TELESCOPIC ASTRONOMY

First Telescope

• 1608- Hans Lippershey. Dutch lens grinder.

• 1609- Galileo. Built his own telescope, used it for scientific study.– Mountains and valleys on Moon– Moons of Jupiter– Phases of Venus– Saturn’s rings– Sunspots

Galileo’s Telescope

Reflecting Telescope• 1663- James Gregory

designed a telescope with a large concave primary mirror and a smaller concave secondary mirror

• 1666- Newton found that a prism breaks up white light into a rainbow of colours– Telescope lenses do the same

• 1672- Newton modified the design, and it won huge acclaim

Newton’s Telescope

Optical Telescopes

Refracting telescope uses a Refracting telescope uses a large large lenslens to gather and focus to gather and focus light.light.

Reflecting telescope uses a Reflecting telescope uses a concave concave mirror.mirror.

Lenses vs Mirrors• Lenses allow the

light to pass through.• As light passes

through the lens it slows down and bends (REFRACTION).

• The bend depends on the shape and material of the lens.

Lenses vs Mirrors• Mirrors do not allow

the light to pass through.

• The light passes through the glass but is then REFLECTED by the silver backing.

• The shape of the mirror directs the light.

Focal lengthFocal length – –the distance from the lens or the distance from the lens or mirror to the image formed of a mirror to the image formed of a distant light sourcedistant light source

Primary lens:Primary lens:

the main lens in the main lens in a a refracting refracting telescope. It telescope. It is also called is also called an an objectiveobjective lens.lens.

Primary mirror:Primary mirror:

the main mirror the main mirror in a in a reflectingreflecting telescope. It is telescope. It is also called an also called an objectiveobjective mirror.mirror.

Eyepiece:Eyepiece:A small lens to A small lens to

magnify the magnify the image image produced by produced by the the objective objective (primary)(primary) lenslens

Lens and Mirror Activity

• STATION 1: Making RainbowsUse the overhead as a light sourceUse the objects to try to create rainbows on the

white paper

Observation: What direction do you need to hold the prisms to

make the rainbow?What happens to the edge of the image created by

the lens?

Lens and Mirror Activity

• STATION 2: Looking Through Lenses

Look at the writing on the worksheet with the various lenses

Observation:

What happens to the size and orientation of the writing?

Lens and Mirror Activity

• STATION 3: Focusing the Flame

Line up the paper with the flame. Move the lens back and forth until you are able to focus an image of the flame on the paper.

Use the different lenses.

Observation:

Measure the focal length of each lens.

What happened to the orientation of the flame?

Lens and Mirror Activity

• STATION 4: Mirror Mirror on the Wall

Attach the mirror to the wall at eye level.

Move your positions until you can see each other and not yourself.

Observation:

Draw a diagram that shows your angles relative to the mirrow.

What do you notice about these angles?

Lens and Mirror Activity

• STATION 5: Reflecting the Image

Stand by the window

Hold a mirror in each hand. Move the two arms until you are able to focus a distance image outside.

Observation:

Measure the distance between your hands

What happens to the orientation of the image?

Powers of a Telescope

1.1. Light-gathering Light-gathering power:power:

The ability of a The ability of a telescope to telescope to collectcollect light.light.

Powers of a Telescope

2.2. Resolving Resolving PowerPower – –

the ability of a the ability of a telescope to telescope to reveal fine reveal fine detail.detail.

Resolving PowerResolving Power (con’t) (con’t)• When light is focused into When light is focused into

an image, a blurred fringe an image, a blurred fringe surrounds the image surrounds the image (diffraction fringe).(diffraction fringe).

• We can never see any detail We can never see any detail smaller than the smaller than the fringe.fringe.

• Large diameter telescopes Large diameter telescopes have small fringes and we have small fringes and we can see can see smaller smaller details. details. Therefore the larger the Therefore the larger the telescope, the better its telescope, the better its resolving resolving power.power.

Resolving PowerResolving Power (con’t) (con’t)• Optical Optical quality quality and and

atmosphericatmospheric conditions conditions limit the detail we can limit the detail we can see.see.

• SeeingSeeing is the blurred is the blurred image caused by image caused by unsteady, turbulent unsteady, turbulent atmosphereatmosphere. A star near . A star near the horizon will the horizon will twinkle twinkle more than an overhead more than an overhead star. Most telescopes are star. Most telescopes are built on high built on high mountains.mountains.

Powers of a Telescope

3.3. Magnifying Magnifying power –power –

the ability to the ability to make the make the image image biggerbigger

Magnifying power con’tMagnifying power con’tMagnification of a Magnification of a

telescope can be telescope can be changed by changed by changing thechanging the eyepiece.eyepiece. We cannot We cannot alter thealter the telescope’s telescope’s light-gathering light-gathering or or resolving power.resolving power.

Astronomers identify telescopes byAstronomers identify telescopes by diameterdiameter because that determines because that determines both light-gathering power andboth light-gathering power and resolvingresolving power.power.

Observatories are built on top of Observatories are built on top of mountaintops because: mountaintops because:

1. Air is thin and more 1. Air is thin and more transparenttransparent2. The sky is 2. The sky is darkdark3. Stars are 3. Stars are brighterbrighter4. Wind blows 4. Wind blows smoothlysmoothly over some over some

mountain topsmountain tops5. There is less 5. There is less pollutionpollution

• Refracting Telescopes have limitations…

1. Chromatic Aberration

When light is refracted through glass, When light is refracted through glass, shorter wavelengths bend shorter wavelengths bend moremore than than longerlonger wavelengths, and blue light wavelengths, and blue light comes to a focus closer to thecomes to a focus closer to the lenslens than than does red light. does red light.

