Astronomical Tools Optics

Telescope Design

Optical Telescopes

Radio Telescopes

Infrared Telescopes

X Ray Telescopes

Gamma Ray Telescopes

Laws of Refraction and Reflection

Law of Refraction

n1 sin θ1 = n2 sin θ2

where

n = c/v

Law of Reflection

θ1 = θ2

Lenses and Mirrors

A focusing lens can be

designed using the law of

refraction

A focusing mirror can be

designed using the law of

reflection

Refracting / Reflecting Lenses A lens can

focus an image

on a plane. A

source at

infinity focuses

on the focal

plane.

A concave

mirror can

focus an image

on a plane. A

source at

infinity focuses

on the focal

plane.

Focal length

Focal length

The Focal Length

Focal length = distance from the center of the lens to

the plane onto which parallel light is focused.

Telescope Design Reflecting and Refracting Telescopes

Newtonian

Galilean

Secondary Optics

In reflecting

telescopes:

Secondary

mirror, to re-

direct light

path towards

back or side of

incoming light

path.

Eyepiece: To

view and

enlarge the

small image

produced in

the focal

plane of the

primary

optics.

Galilean Cassegrainian

Disadvantages of

Refracting Telescopes Chromatic aberration:

Different wavelengths are

focused at different focal

lengths (prism effect).

Can be improved, but not

eliminated by a second

lens out of different

material.

Difficult and expensive to

produce: All surfaces

must be perfectly shaped;

glass must be flawless;

lens can only be

supported at the edges.

Reflectors Most research telescopes are reflectors.

Types of reflecting telescopes

The Powers of a Telescope: Bigger is better

1. Light-gathering

power: Depends on

the surface area A of

the primary

lens/mirror, which is

proportional to the

diameter squared:

A = π (D/2)2

D

The Powers of a Telescope

2. Resolving power: Wave nature of light

the telescope aperture produces

fringe rings that set a limit to the

resolution of the telescope.

Astronomers cannot eliminate these

diffraction fringes, but the larger the

telescope diameter, the diffraction

fringes are smaller. Thus the larger

the telescope, the better its resolving

power.

αmin = 1.22 (λ/D)

For optical wavelengths, this gives

αmin ≈ 11.6 arcsec / D [cm] amin

Effect of improving resolution:

(a) 10′; (b) 1′; (c) 5″; (d) 1″

Resolving Power

Weather conditions and turbulence in the atmosphere

set further limits to the quality of astronomical images.

Atmospheric motion blurs the image.

Seeing

Bad seeing Good seeing

The Powers of a Telescope

3. Magnifying Power: ability of the telescope to

make the image appear bigger. Magnification is

usually changed by changing the focal length of

the eyepiece.

A larger magnification does not improve

the resolving power of the telescope!

Higher magnification is useful for extended bodies

such as the Sun, the Moon and planets—not stars,

which are seen as points of light.

The Best Location for a Telescope

Far away from civilization – to avoid light pollution

The Best Location for a Telescope

On high mountain-tops—to avoid atmospheric turbulence (i.e.

improve seeing) and other weather effects

Paranal Observatory (ESO), Chile

Traditional Telescopes

Traditional primary

mirror: sturdy, heavy

to avoid distortions.

Secondary mirror

Traditional

Telescopes

Mount Wilson Observatory

Hooker 100 inch reflector

Mount Palomar Observatory

Hale 200 inch (5.1 m) reflector

Traditional

Telescopes

Kitt Peak National Observatory

Mayall (4 m) Telescope Mount Palomar Observatory

Schmidt Camera (48 inch)

Traditional Telescopes

Hubble (2.4 m) Space Telescope

Advances in Modern

Telescope Design

Modern computer technology has made possible

significant advances in telescope design:

1. Simpler, stronger mountings (―alt-azimuth

mountings‖) to be controlled by computers

Advances in Modern Telescope Design

2. Lighter mirrors with lighter support structures,

to be controlled dynamically by computers

Floppy mirror

Segmented mirror

Prime Focus Cage

High-Resolution Astronomy

Adaptive optics: track atmospheric changes with

a laser, adjust mirrors in real time

Adaptive Optics

Computer-controlled mirror supports adjust the mirror

surface (many times per second) to compensate for

distortions by atmospheric turbulence

Interferometry

Recall: Resolving power of a telescope depends on diameter D.

