Chapter 5:Telescopes “Nature composes some of her loveliest poems for the telescope” - Theodore Roszah
Invention of the Telescope Galileo did not invent the telescope! The first idea was credited to the Dutch optician Hans Lippershey in 1608.
Early Telescopes The first telescope to use a mirror to gather light invented by Newton in 1670 Galileo with his early telescope 1609
Two Main Types 2) Reflecting Telescope- Uses mirrors to bring light to a focus and form an image 1) Refracting Telescope- Uses lenses to bring light to a focus and form an image
Refracting Telescopes The ideal distance between the two lenses is just the sum of the focal lenses!!
Building Your Own Telescope is Easy! Materials • 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
Building Your Own Telescope is Easy! Assembling • Hold one magnifying glass (the bigger one) between you and the paper. The image of the print will be blurry. • Get the two magnifying glasses and a sheet of printed paper. • Place the second magnifying glass between your eye and the first magnifying glass. • 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. • Have a friend measure the distance between the two magnifying glass and write the distance down. • 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. • Place the two magnifying glasses in their slots (big one at front, little one at back and tape them in with duct tape) • Leave about 0.5-1 inch (1-2 cm) of tube behind the small magnifying glass and cut off any excess tube remaining. • 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.
Refracting Telescopes Chromatic Aberration: Different colors are focused at different distances from the lens Need to add extra lenses to correct the problem, but the correction is NOT 100 %!
Refracting telescopes have disadvantages • Chromatic aberration • More expensive! • Can’t be built too large • Sagging due to gravity distorting the lens
Cassegrain focus Newtonian Focus coude’ focus Prime Focus Reflecting telescopes use mirrors to concentrate incoming starlight
Radio Telescopes Arecibo Observatory, Puerto Rico Very Large Array
Three Main Functions of a Telescope • most important • brighten • (called light gathering power) • see fine detail • (called resolving power) • and least important, • magnify
Bigger is Better! • the functions that depend upon the diameter of the telescope are: • Light gathering power • Resolving power (or resolution) • The function that depends upon the focal • length of the telescope is, • Magnification: M= fobj/feye
Figure 4.1 Light Gathering Power 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!
Telescope2 Telescope3 Telescope1 Unresolved Barely resolved Fully resolved D Resolving Power of a Telescope Resolving Power: The ability to detect fine detail 2 X D 4 X D D = Diameter of Telescope1 The bigger the telescope, the higher the resolving power, but the maximum resolution is limited by the Earth’s atmosphere
Group Activity • What is the difference between a reflecting and a refracting telescope? • Which type of telescope do professional astronomers prefer to build and why? • What are the two most important functions of a telescope? • 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 Question What aspects of a telescope depends upon the diameter of the telescope? A) light gathering power and resolution B) light gathering power and magnification C) resolution and magnification D) light gathering power, magnification, resolution
Telescopes you might buy Refracting Telescope Newtonian Reflecting Telescope Cassegrain Reflecting Telescope
Major Obstacles in Observing the Stars • Light Pollution from Cities • Scattering of light by Earth’s atmosphere
Earth’s Atmosphere Hinders Astronomical Research Same picture taken with Hubble Space Telescope high above Earth’s blurring atmosphere Image of stars taken with a telescope on the Earth’s surface
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!
Cerro-Telolo Inter-American Observatory(CTIO) La Serena, Chile
Hubble Space Telescope 2.4 meter diameter reflector Launched in 1990 and still operational
A Charge-Coupled Device (CCD) Ordinary Photographs vs. CCDs
Adaptive Optics uses a deformable mirror to correct for the loss of resolution due to Earth’s atmosphere
Building Your Own Observatory • What kind of telescope (reflecting or refracting) and why? • What is the difference between a reflecting and refracting telescope? • Diameter of objective (express in meters)? • Where would you build it and why? • What would you observe with it? • Draw and name your telescope or observatory.