TELESCOPIC ASTRONOMY

What is a telescope?

•

What are telescopes used for?

• View distant objects

• Collect light

First Telescope

• 1608- Hans Lippershey• Hans Lippershey was a Dutch lens maker.

• 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

Galileo’s Telescope

Optical Telescopes

Optical telescopes use a lens Optical telescopes use a lens and light.and light.

One type: Refracting telescopeOne type: Refracting telescopeuses two largeuses two large

lenses lenses to gather and focus light.to gather and focus light.

Primary lens:Primary lens:the main lens the main lens

in a in a refractingrefracting

telescope. It telescope. It is also called is also called

ananobjectiveobjective

lens.lens.

Eyepiece:Eyepiece:A small lens A small lens

to magnify to magnify the image the image produced produced

by theby theobjective objective (primary)(primary)

lenslens

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

Pros

• Simple design

• Minimal maintenance

• Good for planets and moons

• Good for photography

• High contrast

Cons

• Costly

• Bulky and large (large focal length)

• Chromatic aberration

Refraction Limitation

When light is When light is refracted through refracted through glass, shorter glass, shorter wavelengths bend wavelengths bend more more than longerthan longer wavelengths, and wavelengths, and blue light comes to blue light comes to a focus closer to a focus closer to thethe lenslens than does than does red light. red light.

Refraction Limitation

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

An achromatic An achromatic lenslens is made of two is made of two components made of components made of different kinds of different kinds of glass glass and brings the and brings the two different two different wavelengths to the wavelengths to the same same focus. focus. Other Other wavelengths are still wavelengths are still out ofout of focus.focus.

Do we still use these types of telescopes??

Yerkes Refracting Telescope• Largest refracting

telescope in the world is at Yerkes Observatory in Wisconsin

• Lens is 1m in diameter

• ½ tonne• The glass sags under

its own weight

Reflecting Telescope

• 1666- Newton found that a prism breaks up white light into a rainbow of colours

• Telescope lenses do the same– Creates haloes of coloured light around

objects being viewed

Newton’s Telescope

Primary mirror:Primary mirror:the main mirror in athe main mirror in areflecting reflecting telescope. It telescope. It

is larger than the is larger than the secondary mirror. It secondary mirror. It is also called an is also called an objective objective mirror.mirror.

• NOTE: The primary NOTE: The primary mirror is in a mirror is in a different spot than different spot than the primary lens of a the primary lens of a refracting telescope.refracting telescope.

Secondary Secondary mirror:mirror:

the smaller the smaller mirror in amirror in areflectingreflecting

telescope. It telescope. It directs the directs the

light through a light through a small hole in small hole in the primary the primary mirror to the mirror to the eyepiece.eyepiece.

Benefits of Reflecting Telescopes

Less expensive. Only the front surface of Less expensive. Only the front surface of the mirror must bethe mirror must be groundground..

The mirror can be supported to reduce The mirror can be supported to reduce sagging. sagging.

They do not suffer from chromatic aberration They do not suffer from chromatic aberration because the light is because the light is reflected reflected toward the toward the focus before it can enter the glass. focus before it can enter the glass.

Cons of Reflecting Telescopes

• More maintenance

William Hershel (1757-1822)

• By late 1770s, he was making the best metallic mirrors and telescopes in the world.

• 1781- Discovered Uranus

• His telescope had a 125cm mirror and was 40ft in length

Hershel’s Telescope

Large Telescopes

• Up until mid-1800s telescopes were generally small and not very powerful

• 1838- Earl of Rosse, Ireland, taught himself art of 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

Rosse’s Telescope

Rosse is important because he built the largest telescope the world had ever seen and he saw further into space than anyone had before him.

Cassegrain Telescope

• Specific type of reflecting telescope

• a wide-angle reflecting telescope

• the eyepiece or camera is mounted at the back end of the tube

• developed in 1672

Schmidt-Cassegrain Telescope

• Specific type of reflecting telescope

• a wide-angle reflecting telescope (Cassegrain telescope) with a correcting lens that minimizes spherical aberration

• The correcting plate (a lens) was added in 1930 by the Estonian astronomer and lens-maker Bernard Schmidt (1879-1935).

New Generation Telescopes

• Refraction and Reflection telescopes are OPTICAL telescopes (uses light)

• New generation telescopes are RADIO telescopes (uses waves)

• Radio telescopes use dishes to pick up radio waves. Sensors on the dishes collect the waves and turn them into a picture.

Radio Telescopes

New Generation Telescopes

• 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

Keck Telescope

Handicaps to Radio Telescopes

1) Poor resolution: To improve resolution, two or more radio telescopes can be combined to improve the resolving power

Handicaps to Radio Telescopes

2.2. Low intensityLow intensityIIn 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

• Interference: This occurs because of poorly designed transmitters in Earth satellites to automobiles with faulty ignition systems.

Powers of a Telescope

1.1. Light-Light-gathering gathering

power-power-This is the ability of This is the ability of

a telescope to a telescope to collectcollect

light.light.The larger the The larger the

telescope’s telescope’s lens, the more lens, the more

light it can light it can gather.gather.

Powers of a Telescope2.2. Resolving Power –Resolving Power –the ability of a telescope the ability of a telescope

to reveal fine detail. to reveal fine detail.

• The larger the The larger the telescope, the better telescope, the better its resolving power. its resolving power. However, optical However, optical quality of the lens and quality of the lens and atmospheric atmospheric conditions limit the conditions limit the detail we can see.detail we can see.

Powers of a Telescope

Magnifying power –Magnifying power –

the ability to make the the ability to make the image image bigger. bigger.

Magnification of a Magnification of a telescope can be telescope can be changed by changing changed by changing the eyepiece.the eyepiece.

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.

Telescopes and Observatories

• An observatory is a place for viewing the sky through telescopes.

• Observatories can be small with just a single telescope outside or they can be large buildings housing a number of telescopes, each in their own room.

Hale Observatory, California

• The traditional image of an observatory is probably that of a large building with domed rooms housing telescopes. But an observatory can be just a single room with a telescope, or nothing more than an open area with telescopes.

Observatories are built on top of mountains because:

1)1) air is thin and moreair is thin and more transparenttransparent

2)2) the sky isthe sky is darkerdarker

3)3) stars arestars are brighterbrighter

4) wind blows4) wind blows smoothly smoothly over some over some mountaintopsmountaintops

5)5) there is lessthere is less pollutionpollution

Telescopes and the Scientific Method

The Scientific Method

The Scientific Method’s 4 Steps

1) Observation and description of a phenomenon.

The observations are made visually or with the aid of scientific equipment (like a telescope).

2) Formulation of a hypothesis to explain the phenomenon (usually in the form of math or a causal mechanism or a mathematical relation.

• Test the hypothesis by analyzing the results of observations or by predicting and observing the existence of new phenomena that follow from the hypothesis. If experiments do not confirm the hypothesis, the hypothesis must be rejected or modified (Go back to Step 2).

• Establish a theory based on repeated verification of the results.