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 Galileo with his early telescope 1609 The first telescope to use a mirror to gather light invented by Newton in 1670 Two Main Types 1) Refracting Telescope2) Reflecting TelescopeUses lenses to bring light Uses mirrors to bring light to to a focus and form an image 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 1. 2. 3. 4. 5. 6. 7. 8. 9. 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 Reflecting telescopes use mirrors to concentrate incoming starlight Newtonian Focus Prime Focus Cassegrain focus coude’ focus Inside the Cage! 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! A larger diameter provides a brighter (not bigger) image Resolving Power of a Telescope Resolving Power: The ability to detect fine detail Telescope1 Unresolved D Telescope2 Barely resolved 2XD Telescope3 Fully resolved 4XD 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 1. 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 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 Light pollution as viewed from space Earth’s Atmosphere Hinders Astronomical Research 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 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 Observatory San 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 Hubble Space Telescope 2.4 meter diameter reflector Launched in 1990 and still operational Hubble Ultra Deep Field The James Webb Space Telescope will replace HST in 2014 Innovations and Advancements in Technology 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 Adaptive Optics: Laser guide star Adaptive Optics Thirty Meter Telescope (TMT) with Adaptive Optics Interferometry Building Your Own Observatory 1. 2. 3. 4. 5. 6. 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.