observational astronomy 6

7/28/2019 Observational Astronomy 6

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19-Apr-13

IESO

Observational Astronomy

Part 6


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Light (Electromagnetic Spectrum) and

Telescopes[week 2 and 3]


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A Telescope is a tool used

to gather light from objects

in the universe


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Our Objective

Treat the telescopes as an instrument

Learn telescope parameters

What makes a telescope useful?

Telescope operation

Different telescope types

OBJECTIVES


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There are two different types of

telescopes A refracting telescope uses aglass lens to concentrate

incoming light

A reflecting telescope uses mirrors to concentrateincoming starlight


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Telescope Optics


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The Refractor

eyepiece

Objective (lens)

2.4 Amateur Refractor 40 Refractor

Common as smalltelescopes

$$$ in large apertures

Superb image quality


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The Newtonian Reflector

6 Amateur Newtonian

Common as amateur

telescopes

Lower cost

Simple optical design

Good image quality

Central obstruction


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The Cassegrain Reflector

Large f-number in small package

F/10 in a 24 long tube

Good imagery for large f/#

Design used in large telescopes


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The Schmidt Cassegrain

Large f-number in small package

F/10 in a 24 long tube

Good imagery for large f/#

Better spherical aberration control

Spherical

primary

mirror

Corrector plate 8 Schmidt Cassegrain


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Chromatic Aberration

The Problem


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Chromatic Aberration

The Solution


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Spherical Aberration


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Spherical Aberration


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A ____________ telescope uses a

lens to concentrate incoming light


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Reflecting telescopes use mirrors to

concentrate incoming starlight


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a e escope

Most important!! Light Gathering Power:bigger aperture is better

making objects appear brighter

followed by

Resolving Power:to see fine detail

and least important,

Magnifying Power:

magnification = M

2

4LGP Area d

( ) ( )(in arcsec) = .2516

d (mm) ( )

nm wavelength nmRP

diameter mm

o

e

fM

f


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SENSITIVITY

HOW MUCH LIGHT CAN THE TELESCOPE GATHER

DEPENDS ON THE

-APERTURE-

SIZE OF THE MIRROR OR

LENS


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g a er ng ower The Power of a Telescope

eye

Eye behind telescope


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Telescope Resolving Power

Star

Double Star


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DIFFRACTION


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RESOLUTION AND THE AIRY DISC


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RESOLUTION

= 4.56 / D

is the separation in arc seconds

D is the diameter of lens/mirror

in inches


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A larger

objective lensprovides a

brighter (not

bigger) image


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Magnification

Magnification = Telescope focal length eyepiece focal length

2000 mm 76 mm = 78 X 2000 mm 10 mm = 200 X

2000 mm 1 mm = 2000X

Maximum useful magnification:

- 60X per 1 of aperturePractical magnification depends on

- Optics and seeing


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The Job of a TelescopeSee faint objects - Light gathering power

See detail on objects - Resolving power

Magnify otherwise small objects - Magnification


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Reflecting telescopes use mirrors to

concentrate incoming starlight

Advantages and Disadvantages of


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Advantages and Disadvantages of

Various Telescopes

See One-Minute Astronomer worksheet and notes

below previous slide.


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If you pass white light through a prism, it

separates into its component colors.

R

O

Y

G

B

I

V

spectrum

long wavelengths

short wavelengths


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But visible light is only one

type of electromagnetic

radiation (light) emitted by stars

Astronomers are trulyinterested in the entire

spectrum of Light!


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C id Thi Cl S i Diff t W l th f Li ht!


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Consider This Class as Seen in Different Wavelengths of Light!

Consider Orion as Seen in Different Wavelengths of Light!


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Consider Orion as Seen in Different Wavelengths of Light!


