“Nature composes some of her loveliest poems for the telescope”- Theodore Roszah

Chapter 5:Telescopes

Invention of the Telescope

Galileo did not invent the telescope! The first idea was credited to the Dutch optician Hans Lippershey in 1608.

Galileo with his early telescope 1609

The first telescope to use a mirror to gather light invented by Newton

in 1670

Early Telescopes

Two Main Types1) Refracting Telescope- Uses lenses to bring light to a focus and form an image

2) Reflecting Telescope- Uses mirrors to bring light to a focus and form an image

The ideal distance between the two lenses is just the sum of the focal lenses!!

Refracting Telescopes

Building Your Own Telescope is Easy!

• two magnifying glasses: perhaps 1-1.5 inches in diameter (it works best if one is larger than the other)

• a cardboard tube: paper towel roll or gift wrapping paper roll (it helps if it is long)

• duct tape• scissors• a ruler, yard stick, or tape measure• sheet of printed paper: e.g. newspaper or

magazine

Materials

Building Your Own Telescope is Easy!

1. Hold one magnifying glass (the bigger one) between you and the paper. The image of the print will be blurry.

2. Get the two magnifying glasses and a sheet of printed paper.3. Place the second magnifying glass between your eye and the first magnifying

glass.4. Move the second magnifying glass forward or backward until the print comes into

sharp focus. You will notice that the print appears larger and upside down.5. Have a friend measure the distance between the two magnifying glass and write

the distance down.6. Cut a slot in the tube the same distance from the first slot as your friend wrote

down. This is where the second magnifying glass will go.7. Place the two magnifying glasses in their slots (big one at front, little one at back

and tape them in with duct tape)8. Leave about 0.5-1 inch (1-2 cm) of tube behind the small magnifying glass and cut

off any excess tube remaining.9. Check to see that it works by looking at the printed page; you may have to play

slightly to get the exact distances between the two glasses right so that the image comes into focus.

Assembling

Refracting TelescopesChromatic Aberration: Different colors are focused at different distances from the lens

Need to add extra lenses to correctthe problem, but the correction is NOT 100 %!

• Chromatic aberration• More expensive!• Can’t be built too

large• Sagging due to

gravity distorting the lens

Refracting telescopes have disadvantages

Newtonian FocusPrime Focus

Cassegrain focuscoude’ focus

Reflecting telescopes use mirrors to concentrate incoming starlight

Inside the Cage!

Radio TelescopesArecibo Observatory, Puerto Rico

Very Large Array

most important• brighten

(called light gathering power)• see fine detail

(called resolving power)and least important,

• magnify

Three Main Functions of a Telescope

the functions that depend upon the diameter of the telescope are:

• Light gathering power• Resolving power (or resolution)

The function that depends upon the focallength of the telescope is,

• Magnification: M= fobj/feye

Bigger is Better!

Figure 4.1

Light Gathering Power: A measure of how much light a telescope collects

The bigger the telescope, the more light it will collect,and the brighter the image will be!

Light Gathering Power

A larger diameter

provides a brighter (not

bigger) image

Resolving Power: The ability to detect fine detail

The bigger the telescope, the higher the resolving power, but the maximum resolution is limited by the Earth’s atmosphere

2 X D 4 X D

D = Diameter of Telescope1

Unresolved Barely resolved Fully resolvedD

Telescope1 Telescope2 Telescope3

Resolving Power of a Telescope

Group Activity1. What is the difference between a reflecting and

a refracting telescope?2. Which type of telescope do professional

astronomers prefer to build and why?3. What are the two most important functions of a

telescope?4. If the focal length of the objective is 20 cm and

the focal length of the eyepiece is 5 cm, what is the magnification of the telescope?

Concept QuestionWhat aspects of a telescope depends uponthe diameter of the telescope?

A) light gathering power and resolutionB) light gathering power and magnificationC) resolution and magnificationD) light gathering power, magnification,

resolution

Telescopes you might buy

Refracting Telescope

Cassegrain Reflecting Telescope

Newtonian Reflecting Telescope

• Light Pollution from Cities

• Scattering of light by Earth’s atmosphere

Major Obstacles in Observing the Stars

Light pollution as viewed from space

Image of stars taken with a telescope on the Earth’s surface

Same picture taken with Hubble Space Telescope high above Earth’s blurring atmosphere

Earth’s Atmosphere Hinders Astronomical Research

Best places to build observatories

• air has to be very dry!• little to no light pollution!• calm air currents! - on mountain tops! - on mountain tops in deserts! - on mountain tops on islands! - in space!

Fremont Peak Observatory, San Juan Bautista, CA

Lick ObservatorySan Jose, CA

Kitt Peak National Observatory (KPNO)Tucson Arizona

Cerro-Telolo Inter-American Observatory(CTIO) La Serena, Chile

Keck Observatory Mauna Kea Hawaii

Keck Observatory

Gran Telescopia Canarias (GTC), Canary Islands

Not all radiation can penetrate Earth’s atmosphere

Space Telescopes

2.4 meter diameter reflector

Launched in 1990 and still operational

Hubble Space Telescope

Hubble Ultra Deep Field

The James Webb Space Telescope will replace HST in 2014

Innovationsand

Advancementsin Technology

Ordinary Photographs vs. CCDs

A Charge-Coupled Device (CCD)

Adaptive Optics uses a deformable mirror to correct for

the loss of resolution due to Earth’s atmosphere

Adaptive Optics: Laser guide star

Adaptive Optics

Thirty Meter Telescope

(TMT)with

Adaptive Optics

Interferometry

Building Your Own Observatory

1. What kind of telescope (reflecting or refracting) and why?

2. What is the difference between a reflecting and refracting telescope?

3. Diameter of objective (express in meters)? 4. Where would you build it and why?5. What would you observe with it?6. Draw and name your telescope or observatory.