Light and TelescopesLight and Telescopes

Why light?Why light?

With only a very few exceptions, all the With only a very few exceptions, all the information about stars and galaxies information about stars and galaxies reaches the Astronomers in the form of reaches the Astronomers in the form of lightlight

So what is light?So what is light? Light is an Light is an electroelectromagneticmagnetic wave wave It can also be thought of as a particle called a It can also be thought of as a particle called a

photon: photon: This apparent contradiction is called the This apparent contradiction is called the

particle/wave dualityparticle/wave duality The apparent contradiction is due to our inability The apparent contradiction is due to our inability

to describe something that is neither entirely a to describe something that is neither entirely a wave nor a particlewave nor a particle

At times it’s convenient to speak of light as a At times it’s convenient to speak of light as a wave; at other times convenience begs for a wave; at other times convenience begs for a particleparticle

Early Studies 1Early Studies 1

Euclid, circa 300BC, Euclid, circa 300BC, noted that light travels noted that light travels in straight lines and in straight lines and derived the law of derived the law of reflectionreflection

Early Studies 2Early Studies 2

Roger Bacon 1214-Roger Bacon 1214-1294 was a pioneer in 1294 was a pioneer in the study of opticsthe study of optics

On the right is a page On the right is a page from his treatise from his treatise Opus Opus MajusMajus describing the describing the path of lightpath of light

Early Studies 3Early Studies 3

Isaac Newton 1642-Isaac Newton 1642-1727 showed that 1727 showed that white light is made up white light is made up of many colorsof many colors

He used a prism to He used a prism to break light into its break light into its constituent colorsconstituent colors

The Speed of LightThe Speed of Light

The speed of light, denoted ‘c’, is a fixed The speed of light, denoted ‘c’, is a fixed speed (speed (we’ve “c’een this before!)we’ve “c’een this before!)

We use We use cc because the speed of a wave is because the speed of a wave is cconstant in a particular mediumonstant in a particular medium

The ultimate speed limit is 300,000 The ultimate speed limit is 300,000 km/sec, about 186,000 miles a km/sec, about 186,000 miles a second~670 million miles an hour.second~670 million miles an hour.

Light can travel more slowly than this, for Light can travel more slowly than this, for instance in glass or water, but never fasterinstance in glass or water, but never faster

Frequency and WavelengthFrequency and Wavelength

Frequency Frequency ff is how many waves occur is how many waves occur each second, and wavelength each second, and wavelength is, is, obviously, the length of the waveobviously, the length of the wave

Frequency is measured in Hz and Frequency is measured in Hz and wavelength is measured in meterswavelength is measured in meters

The product of those two terms gives you The product of those two terms gives you the speed of the wavethe speed of the wave

c = fc = f

The above equation is for demonstration The above equation is for demonstration purposes—don’t panic!purposes—don’t panic!

It demonstrates that, if It demonstrates that, if cc can’t change but can’t change but ff does, then does, then must change reciprocally must change reciprocally

Remember thisRemember this, because when we talk , because when we talk about Red Shift this relationship will be about Red Shift this relationship will be citedcited

The Full SpectrumThe Full Spectrum

SourcesSources

AnimationAnimation

Other SourcesOther Sources

Electric Charges running along a wire—Electric Charges running along a wire—long waves like radiolong waves like radio

Hot ionized atoms—short waves like X-Hot ionized atoms—short waves like X-raysrays

Splitting of the atomic nucleus—very short Splitting of the atomic nucleus—very short waves like gamma rayswaves like gamma rays

Changing Directions 1Changing Directions 1

Reflection: bouncing Reflection: bouncing light so that it focuses light so that it focuses to a point.to a point.

There are other There are other mirrors of course, but mirrors of course, but less useful for less useful for AstronomyAstronomy

Changing Directions 2Changing Directions 2

Refraction: the Refraction: the bending of light as it bending of light as it goes from one goes from one medium to another.medium to another.

Changing Directions 3Changing Directions 3

Diffraction: the bending Diffraction: the bending of a wave as it goes of a wave as it goes around cornersaround corners

Alternative to a prismAlternative to a prism

Qualities that make a good Qualities that make a good telescope:telescope:

Light-gathering ability (aperture): the bigger the Light-gathering ability (aperture): the bigger the lens, mirror, or dish, the more light it can gatherlens, mirror, or dish, the more light it can gather

Resolution: the ability to distinguish to distant Resolution: the ability to distinguish to distant objects that are close to each otherobjects that are close to each other

Tracking: the ability to follow an object in the sky Tracking: the ability to follow an object in the sky for long exposuresfor long exposures

Adaptive OpticsAdaptive Optics MagnificationMagnification Location, Location, LocationLocation, Location, Location PDF

Don’t forget the mount!Don’t forget the mount!