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Observations at other wavelengths are

revealing previously invisible sights

UV

Ordinary

visible

infrared

Map of

Orion

region


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Hubble Space Telescope Views of Orion Nebula showing stars hidden in clouds

http://oposite.stsci.edu/pubinfo/pr/97/13/A.html


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TODAYS Sun as seen in visible light from Earth and

from space in X-rays by satellites

Radio wavelength observations are


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Radio wavelength observations are

possible from Earths surface


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The Very Large Array (VLA) in New Mexico

One such array is called the Very Large Baseline Array (VLBA): it consists of ten radio

telescopes which reach all the way from Hawaii to Puerto Rico: nearly a third of the way

around the world! By putting a radio telescope in orbitaround the Earth, radio astronomerscould make images as if they had a radio telescope the size of the entire planet!


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Differences in the temperature and density of small portions of Earths

atmosphere cause passing starlight to quickly change direction, making

stars appear to twinkle.

Why do some stars, sky objects appear to twinkle?


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Earths atmosphere hinders astronomical researchImage of stars taken

with a telescope onthe Earths surface

Same picture taken with

Hubble Space Telescopehigh above Earths blurring

atmosphere


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High above

Earths

atmosphere,

the HubbleSpace

Telescope

providesstunning

details about

the universe


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Hubble

orbits theEarth at an

altitude of

about 353

miles and in97 minutes.


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But visible light is only one

type of electromagnetic

radiation (light) emitted by stars

Astronomers are trulyinterested in the entire

spectrum of Light!


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Astronomers use different instruments to look at

light of different wavelengths - sometimes, we even

have to go above Earths atmosphere.

Not all EM radiation can penetrate Earths atmosphere


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Not all EM radiation can penetrate Earth s atmosphere.


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Which is the correct reasoning for why a gamma ray

telescope located in Antarctica that is to be used to look for

evidence of black holes in the centers of galaxies would not

get funded?

A. There is no way to detect the presence of a

black hole.

B. Gamma rays are too energetic to detect with atelescope.

C. You cant build a functioning telescope in

Antarctica.

D. Gamma rays dont penetrate Earthsatmosphere.

th t h ti ht b d t f b ildi


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that has a very tight budget for building a

telescope. Which of the three proposed

telescopes below would be best to support?

A. A gamma ray telescope in Antarctica

B. A radio telescope in orbit above the Earth

C. A visible telescope located high on a

mountain in Peru

D. An ultraviolet telescope located in the

Mojave desert


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Poor Seeing:

Aberration introduced

by the Atmosphere

IdealAberrated


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ACTIVE OPTICS

ADAPTIVE OPTICS - CORRECTS ATMOSPHERIC EFFECTS


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ALTAZIMTH MOUNT


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EQUATORIAL MOUNT


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Telescope Field of View

Field of View = Eyepiece apparent FOV Magnification

Large Magnification = Small field of view

Small Magnification = Large Field of view

Moon=0.5 deg.


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Plane Mirror

Normal:

Angle of Incidence

Angle of Reflectioni

r

The Law of Reflection:

normal

i r

i

r

Concave & Convex Mirrors


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Focal Length:

The radius of curvature of the

mirror is twice the

focal length.

Concave & Convex Mirrors

Radius of Curvature:

Diverging (of light):

Converging (of light):

Concave

Convex


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Snells Law

[The Law of Refraction]

Normal:

Angle of Incidence:

Angle of Refraction:

Refraction: The bending of light that occurs when it is incident on a

plane surface & is going from one medium to another medium. Why

does this happen?

ANSWER: Because the speed of light changes in various mediums!


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Snells Law (carpet & hardwood analogy) If the light slows down, what happens to its wavelength, frequency and color? (do

they stay constant, increase or decrease)

Recall .

Different colors of light have different speeds and thus bend different amounts. (n =v/c)

If light is going from a less dense medium to a more dense one, it bends_____________ the normal.

v f

If light is going from a more dense medium to a less dense one, it bends_____________ from the normal.

Be sure to observe Snells Law in lab today as it explains why the

lenses bend light or refract light.

Diverging & Converging Lenses


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Focal Length:

The radius of curvature of the

mirror is twice the

focal length.

Diverging & Converging Lenses

Radius of Curvature:


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Refractor and Reflector

Lens Mirror

Astrology: The belief that the


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Astrology: The belief that the

positions of the stars and planets as

seen from Earth impact humanevents.

observational astronomy 6

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