathering-Power (LGP) is proportional to the area of the lens or the mirror, i.e., LGP  D 2 Resolving-Power (RP) is inversely proportional to the linear size of the lens or the mirror, i.e., RP  1/D -- Resolving power:Resolving power: Magnifying-Power (MP or M) is just the ratio of the focal length of the objective over the focal length of the lens or the mirror, i.e., M=f o /f e

Let us compare….. A student named “Antar” has a reflecting telescope with a diameter of 20 cm having a focal length of 100 cm equipped with an eyepiece of focal length of 0.4 cm. Another student named “Kais” has a refracting telescope with a diameter of 10 cm having a focal length of 100 cm and equipped with an eyepiece of focal length of 0.2 cm. 1.Antar’s telescope has better light gathering power, but less magnifying power than Kais’s telescope 2.Antar’s telescope has less light gathering power, but better magnifying power than Kais’s telescope 3.Kais’s telescope has better light gathering power, but less magnifying power than Antar’s telescope 4.Kais’s telescope has less light gathering power, and less magnifying power than Antar’s telescope

D. Interferometry Recall: Resolving power of a telescope depends on diameter D:  min = 1.22 /D. This holds true even if not the entire surface is filled out. Combine the signals from several smaller telescopes to simulate one big mirror  Interferometry

Examples…. Very Large Array 27 Radio Telescopes NM (USA) Keck Observatory Hawaii (USA) 10m mirror ESO Paranal Observatory 8.2m mirror

CCD Imaging: Video 4Video 4 CCD = Charge-coupled device More sensitive than photographic plates Data can be read directly into computer memory, allowing easy electronic manipulations Negative image to enhance contrasts False-color image to visualize brightness contours

CCD Chip: Charge-coupled device

Scientific Argument Why do astronomers build optical observatories at the tops of mountains? What considerations do astronomers make in choosing the location of a new radio telescope?

Chapter Summary (4 ) Interferometry refers to the technique of connecting two or more separate telescopes to act as a single large telescope. Modern electronic systems such as charge-coupled devices (CCDs) have replaced both photographic plates and photometers.

III. Astronomy from Space

Radiation that cannot reach Earth’s surface: –Infrared –Ultraviolet –X-rays –Gamma ray Need Space Telescopes Video 5

RadioInfraredVisibleUltravioletX-raysGamma

Infrared Astronomy However, from high mountain tops or high-flying air planes, some infrared radiation can still be observed. NASA infrared telescope on Mauna Kea, Hawaii Most infrared radiation is absorbed in the lower atmosphere. Infrared cameras need to be cooled to very low temperatures, usually using liquid nitrogen.

NASA’s Space Infrared Telescope Facility (SIRTF) Infrared light with wavelengths much longer than visible light (“Far Infrared”) can only be observed from space.

Ultraviolet Astronomy Ultraviolet radiation with < 290 nm is completely absorbed in the ozone layer of the atmosphere. Ultraviolet astronomy has to be done from satellites. Several successful ultraviolet astronomy satellites: IUE, EUVE, FUSE Ultraviolet radiation traces hot (tens of thousands of degrees), moderately ionized gas in the Universe.

The Hubble Space Telescope Avoids turbulence in the Earth’s atmosphere Extends imaging and spectroscopy to (invisible) infrared and ultraviolet Launched in 1990; maintained and upgraded by several space shuttle service missions throughout the 1990s and early 2000’s

Gamma-Ray Astronomy Gamma-rays: most energetic electromagnetic radiation; traces the most violent processes in the Universe The Compton Gamma-Ray Observatory

X-Ray Astronomy X-rays are completely absorbed in the atmosphere. X-ray astronomy has to be done from satellites. NASA’s Chandra X-ray Observatory X-rays trace hot (million degrees), highly ionized gas in the Universe.

False Color Images

Chapter Summary (1) Light is a form of electromagnetic wave. Complete electromagnetic spectrum includes: gamma rays, X-rays, ultraviolet (UV), visible light, infrared (IR), microwaves, and radio waves. Earth’s atmosphere is transparent in only two atmospheric windows: visible light and radio. There are two types of optical telescopes: refracting and reflecting telescopes. Refracting telescopes use a primary lens, and a reflecting telescope use a primary mirror.

Chapter Summary (2) Refracting telescopes suffer from chromatic aberration and expensive to make. Reflecting telescopes are easier to build and less expensive than refracting telescopes of the same diameter. Light-gathering power refers to the ability of a telescope to produce bright images. Resolving power refers to the ability of a telescope to resolve fine detail. Magnifying power, the ability to make an object look bigger, is a less important telescope power.

Chapter Summary (3) Modern electronic systems such as charge-coupled devices (CCDs) have replaced both photographic plates and photometers. Adaptive optics techniques involve measuring seeing distortions caused by Earth’s atmosphere and corrected by using laser technologies. Interferometry refers to the technique of connecting two or more separate telescopes to act as a single large telescope.

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