What you place the telescope on is What you place the telescope on is vital!vital! Any stray vibrations distort the imageAny stray vibrations distort the image

Also, the sky moves (as seen from Also, the sky moves (as seen from Earth)Earth)

You need a mount that is both You need a mount that is both stable and tracks the skystable and tracks the sky

Enter Joseph FraunhofferEnter Joseph Fraunhoffer We’ll meet him again later concerning We’ll meet him again later concerning

the solar spectrumthe solar spectrum

Alt/Azimuth MountsAlt/Azimuth Mounts

Altitude (up and down Altitude (up and down angle)angle)

Azimuth (around Azimuth (around motion)motion)

If you take the A105L If you take the A105L lab you’ll use lab you’ll use Dobsonians like thisDobsonians like this

Good and cheap, but Good and cheap, but you do all the work you do all the work slewing the ‘scopeslewing the ‘scope

How the sky movesHow the sky moves

The picture show a v-e-r-y long exposure centered on PolarisThe picture show a v-e-r-y long exposure centered on Polaris It demonstrates that the farther from the pole star you observe, the It demonstrates that the farther from the pole star you observe, the

more the sky move in a given timemore the sky move in a given time Alt/azimuth, even if motorized, won’t get the job doneAlt/azimuth, even if motorized, won’t get the job done

German Equatorial MountGerman Equatorial Mount Invented by Fraunhoffer, the GEM Invented by Fraunhoffer, the GEM

uses a series of gears that adjust for uses a series of gears that adjust for the motion of the sky in the direction the motion of the sky in the direction of observationof observation

Early models used falling weights to Early models used falling weights to drive the systemdrive the system

Astronomer F.G. Wilhelm Struve Astronomer F.G. Wilhelm Struve used this instrument to measure over used this instrument to measure over 3,000 double stars with precision of 3,000 double stars with precision of less than one arc second. less than one arc second.

“ “...undetermined which to admire most, ...undetermined which to admire most, the beauty and elegance of the the beauty and elegance of the workmanship in its most minute parts, the workmanship in its most minute parts, the propriety of its construction, the ingenious propriety of its construction, the ingenious method of moving it, or the incomparable method of moving it, or the incomparable optical power of the telescope, and the optical power of the telescope, and the precision with which objects are defined.”precision with which objects are defined.”

An Early Optical InstrumentsAn Early Optical Instruments

Tycho Brahe’s astrolabe Tycho Brahe’s astrolabe was used before the was used before the telescope to locate telescope to locate objects in the sky to objects in the sky to within 1 minute of arcwithin 1 minute of arc

Leonard Diggs, a16th C. Leonard Diggs, a16th C. English mathematician, English mathematician, invented a device called a invented a device called a ‘perspective glass’ to aid ‘perspective glass’ to aid navigationnavigation Early reflectorEarly reflector Kept secretKept secret

Early TelescopesEarly Telescopes

Galileo didn’t invent Galileo didn’t invent the telescope, but he the telescope, but he improved on the improved on the Flemish designFlemish design

Circa 1600Circa 1600 A A refractorrefractor

Diagram: GalileanDiagram: Galilean

Largest Refractor:

Yerkes 40”

Diagram: NewtonianDiagram: Newtonian

Friedrich Wilhelm Herschel Friedrich Wilhelm Herschel

Late 18Late 18thth C. C. 20 ft20 ft A A reflectorreflector

Diagram: Schmitt-CassegrainDiagram: Schmitt-Cassegrain

Mt. Wilson 100” Hooker reflectorMt. Wilson 100” Hooker reflector

Mt. Palomar 200” Hale telescopeMt. Palomar 200” Hale telescope

Mauna Kea Keck (ex) interferometerMauna Kea Keck (ex) interferometer

Large Binocular Telescope, Mt. GrahamLarge Binocular Telescope, Mt. Graham

Space Telescopes: HubbleSpace Telescopes: Hubble

Kepler Space TelescopeKepler Space Telescope

James Webb Space TelescopeJames Webb Space Telescope

2018 launch

Radio TelescopesRadio Telescopes

Recall the spectrum Recall the spectrum slide: light is not slide: light is not confined to the visible confined to the visible rangerange

The sky is transparent The sky is transparent to both visible light to both visible light and radio wavesand radio waves

The First Radio TelescopeThe First Radio Telescope

Karl Jansky, in 1932, Karl Jansky, in 1932, discovered radio discovered radio waves impinging the waves impinging the Earth from outer Earth from outer spacespace

Modern Telescopes: Owens ValleyModern Telescopes: Owens Valley

Modern Telescopes: AreciboModern Telescopes: Arecibo

Modern Telescopes: VLAModern Telescopes: VLA

Hess II CherenkovHess II Cherenkov Gamma rays don’t reach the earth’s surfaceGamma rays don’t reach the earth’s surface But (!) they But (!) they dodo produce superluminal particles in the produce superluminal particles in the

airair These give off Cherenkov radiation which is These give off Cherenkov radiation which is

detectable on the grounddetectable on the ground