telescopes lecture. standards understand how knowledge about the universe comes from evidence...

Telescopes Lecture

Standards

• Understand how knowledge about the universe comes from evidence collected from advanced technology (e.g., telescopes, satellites, images, computer models)

• Understand the scientific foundations of common technologies (e.g., telescopes)

Introduction

• Telescopes are used to look at astronomical objects, such as

planets stars nebulae galaxies details of the universe

Introduction

• Astronomy is not an experimental science, it is an observational science.

Can’t bring a star into the lab to study

• Experimentation is done indirectly in one of two ways:

1. Different observations of the same object in different parts of the spectrum

2. Theoretical models – using computers and technology

Introduction

• Telescopes are time machines.• When we look at stars, we are seeing light that is decades, centuries or millions to billions of years old.

• We are looking into the past.• We never observe the universe as it is, but as it was

Introduction

• Light from our sun takes 8 minutes to reach Earth.

We therefore see the sun as it existed 8 minutes in the past.

• The closest galaxy to us is Andromeda. The light we receive from it is 3 million years old.

We are seeing it as it was 3 million years ago!

Introduction

• Without optical telescopes, we can see 6000 stars in both hemispheres.

• With a telescope, we can see billions of stars.

Optical Telescopes

• Optical telescopes observe the universe in the visible portion of the electromagnetic spectrum.

• An optical telescope is essentially a “light bucket”:

It gathers lightResolves the objectAnd magnifies the object

Optical Telescopes

• The telescope’s primary function is to capture as many photons as possible from a given region of the sky.

• Use of telescopes to observe space started with Galileo and the refracting telescope.

Refracting Telescopes

Parts of a refracting telescope:• Objective – main lens used to bring light to the focus. Mounted on end of telescope.

• Focus – a point in a telescope that all light rays pass through.

Focal length = distance from the lens to the focus. It determines the angular field of view. Longer focal length = narrower field of view and vice versa.

Refracting Telescopes

• Eyepiece - magnifies the image.The magnification = the telescope focal length divided by the eyepiece focal length

Refracting Telescopes

Reflecting Telescopes

Parts of a reflecting telescope• Curved mirror = objective.Called the primary mirror because there is usually more than one

The primary mirror reflects the light rays to the focus (called the prime focus)

• Focal length = distance from the primary mirror to the prime focus.

Reflecting Telescopes

Reflecting Telescopes

• Most telescopes in use today are reflecting telescopes because there are disadvantages to refracting telescopes:

1. Light must pass through the lens on a refractor. Some of this light does not pass through uniformly – called chromatic aberration – which causes a colored halo around the object being observed.

Reflecting Telescopes

2. Some light is absorbed by the glass as it passes through the lens. This is severe for IR and UV observations.

3. Large lenses can be very heavy. In a telescope, they can only be supported by their sides and can deform under their own weight. Mirrors, on the other hand, can be supported along their back.

4. Lenses have two surfaces, which must be accurately machine polished. A mirror has only one surface.

Telescopes in Use Today

• Largest refracting telescope is 1 meter in diameter.

Located at Yerkes Observatory in Wisconsin.

Built in 1897.

Telescopes in Use TodayA few of the largest reflecting telescopes:

• 3.8 m – Kitt Peak, Arizona• 5 m - Hale telescope at Mt. Palomar in California

• 10 m – Keck and Keck II, Mauna Kea, Hawaii Mirror is composed of 36 segment First light in 1993 & 1996, respectively

• 10.4 m – Gran Telescopio Canarias, La Palma, Canary IslandsSegmented mirror based on KeckFirst light in 2007

Kitt Peak

Hale Telescope

Twin Keck Telescopes

Gran Telescopio Canarias

Telescopes in Use Today

• Hubble Space Telescope2.4 meterAbove Earth’s atmosphere, so has good resolution

Telescopes in Use Today

• Use CCD’s (charge coupled devices) instead of photographic plates.

These are electronic detectors hooked up to computers to make digital images

Advantage of CCD’s: they’re more efficient so can show objects 10 to 20 times fainter than the same telescope using a photographic plate.

Telescopes in Use Today

• Adaptive optics (developed because of Star Wars) - deform the shape of mirrors to undo effects of atmospheric turbulence.

Starfire Optical RangeKirtland Air Force base:laser beams probe theatmosphere to allow a computer to adjust forturbulence

Why Build Bigger and Bigger Telescopes?

• Bigger telescopes increase light gathering power.

Can see objects further away

A 5 meter telescope will produce an image 25 times brighter than a 1 meter telescope

Andromeda Galaxy: bottom image take with telescopetwice the size of one used to make top image

Why Build Bigger and Bigger Telescopes?

• Bigger telescopes have better resolving power

Resolution – ability of a device to form distinct, separate images of objects lying close together in the field of view

Better resolution = better detail

Resolving Power

Same objects as resolution improves

Andromeda Galaxyas resolution is improved 600 times

Why Build Bigger and Bigger Telescopes?

• Making mirrors for reflecting telescopes:

Use materials that can be polished and withstand large temperature changes with little expansion or contraction, such as quartz, glass or ceramics

Molten material is poured into a cast and cooled over several years. Then it is ground and polished

Surface is coated with thin film of aluminum to provide a reflecting surface

Earth’s Turbulent Atmosphere

• Reflects and refracts incoming energy (light), so images are blurred

• Light from a star is refracted – this causes it to twinkle

• Hubble Space Telescope is above atmosphere, so it allows astronomers to view the universe 20 times sharper than from ground based telescopes