Learning ObjectivesLearning Objectives

Explain the significance of light and Explain the significance of light and astronomyastronomy

Explain how telescopes workExplain how telescopes work Demonstrate how to correctly select and Demonstrate how to correctly select and

use a telescopeuse a telescope Explain why different telescopes are used Explain why different telescopes are used

for observing the universefor observing the universe

Main ThreadsMain Threads

• the connection between sunlight, radio waves, and the connection between sunlight, radio waves, and other kinds of electromagnetic radiationother kinds of electromagnetic radiation

• what telescopes can and cannot do and how what telescopes can and cannot do and how telescopes collect and focus lighttelescopes collect and focus light

• why different types of telescopes are used for why different types of telescopes are used for different types of research and how astronomers use different types of research and how astronomers use the entire spectrum of electromagnetic radiation to the entire spectrum of electromagnetic radiation to observe the stars and other astronomical eventsobserve the stars and other astronomical events

• what new generations of land-based and space-what new generations of land-based and space-based high-technology telescopes being developed based high-technology telescopes being developed can docan do

Reflector vs. RefractorReflector vs. Refractor

Even Dogs Love AstronomyEven Dogs Love Astronomy

CHAPTER 3:CHAPTER 3:Light and Telescopes

Radio wave image of Saturn. False color is used to represent different wavelengths of radio waves.

WHAT DO YOU THINK?

What is light? What is the electromagnetic spectrum? Which type of electromagnetic radiation is

most dangerous to life? What is the main purpose of a telescope? Why do stars twinkle? Which type(s) of electromagnetic radiation

can telescopes currently detect?

PrismsPrisms

Newton found that the prism itself does not add the colors to the light, but that color is an intrinsic property of light.

When light of a single color is passed through two slits, the light from the two sources creates an interference pattern of bright and dark regions. This shows that light is a wave.

In 1860, James Clerk Maxwell combined and unified the current theories of electricity and magnetism and showed that electric and magnetic fields should travel through space together in the form of electromagnetic waves.

These waves are characterized by their wavelength, the distance between two peaks in a wave.

There are a wide range of wavelengths of electromagnetic waves plotted on the electromagnetic spectrum. We classify these waves by their source, use, or interactions with other matter. Only a very small range of wavelengths, 400nm to 700nm, are visible to humans. (Since these wavelengths are small we describe them in terms of nanometers (10-9m) or Angstroms (10-10m).

Other wavelengths are classified as Gamma Rays, X-Rays, Ultraviolet, Infrared, Microwaves or Radio waves.

Ole Romer used two eclipses of one of Jupiter’s moons to show that light does not travel infinitely fast and to measure its speed. One can also use Maxwell’s equations to calculate the speed of light.

The speed of light, denoted by the letter c, has since been measured to be 299,792.458km/s which we generally round to

cc = 300,000 km/s = 180,000 mi/s = 300,000 km/s = 180,000 mi/s

The Doppler EffectThe Doppler Effect

Sources moving toward the observer squeeze light waves in front of them, causing them to be shorter. We call this a

blueshift.

Sources moving away from the observer stretch the light waves behind them, causing them to be longer. We call this a

redshift.

Einstein showed that light also behaves as a particle. He found that light behaves as discreet packets called photons. The energy of a photon decreases with its wavelength and is calculated by:

Photon energy = PlanckPlanck’’s constant s constant ×× speed of light speed of light

wavelengthwavelength

Questions--LightQuestions--Light What is light?What is light? Why is light referred to as a spectrum?Why is light referred to as a spectrum? How is light distinguished or recognized?How is light distinguished or recognized? What is the speed of light?What is the speed of light? What is the Doppler Effect and why does it relate to What is the Doppler Effect and why does it relate to

light?light? What does it mean for an astronomical object to be What does it mean for an astronomical object to be

redshifted or blueshifted?redshifted or blueshifted? Which two properties does light exhibit? (or in which Which two properties does light exhibit? (or in which

two ways can light be modeled?)two ways can light be modeled?) What is a photon?What is a photon?

What is a photon bucket?

