Why Use a Telescope?

• All astronomical objects are distant so a

telescope is needed to

– Gather light -- telescopes sometimes referred to

as “light buckets”

– Resolve detail

– Magnify an image (least important of the three)

• Telescopes accomplish these by using a

combination of lenses and/or mirrors

1

Refracting Telescope

• Refraction = as light passes from one medium

to another (e.g. air to glass) it is bent

• Light is gathered and focused by a curved lens.

2

Refracting Telescope

• First telescopes were of this type

• No longer used for astronomical research

– Very difficult to make large, defect-free lenses

– Weight of large lenses make them deform over

time.

– Lenses exhibit chromatic aberration and

spherical aberration

3

Reflecting Telescope

• A curved mirror is used to collect and focus

the light.

4

Reflecting Telescope

• Used in modern telescopes

– Mirror can be supported from the back, allowing

the mirror to be much less massive than a

comparable lens

– Do not need large, defect free glass because

surface is coated with reflective material.

– Mirrors can be constructed in a parabolic shape,

minimizing spherical aberration

– Mirrors do no exhibit chromatic aberration

5

Telescope

Designs

• Distance from lens or

mirror to focus =

focal length.

• Objective produces

an image at the focus

• An eyepiece (usually

a lens) is used to

magnify the image.

6

Reflecting Telescope Designs• For a reflecting telescope, a secondary mirror is used to

reflect the image to a detector outside of the telescope.

Prime Focus Newtonian Cassegrain

7

Eyepiece and Magnification

focal length of objective

focal lengtMagnificat

h of eyepion

iece=

8

Animation

Telescope Properties: Gathering Light

• The larger the area of

the primary mirror, the

more light can be

collected and the fainter

the object we can

detect.

2Light Gathering P Diameower ter

9

Gathering Light• Can combine many

smaller mirrors to make

one large area

– Keck telescope – primary

mirror made of 36

hexagonal mirrors

10

Keck Observatory

Keck Observatory

Comparing

Telescopes

11

Telescope Properties:

Angular Resolution• The smallest separation in angle which can

be observed by the telescope

12

Animation

Angular Resolution

• Absolute limit of angular resolution:

D

5105.2 =

θ = best angular resolution in

arcseconds

= wavelength (meters)

D = diameter of mirror (meters)

• Small (the minimum resolvable angle) means

good resolution

• Smaller wavelengths = better resolution

• Larger mirror = better resolution

13

Angular Resolution

• Resolution usually limited by motions in the

atmosphere (“twinkling”)

– Need site with calm, dry weather, little

atmosphere above the telescope to reduce effect.

– Adaptive optics: sensors monitor distortions due

to atmosphere and correct shape of mirror 10-100

times per second

• Very Large Telescope (8.2 meters), many others

14

Adaptive Optics

Object viewed with

adaptive opticsObject viewed through

typical telescope

15

Angular Resolution16

Observing the Image• Astronomers rarely look through a research telescope

• Photographic film is now replaced by CCD detectors

• Detector is put at the focus to record a digital image.

• Data is processed by a computer

CCD Chip

17

900 megapixel CCD detector

for the Subaru 8.2-m telescope

LBT

18

The Large Binocular Telescope (LBT) is an optical telescope for astronomy located in southeastern Arizona, and is a part of

the Mount Graham International Observatory. The LBT is currently one of the world's most advanced optical telescopes;

using two 8.4-m (27-ft) wide mirrors, with a 14.4 m center-center separation, it has the same light gathering ability as an

11.8-m (39-ft) wide single circular telescope and resolution of a 22.8-m (75-ft) wide one.

Hubble Space Telescope19

Coming soon: The James

Webb Space Telescope

Animation of deployment

Full scale model at

SXSW 2013

Image credit

Spectroscopy

• Light coming though telescope is often separated

by a prism or a diffraction grating to produce a

spectrum of intensity vs. wavelength (color).

20

Observing at Other Wavelengths

• All telescopes discussed so far have been

optical (visible wavelengths) telescopes.

• Other wavelengths are very useful because

they often yield information that visible

light cannot

• Earth’s atmosphere blocks some

wavelengths of light

21

Atmospheric Opacity

• Opacity = percentage of light blocked by

atmosphere

– Low opacity: Optical and Radio

– Medium opacity: Infrared and UV

– High opacity: Gamma Rays, X-rays & some UV

22

Infrared Telescopes

• IR light can pass through dust

• Used to observe star formation, center of

galaxies, low T objects (i.e. planets)

• Telescope design much like optical telescope,

but with different detector.

• Best results if telescope is placed above much

of the atmosphere (Spitzer Space Telescope,

Herschel Space Observatory)

23

Spitzer Space Telescope24

Radio Telescopes

• Can observe day or night

• Not affected by Earth’s atmosphere

• Radio light can pass though dust in space

• Because wavelength is long, we need large

telescope to get good resolution

• 21-cm wavelength allows astronomers to

map the hydrogen concentration

25

64 m telescope at Parkes

Observatory in Australia

305 m telescope at Arecibo

Observatory in Puerto Rico26

Interferometers• More than one radio telescope is used to increase

resolution.

– Large effective diameter

– Image made only after much computer processing

Very Large

Array in New

Mexico

27

Ultraviolet, X-rays and Gamma Rays

• All blocked by atmosphere

• Telescopes must be above atmosphere

Chandra X-ray Telescope

28

Sky at Many

Wavelengths

29

Figure 5.36 Chaisson/McMillan Astronomy Today, 7th ed., (Pearson Education, 2011)