Light and TelescopesLight and Telescopes

6-1 Radiation: Information from Space

Light as a Wave and a Light as a Wave and a ParticleParticleElectromagnetic radiation –

changing electric and magnetic fields that travel through space and transfer energy from place to another

Ex. Light or radio waves Moves at the speed of light (186,000 mi/sec)

Light as a Wave and a Light as a Wave and a ParticleParticleElectromagnetic radiation is a

wave phenomenon so we use the common measurements of wavelength and frequency◦Wavelength is the distance between

successive peaks of a wave◦Frequency is the number of cycles

that pass in one second

Light as a Wave and a Light as a Wave and a ParticleParticle

http://encyclopedia2.thefreedictionary.com/ultrasound+frequency

http://www.chemguide.co.uk/analysis/uvvisible/theory.html

An easy way to remember the relationship between wavelength and frequency is the larger (higher) the frequency the smaller (shorter) the wavelength and vise versa.

Light as a Wave and a Light as a Wave and a ParticleParticleCouple other terms dealing with

light of importance:◦Photon is a particle of light◦Nanometer (nm) is the unit used to

measure light particles This is used because the wavelength of light

particles are so much smaller (shorter) than other waves measured

Electromagnetic SpectrumElectromagnetic SpectrumThis spectrum is used to classify

electromagnetic radiation We are most familiar with the

spectrum of visible light◦ Light near the short-wavelength end of the visible spectrum

(400 nm) looks violet to our eyes, and light near the long-wavelength end (700 nm) looks red

Most electromagnetic radiation is absorbed in Earth’s atmosphere, and only radiation in the visual window and the radio window can reach Earth’s surface

Pg 101 Figure 6-2

Electromagnetic SpectrumElectromagnetic Spectrum

http://www.lcse.umn.edu/specs/labs/glossary_items/em_spectrum.html

MAKE SURE YOU KNOW SPECTRUM ORDER FOR TEST…MULTIPLE QUESTIONS

Light and TelescopesLight and Telescopes

6-2 Astronomical Telescopes

Refracting TelescopesRefracting TelescopesThe main element in a refracting

telescope is a lens, a piece of glass carefully shaped so that light striking it is refracted, or bent, into an image

http://www.opticsplanet.net/vivitar-60-120x-refractor-telescope-with-full-size-expandable-tripod.html

http://www.bro.lsu.edu/telescope/Classroom/2.How%20Telescopes%20Work/lesson.html

Refracting TelescopesRefracting TelescopesTerms:

◦Focal length is the distance from a lens to the point where it focuses parallel rays of light

◦Objective lens is the long-focal-length lens that forms an image of the object viewed

This lens is closest to the object

◦Eyepiece is a short-focal-length lens used to enlarge the image in a telescope

This lens is nearest the eye of the observer

Refracting TelescopesRefracting TelescopesPositives of Refracting Telescopes:

◦They are great for viewing planets, the moon, and star clusters

Negatives of Refracting Telescopes:◦They are not very good for viewing very

faint objects such as galaxies  

REFRACTING ANIMATION

Reflecting TelescopesReflecting TelescopesIn a reflecting telescope, a concave

mirror, the objective mirror, focuses the starlight into an image that can be viewed with an eyepiece

http://www.bro.lsu.edu/telescope/Classroom/2.How%20Telescopes%20Work/lesson.html

http://www.telescopes.com/telescopes/reflecting-telescopes/celestronc10ngtreflector.cfm

Reflecting TelescopesReflecting TelescopesPositives for Reflecting Telescopes:

◦They are great for viewing faint objects such as galaxies. Very affordable.

Negatives for Reflecting Telescopes:◦They are larger and this often makes them

more difficult to store. They also need to be maintained often to make sure the optics are properly aligned.

REFLECTING ANIMATION

The Powers of a TelescopeThe Powers of a TelescopeA telescope can aid our eyes in

only three ways These three ways are the three powers of

a telescope They make images brighter, more

detailed, and larger

The Powers of a TelescopeThe Powers of a TelescopeLight-gathering power refers to the

ability of a telescope to collect light◦Catching light in a telescope is like

catching rain in a bucket – the bigger the bucket, the more rain it catches

Light-gathering Power = (DA/DB)2

For example, suppose we compare a telescope 24 cm in diameter with a telescope with a 4 cm diameter. The ratio of the diameters is 24/4, or 6, but the larger telescope doesn’t gather 6 times as much light. Light-gathering power increases as the ratio of diameters squared (in this case 62 or 6x6), so it gathers 36 times more light than the smaller telescope.