(Another name for a telescope)

TWO BASIC TYPES OF TWO BASIC TYPES OF TELESCOPESTELESCOPES

1.1. Reflecting Telescopes use a mirror to Reflecting Telescopes use a mirror to gather light. gather light.

2.2. Refracting Telescopes use a lens to Refracting Telescopes use a lens to gather light.gather light.

Functions of a TelescopeFunctions of a Telescope

BrightenBrighten (increase the number of photons gathered) (increase the number of photons gathered) Increase brightness by increasing the area of primary Increase brightness by increasing the area of primary mirror or lens.mirror or lens.

ResolveResolve (angular resolution, best resolution on Earth (angular resolution, best resolution on Earth is 0.1 arcseconds). Resolution is improved by is 0.1 arcseconds). Resolution is improved by increasing the area of the primary mirror or lens; the increasing the area of the primary mirror or lens; the manufacturing quality is also important.manufacturing quality is also important.

MagnifyMagnify (focal length of primary/focal length of (focal length of primary/focal length of eyepiece) Magnification makes image appear larger, eyepiece) Magnification makes image appear larger, the larger the image, the greater the detail seen.the larger the image, the greater the detail seen.

Your Best ViewYour Best View

If your original image is blurry, and you magnify If your original image is blurry, and you magnify this, will the magnification improve the image this, will the magnification improve the image resolution?resolution?

No.No. The two most important aspects of a telescope are The two most important aspects of a telescope are

light-gathering powerlight-gathering power and and resolving powerresolving power.. Greater magnification will allow for more detail to be Greater magnification will allow for more detail to be

seen, but will eventually lead to greater and greater seen, but will eventually lead to greater and greater distortion (distortion (maximum useful magnificationmaximum useful magnification).).

Your Best ViewYour Best View

If you had a 3 in diameter reflecting If you had a 3 in diameter reflecting telescope, how would you to improve the telescope, how would you to improve the quality of the image?quality of the image?

Increase the diameter of the primary mirror.Increase the diameter of the primary mirror. Increase the quality of the mirror.Increase the quality of the mirror.

Making a Large Parabolic MirrorMaking a Large Parabolic Mirror

40,000 pounds of glass are loaded into a rotated furnace and heated to 1500K.

After melting, spinning and cooling, the surface is ready to be coated with a highly reflective material.

FUNCTIONS OF A TELESCOPEFUNCTIONS OF A TELESCOPE

Telescopes brighten objects by collecting light. The ability of a telescope to collect light is called its light-gathering power.

SMALLER SMALLER DIAMETERDIAMETER

LESS LIGHT LESS LIGHT GATHERING GATHERING POWERPOWER

DIMMER DIMMER IMAGEIMAGE

LARGER LARGER DIAMETERDIAMETER

MORE LIGHT MORE LIGHT GATHERING GATHERING POWERPOWER

BRIGHTER IMAGEBRIGHTER IMAGE

FUNCTIONS OF A TELESCOPEFUNCTIONS OF A TELESCOPE

Telescopes reveal the details of extended celestial objects. The clarity of the image and the amount of detail revealed is called the angular resolution.

Smaller diameter telescopes have less resolution and produce images that are blurred.

Larger diameter telescopes have more resolution and produce images that are clear.

FUNCTIONS OF A TELESCOPEFUNCTIONS OF A TELESCOPE

Telescopes make objects appear larger. This is called magnification. The magnification is the focal length of the primary mirror divided by the focal length of the eyepiece.

magnification =magnification =focal length of the primaryfocal length of the primary

focal length of the eyepiecefocal length of the eyepiece

A common misconception is that the magnification is the most important factor determining the quality of a telescope’s images. Actually, the light-gathering and resolving powers of a telescope are much more important, as the magnification can be changed simply by switching eyepieces. Telescopes have a maximum useful magnification beyond which images will be larger but will no longer be clearly focused. In general, the larger the diameter of a telescope, the greater the maximum magnification will be.

Light rays approaching the surface of a mirror will bounce off at the same angle at which it approaches. This is the principle of reflection.

The reflected angle is the same as the incident angle.

REFLECTING REFLECTING TELESCOPESTELESCOPES

The large mirror used to gather and focus the light in a reflecting telescope is called the primary mirror.

