TELESCOPIC ASTRONOMY

First Telescope Hans Lippershey. Dutch lens grinder 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 James Gregory designed a telescope with a large concave primary mirror and a smaller concave secondary mirror Newton found that a prism breaks up white light into a rainbow of colours –Telescope lenses do the same Newton modified the design, and it won huge acclaim

Newton’s Telescope

Optical Telescopes Refracting telescope uses a large lens to gather and focus light. Reflecting telescope uses a concave 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 length – the distance from the lens or mirror to the image formed of a distant light source

Primary lens: the main lens in a refracting telescope. It is also called an objective lens.

Primary mirror: the main mirror in a reflecting telescope. It is also called an objective mirror.

Eyepiece: A small lens to magnify the image produced by the objective (primary) lens

Lens and Mirror Activity STATION 1: Making Rainbows Use the overhead as a light source Use 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.Light-gathering power: The ability of a telescope to collect light.

Powers of a Telescope 2.Resolving Power – the ability of a telescope to reveal fine detail.

Resolving Power (con’t) When light is focused into an image, a blurred fringe surrounds the image (diffraction fringe).When light is focused into an image, a blurred fringe surrounds the image (diffraction fringe). We can never see any detail smaller than the fringe.We can never see any detail smaller than the fringe. Large diameter telescopes have small fringes and we can see smaller details. Therefore the larger the telescope, the better its resolving power.Large diameter telescopes have small fringes and we can see smaller details. Therefore the larger the telescope, the better its resolving power.

Resolving Power (con’t) Optical quality and atmospheric conditions limit the detail we can see.Optical quality and atmospheric conditions limit the detail we can see. Seeing is the blurred image caused by unsteady, turbulent atmosphere. A star near the horizon will twinkle more than an overhead star. Most telescopes are built on high mountains.Seeing is the blurred image caused by unsteady, turbulent atmosphere. A star near the horizon will twinkle more than an overhead star. Most telescopes are built on high mountains.

Powers of a Telescope 3.Magnifying power – the ability to make the image bigger

Magnifying power con’t Magnification of a telescope can be changed by changing the eyepiece. We cannot alter the telescope’s light-gathering or resolving power.

Astronomers identify telescopes by diameter because that determines both light-gathering power and resolving power.

Observatories are built on top of mountaintops because: 1. Air is thin and more transparent 2. The sky is dark 3. Stars are brighter 4. Wind blows smoothly over some mountain tops 5. There is less pollution

Refracting Telescopes have limitations…

1. Chromatic Aberration When light is refracted through glass, shorter wavelengths bend more than longer wavelengths, and blue light comes to a focus closer to the lens than does red light.

1. Chromatic Aberration con’t If we focus on the blue image, the red image is out of focus and we see a red blur around the image. This color separation is called chromatic aberration.

SOLUTION…

Achromatic Lens An achromatic lens is made of two components made of different kinds of glass and brings the two different wavelengths to the same focus. Other wavelengths are still out of 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…

Reflecting Telescope Newton found that a prism breaks up white light into a rainbow of colours –Telescope lenses do the same James Gregory designed a telescope with a large concave primary mirror and a smaller concave secondary mirror Newton modified the design, and it won huge acclaim

Benefits of Reflecting Telescopes Less expensive.Less expensive. –Only the front surface of the mirror must be ground. –The glass doesn’t need to be perfectly transparent The mirror can be supported over its back surface to reduce sagging.The mirror can be supported over its back surface to reduce sagging. They do not suffer from chromatic aberration because the light is reflected toward the focus before it can enter the glass.They do not suffer from chromatic aberration because the light is reflected toward the focus before it can enter the glass.

Four ways to look through reflecting telescopes

Prime Focus

Newtonian Focus

Cassegrain Focus

Schmidt-Cassegrain

TELESCOPES DAY 2

Hershel’s Telescope Late 1770s, William Hershel was making the best metallic mirrors and telescopes in the world Discovered Uranus Built a giant telescope which he used with his sister Caroline –His telescope had a 125cm mirror –40ft in length

Rosse’s Telescope Earl of Rosse, Ireland, taught himself mirror-making and built a 91cm telescope attempted to build a 181cm telescope but it broke when moved –built another one that couldn’t be moved

Observatories cm telescope and observatory at University of California (James Lick) cm telescope in France cm telescope at Yerkes Observatory in Wisconsin cm telescope on Mount Wilson California (George Ellery Hale) cm telescope also built on Mount Wilson (John D. Hooker) cm 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 the middle. To prevent this: 1.Mirrors can be made very thick but they are very heavy and very costly.

A large mirror sags in the middle. To prevent this: 2.Spincasting – an oven turns and molten glass flows outward in a mold to form a concave upper surface.

A large mirror sags in the middle. To prevent this: 3.A mirror can be made in segments.

A large mirror sags in the middle. To prevent this: 4.Thin mirrors (floppy mirrors) can have their shape controlled by a computer – called active optics. They cool quickly to adjust to surrounding temperatures.

Radio Telescopes Objects in space emit light waves of many different wavelengths. Radio Telescopes receive very long wavelengths (radio waves) – first Radio telescope; picks up long wave radio emissions from deep space

Handicaps to Radio Telescopes 1.Poor resolution To improve resolution, two or more radio telescopes can be combined to improve the resolving power (called a radio interferometer). Resolving power equals the separation of the telescopes.

Handicaps to Radio Telescopes 2.Low intensity In order to get strong signals focused on the antenna, the radio astronomer must build large collecting dishes. The largest dish is the 300 m dish at Arecibo, Puerto Rico.

Handicaps to Radio Telescopes 3.Interference This occurs because of poorly designed transmitters in Earth satellites to automobiles with faulty ignition systems.

Space Telescopes HubbleHubble SpitzerSpitzer KeplerKepler WebbWebb ChandraChandra HershelHershel PlanckPlanck FermiFermi XMM-NewtonXMM-Newton