The Powers of a TelescopeThe Powers of a TelescopeResolving Power is the 2nd power

and it refers to the ability of a telescope to reveal fine detail

Resolving Power = 11.6 D

For example, the resolving power of a 25-cm telescope is 11.6 divided by 25. This means the resolving power is .46

The Powers of a TelescopeThe Powers of a TelescopeThe third and least important power of

a telescope is magnifying power, the ability to make the image bigger

Magnifying power = Focal length of the objective lens Focal length of eyepiece

For example, if a telescope has an objective with a focal length of 80 cm and we use an eyepiece whose focal length is 0.5 cm, the magnification is 80/0.5 which gives us a magnification power of 160 times

The Powers of a TelescopeThe Powers of a TelescopeThe search for light-gathering power

and high resolution explains why almost all major observatories are located far from major cities and usually on high mountains

Astronomers avoid cities because of light pollution

This is the phenomenon of the brightening of the night sky by light scattered from artificial outdoor lighting, which can make it difficult or impossible to see faint objects like stars in the sky

http://www.nasa.gov/images/content/49261main_usa_nightm.jpe

http://www.astro.caltech.edu/palomar/images/viewjan2006.jpg

Picture above is showing amount of light created at night by cities

Picture to right is showing the glare these lights put into the sky

Light and TelescopesLight and Telescopes

6-3 Special Instruments

Imaging SystemsImaging SystemsMost modern astronomers use

charge-coupled devices (CCDs) to record images◦These devices can be used like a small

photographic plate They can detect both bright and faint objects in a

single exposure, are much more sensitive that a photographic plate, and can be read directly into computer memory for later analysis

http://www.circuitstoday.com/charge-coupled-devices-ccd

The SpectrographThe SpectrographSpectrograph is a device that separates

light by wavelength to produce a spectrum◦Sir Isaac Newton was one of the first people

to understand the changing of color of sunlight after it passes through a prism – he then concluded that white light was made up of a mixture of all colors

https://wiki.engr.illinois.edu/display/BIOE414/Overview+of+the+BIND+Technology

http://astronomy.nmsu.edu/tharriso/ast110/class26.html

Light and TelescopesLight and Telescopes

6-4 Radio Telescopes

Operation of a Radio Operation of a Radio TelescopeTelescope

A radio telescope usually consists of four parts:◦A dish reflector, an

antenna, an amplifier, and a recorder These components, working

together, make it possible for astronomers to detect radio radiation from celestial objects

This page was copied from Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com for the updated and corrected version.

This page was copied from Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com for the updated and corrected version.

The Robert C Byrd Green Bank Telescope is the largest fully steerable radio telescope in the world

Limitations of the Radio Limitations of the Radio TelescopeTelescopeA radio astronomer works under

three handicaps◦Poor resolution

As with an optical telescope the only way to help improve these handicaps is to build a bigger telescope – which is the reason that radio telescopes are so large

To help improve resolution, radio astronomers can combine two or more telescopes

Limitations of the Radio Limitations of the Radio TelescopeTelescope

◦Low resolution To help combat low resolution radio

astronomers must build large collecting dishes

◦Interference To avoid this kind of interference, radio

astronomers locate their telescopes as far from civilization as possible

Keep them hidden deep in mountain valleys, they are able to listen to the sky protected from human-made radio noise

Advantages of Radio Advantages of Radio TelescopesTelescopes

Three factors make Radio Telescopes worth while◦First, and most important, a radio telescope

can show us where clouds of cool hydrogen are located between stars

◦Radio signals have relatively long wavelengths, they can penetrate the vast clouds of dust that obscure our view at visual wavelengths

◦A radio telescope can detect objects that are more luminous at radio wavelengths than at visible wavelengths. This includes everything from the coldest clouds of gas to the hottest stars