1. Chromatic Aberration con’t

If we focus on the blue image, the red If we focus on the blue image, the red image is out of focus and we see a image is out of focus and we see a red red blur around the image. This color blur around the image. This color separation is called separation is called chromatic chromatic aberration.aberration.

• SOLUTION…

Achromatic Lens

An achromatic lensAn achromatic lens is made of two is made of two components made of different kinds of components made of different kinds of glassglass and brings the two different and brings the two different wavelengths to the same wavelengths to the same focus.focus. Other Other wavelengths are still out ofwavelengths are still out of focus.focus.

2. Size• Largest

refracting telescope in the world is at Yerkes Observatory in Wisconsin (1m).

• It weighs ½ ton and the glass sags under its own weight.

• SOLUTION…

Benefits of Reflecting Telescopes

• Less expensive. Less expensive. – Only the front surface of the mirror must Only the front surface of the mirror must

bebe ground.ground.– The glass doesn’t need to be perfectly The glass doesn’t need to be perfectly

transparenttransparent• The mirror can be supported over its The mirror can be supported over its

backback surface to reduce sagging. surface to reduce sagging. • They do not suffer from chromatic They do not suffer from chromatic

aberration because the light is aberration because the light is reflected reflected toward the focus before it can enter the toward the focus before it can enter the glass. glass.

Four ways to look through reflecting telescopes

Prime Focus

Newtonian Focus

Cassegrain Focus

Schmidt-Cassegrain

Hershel’s Telescope• Late 1770s, William

Hershel was making the best metallic mirrors and telescopes in the world.

• 1781- Discovered Uranus• 1789 - Built a giant

telescope which he used with his sister Caroline– His telescope had a 125cm

mirror– 40ft in length

Rosse’s Telescope• 1838- Earl of Rosse,

Ireland, taught himself mirror-making and built a 91cm telescope

• 1842- attempted to build a 181cm telescope but it broke when moved– built another one that

couldn’t be moved

Observatories• 1874- 91cm telescope and

observatory at University of California (James Lick)

• 1880- 76cm telescope in France• 1897- 102cm telescope at Yerkes

Observatory in Wisconsin• 1908- 153cm telescope on Mount

Wilson California (George Ellery Hale)

• 1917- 254cm telescope also built on Mount Wilson (John D. Hooker)

• 1948- 500cm mirror. Hale Observatory, Mount Palomar, California.

New Generation Telescopes

Keck Telescope

• 1993 – Keck telescope 1000cm mirror, made of smaller segments

• Photographic plates were more sensitive and permitted a permanent record of observations– Photographic plates have

since been replaced by electronic imaging devices

A large mirror sags in A large mirror sags in the middle. To prevent the middle. To prevent

this:this:1.1. Mirrors can be Mirrors can be

made very made very thick thick but they but they are are very heavy very heavy and very costly.and very costly.

A large mirror sags in A large mirror sags in the middle. To prevent the middle. To prevent

this:this:2.2. Spincasting – an oven Spincasting – an oven

turns and molten glass turns and molten glass flows outward in a mold to flows outward in a mold to form a form a concave concave upper upper surface.surface.

A large mirror sags in A large mirror sags in the middle. To prevent the middle. To prevent

this:this:3.3. A mirror can A mirror can

be made inbe made in

segments.segments.

A large mirror sags in A large mirror sags in the middle. To prevent the middle. To prevent

this:this:4.4. Thin mirrors (Thin mirrors (floppy floppy

mirrors) can have mirrors) can have their shape their shape controlled by a controlled by a computer – called computer – called active active optics. They optics. They cool quickly to cool quickly to adjust to adjust to surrounding surrounding temperatures.temperatures.

Radio Telescopes

• Objects in space emit light waves of many different wavelengths.

• Radio Telescopes receive very long wavelengths (radio waves).

• 1937 – first Radio telescope; picks up long wave radio emissions from deep space

Handicaps to Radio Telescopes

1.1. Poor resolutionPoor resolutionTo improve resolution, To improve resolution,

two or more radio two or more radio telescopes can be telescopes can be combined to improve combined to improve the resolving power the resolving power (called a radio (called a radio interferometerinterferometer). ). Resolving power Resolving power equals the equals the separation separation of the telescopes.of the telescopes.

Handicaps to Radio Telescopes

2.2. Low intensityLow intensity

IIn order to get strong n order to get strong signals focused on signals focused on the antenna, the the antenna, the radio astronomer radio astronomer must build large must build large collecting dishes. collecting dishes. TheThe largest dish is largest dish is the the 300 m 300 m dish at dish at Arecibo, Arecibo, Puerto Puerto Rico.Rico.

Handicaps to Radio Telescopes

3.3. InterferenceInterferenceThis occurs This occurs

because of because of poorly designed poorly designed transmitters in transmitters in Earth satellites Earth satellites to automobiles to automobiles with faulty with faulty ignition systemsignition systems..

Space Telescopes• HubbleHubble

• SpitzerSpitzer

• KeplerKepler

• WebbWebb

• ChandraChandra

• HershelHershel

• PlanckPlanck

• FermiFermi

• XMM-NewtonXMM-Newton