The surface of the mirror used is bent into a curve. Parallel light rays from distant objects converges to a focal point.

The distance between the mirror and its focal point is called the focal length.

Designs of Reflecting TelescopesDesigns of Reflecting Telescopes

A Newtonian telescope uses a flat secondary mirror to redirect the focused image to the side of the telescope for viewing. The image is viewed through a small focal length lens called an eyepiece.

Reflecting TelescopeReflecting Telescope

See Animation See Animation

Light rays traveling into a transparent medium such as glass bend at the surface. The bending of light rays between two transparent media is called refraction.

REFRACTING TELESCOPESREFRACTING TELESCOPES

If the lens is curved, parallel rays will converge at a focal point, just like the rays in a reflecting telescope.

Like reflecting telescopes, extended objects produce images in a focal plane.

Refracting telescopes use an objective lens to gather light and an eyepiece through which the image is viewed.

Refracting TelescopeRefracting Telescope

See animationSee animation

Refracting TelescopesRefracting Telescopes

Historically speaking, the first telescopes. Historically speaking, the first telescopes. Invented by Hans Lippershey (1608)Invented by Hans Lippershey (1608) Largest refracting telescope in the world—is at Largest refracting telescope in the world—is at

the Yerkes Observatory the Yerkes Observatory

in Wisconsin (N of Chicago)in Wisconsin (N of Chicago)

……40 inches in diameter!40 inches in diameter!

Limitations of Refracting TelescopesLimitations of Refracting Telescopes

•Different colors of light are refracted differently and have different focal points. Thus, all the colors of the image will not be focused at once. This is called chromatic aberration.

•It is difficult to grind a lens into the proper shape to have all parallel rays converge at a single focal point.

•The weight of a large lens can cause the lens to sag and distort the image.

•Air bubbles in the glass cause unwanted refractions, distorting the image.

•Glass is opaque to certain wavelengths of light, meaning they do not go through the glass.

http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/aber2.html

These issues do not affect reflecting telescopes because the light from the stars does not travel through a glass lens before being focused. However, reflecting telescopes do have some problems.

One problem is that the secondary mirror used to deflect the light out the side partially blocks the light from the star.

Problems with Reflecting Telescopes

Another problem with reflecting telescopes is called spherical aberration.

When a spherically-shaped mirror is used, the light rays hitting far from the center will not converge at the same point. One solution is to grind the mirror into a parabolic shape. Another solution is to use a correcting lens to make all the light rays converge at a single point.

Telescopes can yield reversed and Telescopes can yield reversed and inverted imagesinverted images

Which way is up?Which way is up?

Astronomical telescopes invert images.Astronomical telescopes invert images. For instance, this image of Saturn as viewed For instance, this image of Saturn as viewed

from Earth would appear reversed and/or from Earth would appear reversed and/or upside-down.upside-down.

Questions--TelescopesQuestions--Telescopes

What are the two types of telescopes?What are the two types of telescopes? What are the three functions of a telescope?What are the three functions of a telescope? What could you do to better your viewing of What could you do to better your viewing of

the night sky?the night sky? Why is it that increasing the magnification of a Why is it that increasing the magnification of a

telescope does not necessarily help you see the telescope does not necessarily help you see the object any better?object any better?

How do you determine the magnification of How do you determine the magnification of your telescope?your telescope?

Questions--TelescopesQuestions--Telescopes

Why is there a hole in the primary mirror of a Why is there a hole in the primary mirror of a Cassegrain reflecting telescope?Cassegrain reflecting telescope?

What is the focal length of a lens or mirror?What is the focal length of a lens or mirror? Name three limitations of a refracting Name three limitations of a refracting

telescope.telescope. Name two limitations of a reflecting telescope.Name two limitations of a reflecting telescope. Explain why the term Explain why the term photon bucketphoton bucket is such a is such a

good description for a telescope.good description for a telescope.

Even though light rays from stars spread out in all directions, they must travel over huge interstellar distances to reach the Earth. Therefore, the rays which enter our telescopes are essentially traveling in the same direction, and thus they are considered parallel.

If the object we are examining is an extended object, such as the Moon, then the light rays converge in a focal plane rather than a single point.