Optics and TelescopesKathy Cooksey

Acknowledgements Tyler Nordgren & Julie Rathbun University of RedlandsJohnny B. Holmes Christian Brothers University

Outline• Optics and telescopes• Understand different

types• Why does size matter?• Why put them in

space?

Refraction & Lens-users Eq.

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as o approaches infinityi approaches f

Refraction & Lens-users Eq.

f f

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o > 0 AND o > f

i > 0 AND i > f

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€

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Refraction & Lens-users Eq.

f f

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as o gets smaller,i gets larger

€

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= 1o

+ 1i

Refraction & Lens-users Eq.

f f

o i

as o approaches f,i approaches infinity

€

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Refracting Telescope

objective lens

eyepiece

fo fe

Two lenses: How does it work?

Refracting Telescope

objective lens

eyepiece

fo fe

Two lenses:

Model Optical Rail

• http://www.school-for-champions.com/science/experiments/simopticsmirror.htm

• Magnification• Image Inversion• Multiple optics

– Image re-inversion ;)

Purpose of Telescope

1. Increases amount of light we see:Sensitivity is proportional to collecting area.If one telescope is 3 times bigger than another, then it can see 32 = 9 times fainter objects.

2. Increases detail (resolution) we see:Resolution is inversely proportional to telescope diameter.If one telescope is 3 times bigger than another, then it can see 3 times smaller angles (3 times smaller objects or detail).

Andromeda Galaxy Copyright by AURA

Sensitivity

• Telescope size doubled from left to right

Resolution

Andromeda Galaxy Copyright by AURA

(a) 10’(b) 1’ (10 times bigger than a)(c) 5” (12 times bigger than b)(d) 1” (5 times bigger than c)Human eye: 0.5’ in visual range

Side Note: Exposure Time

• Point telescope, open shutter, and expose for a long time (integration)

• Gather more photons and improve image

• Human eye has limited integration time

Telescopes & Light

• Collect “light”– Visible light is just part of whole spectrum of “light”

• Light is all astronomers have!!!• Electromagnetic spectrum (EM): whole range of

varieties of “light”:– Example: visible light comes from lamp in your house

– Example: radio waves come from radio station

– Other types of radiation: microwaves, infrared and ultraviolet light, X-rays, and gamma-rays

EM

Rad

iati

on

Radio Optical and infrared

X-rayUVγ-ray

Types of Telescopes

Refracting Telescope

objectivelens

eyepiece

fo fe

Two lenses (as we made)

Optical

Yerkes 40-in Refractor

All modern astronomy telescopes are reflectors

Use mirrors for primary optic

Reflecting Telescope

Light from far away mirror

focuses

light

Problem: how do we get to focused light without blocking incoming light?

Reflecting Telescope

Light from far away mirror

focuses

eyepiece light

Solution #1: If mirror is big enough (say100 to 200 inches in diameter), we can sit right in middle and won’t block much light - this is called prime focus.

Radio

Arecibo 300-ft Reflector

Reflecting Telescope

Light from far away eyepiece mirror

focuses

light

Solution #2: Use secondary mirror to reflect light off to side of telescope - this is called Newtonian focus.

Reflecting Telescope

Light from far away primary mirror

focuses

light

eyepiece

Solution #3: Use secondary mirror to reflect light out back of telescope - this is called Cassegrain focus.

Optical

Hooker 100-in Reflector

Some Large Optical TelescopesAperture(meters)

Name Location Comments

Keck mirror composed of 36 segments10.0

Keck II

Mauna Kea,Hawaii

future optical interferometry!

9.2 Hobby-EberlyMt. Fowlkes,Texas

very inexpensive: sphericalsegmented mirror; fixedelevation; spectroscopy only

8.3 SubaruMauna Kea,Hawaii

NAOJ

Antu

Kueyen

Melipal8.2

Yepun

Cerro Paranal,Chile

now operate independently in thefuture will be units of Very LargeTelescope

Gemini NorthMauna Kea,Hawaii

8.0

Gemini SouthCerro Pachon,Chile

twins

3.0 ShaneMountHamilton,California

Lick Observatory

Loca

tion

, lo

cati

on

, lo

cati

on

Optical & IR

Keck Twin 10-m

Reflector

Keck Primary Mirrors

• Large!

– Formed by 36 individual

hexagonal segments

• All must be kept positioned relative to their neighbors– To accuracy of four

nanometers, or 1,000 times

thinner than human hair

Lick Observatory

Shane 120-in

Nickel 40-in

Lick Observatory

Adaptive Optics Teaser

Atmospheric Seeing

Ground - KPNO 4.0m Copyright NOAO /AURA/NSF Space - HST – 1.0m

Hubble Space Telescope

Summary• Refracting telescope: primary lens• Reflecting telescope: primary mirror

– All modern telescopes

• Sensitivity: amount of light– Proportional to collecting area

• Resolution: detail– Inversely proportional to telescope

diameter

• Bigger is better• Space is atmosphere free