Combine the signals

from several smaller

telescopes to

simulate one big

mirror

Interferometry

The amount of

radiation collected is

smaller, but the

improved resolution

is important.

Images and Detectors

Image acquisition: Photographic plates are being

replaced by charge-coupled devices (CCDs), which are

electronic devices that can be read out and reset quickly.

Smaller CCDs are used in digital cameras.

Radio Astronomy Recall: Radio waves of l ≈ 1 cm – 1 m also penetrate the

Earth’s atmosphere and can be observed from the ground.

Radio Telescopes

Large dish focuses the

energy of radio waves

onto a small receiver

(antenna)

Amplified signals are stored

in computers and converted

into images, spectra, etc.

Radio Maps

In radio maps, the intensity of the radiation is color-coded:

For example:

Red = high intensity

going to

Black = low intensity

Analogy: Seat prices in a baseball

stadium: Red = expensive going to

Purple = cheap.

Radio Astronomy

Largest radio telescope: 300-m dish at Arecibo

Radio Astronomy

Disadvantage: Longer wavelength means poor

angular resolution—hence astronomical

interferometry began in radio astronomy.

Advantages of radio astronomy:

• Can observe 24 hours

a day. Clouds, rain, and

snow don’t interfere

• Observations at a

different frequency give

different information

Radio Interferometry

The Very Large Array (VLA): 27 dish

antennae are combined to simulate a

large dish of as much as 36 km in

diameter.

Just as for optical

telescopes, the

resolving power of a

radio telescope

depends on the

diameter of the

objective lens or mirror

amin = 1.22 l/D.

For radio telescopes,

this is a big problem:

Radio waves are much

longer than visible light

Use interferometry to

improve resolution!

Science of Radio Astronomy

Radio astronomy reveals several features,

not visible at other wavelengths:

• Neutral hydrogen clouds (which don’t

emit any visible light), containing ~90 %

of all the atoms in the universe.

• Molecules (often located in dense

clouds, where visible light is

completely absorbed).

• Radio waves penetrate gas and

dust clouds, so we can observe

regions from which visible light is

heavily absorbed.

Infrared Astronomy Most infrared radiation is absorbed in the lower atmosphere.

However, from high

mountain tops or

high-flying aircraft,

infrared radiation

can be observed at

some wavelengths.

Infrared astronomy

is best done from

spacecraft.

NASA infrared telescope on Mauna Kea, Hawaii

Infrared Astronomy The Spitzer infrared telescope is in space

Infrared Astronomy Infrared observations of M81 at different wavelengths. The images

a, b, and c are colored blue, green and red respectively and

combined to give an artificial color image in d.

4 μm 8 μm

24 μm

Ultraviolet Astronomy

• Ultraviolet radiation with l <

290 nm is completely absorbed

in the ozone layer of the

atmosphere.

• Ultraviolet astronomy must be

done from spacecraft.

• Several successful ultraviolet

astronomy satellites: IRAS,

IUE, EUVE, FUSE

• Ultraviolet radiation traces hot

(tens of thousands of degrees),

moderately ionized gas in the

universe.

X Ray Astronomy

X rays and gamma rays cannot reflect off mirrors as other

wavelengths do.

X rays can undergo Bragg reflection at very shallow angles and

they can be focused in special telescopes.

X Ray Astronomy X-ray image of a supernova remnant

Gamma Ray Astronomy

Gamma rays cannot be focused at all; therefore

images are coarse.

Compton Gamma Ray Observatory (1991-2000) and an image

made by it.

Much can be

learned from

observing the

same

astronomical

object at many

wavelengths.

Here is the

Milky Way.

Full-Spectrum Coverage