Astronomical Tools

Essential Questions

1. What is Light?

2. How do telescopes work, and how are they limited?

3. What kind of instruments do astronomers use to record and analyze light?

4. Why do astronomers use radio telescopes?

5. Why must some telescopes go into orbit?

A particle and a wave…

• Electromagnetic radiation comes in many types but all have the same speed.

• Speed = frequency x wavelength (c = f x λ)

• Particles of light are called photons.

Light• Light is the visible form of

electromagnetic radiation.• It’s an electric and magnetic disturbance

that transports energy at the speed of light.

• The EM Spectrum includes: Radio, Microwave, Infrared, Visible, Ultraviolet, X-Rays, and Gamma Rays.

• A photon, or particle of light, can be thought of as a bundle of waves that sometimes acts like a wave and sometimes like a particle.

Visible Light• The visible spectrum

ranges from 700 nm red light to 400 nm violet light.

• ROY G BIV• As wavelengths

decrease, energy increases.

• As frequency increases so does energy.

Try a few calculations…• Find the frequency of red light (λ = 700 x 10-9 m)

• Find the frequency of violet light (λ = 400 x 10-9 m)

• Find the wavelength of your favorite radio station… (remember M = mega or 106)

Telescopes

• Earth based or space based, telescopes are designed to do one thing very well…

• Gather and focus as much light as possible.

Bigger is Better• Telescopes allow

astronomers to gather light, see fine detail, and magnify images.

• The first two depend on the telescope’s diameter or aperture.

• Consequently, astronomers always strive for larger scopes.

Reflectors use Mirrors

Refractors use Lenses

Both reflectors and refractors use lenses in the eyepiece.

Refracting Telescopes• A refracting telescope uses a lens to

gather and focus light.• Because of chromatic aberration,

refractors cannot bring all colors to focus at the same spot—color fringes result.

• An achromatic lens partially corrects for this, but such lenses are expensive.

• Refracting lenses cannot be made larger than 1 meter—they warp if too heavy.

Chromatic Aberration

Yerkes Refractor• The world’s largest

refracting telescope is located in Wisconsin.

• The diameter of the lens is 40 inches.

• Any more than this, and the lens will warp and change shape due to its own weight.

Reflecting Telescopes• Use a mirror to gather and focus light.• Less expensive than refractors of the

same diameter.• Do not suffer from chromatic

aberration.• Most recently built large telescopes are

reflectors.• The mirror can be supported from

below, so there is no limit to size (except for $).

The Keck Observatory• The world’s largest

reflecting telescopes are the twin Keck reflectors in Hawaii.

• Each scope has a 10 meter diameter mirror!

• Actually a group of segmented mirrors put together.

Types of Reflectors

Types of Reflectors• Prime focus—instruments/eyepiece on the

light gathering end of scope.• Newtonian Focus—instruments/eyepiece

on side of scope, a secondary mirror bounces light out the side.

• Cassegrain Focus—instruments/eyepiece behind the primary mirror, light enters through a hole in the primary.

Observatories on Mountains

WIRO 2004

Telescopes High and Dry

• Astronomers build observatories on mountains for two reasons:

• Better “seeing” due to less turbulence in the atmosphere.

• Air is thinner and dryer and is therefore more transparent especially to infrared.

• After all, WIRO—Wyoming Infrared Observatory

Advances in Optical Telescopes

• Sometimes telescopes are linked through a technique called interferometry.

• Resolution is equal to the separation of the two scopes.

• Adaptive optics have also been used to compensate for atmospheric distortion.

Modern Instruments• For many years astronomers used

photographic plates to record images at the telescope.

• Modern electronic systems such as CCDs (advanced digital camera chips) have replaced most photographic plates.

WIRO Camera and Control

Spectrographs

• Spectrographs using prisms or a grating spread starlight out according to wavelength.

• This reveals a spectrum showing hundreds of spectral lines produced by atoms in the object being studied.

Radio Astronomy

• Astronomers use radio telescopes for three reasons:

1. Can detect cool hydrogen

2. Can see through dust clouds

3. Can detect objects invisible at other wavelengths

Radio astronomy details• Radio telescopes can be used during the

day• Consist of a dish reflector, an antenna, an

amplifier and a data recorder.• The largest single, fully-steerable, radio

telescope is in Green Bank, WV. (100 m)• The largest single radio telescope is

located in Puerto Rico and at Arecibo. (300 m) Maybe you’ve seen it in a 007 movie…

Arecibo Radio Telescope

Radio Interferometry

• Radio telescopes have poor resolution due to the long wavelength of radio waves.

• Often linked to get better resolution.• The VLA in New Mexico can be 30 km across!

Atmospheric Windows

Our troublesome atmosphere• Although our atmosphere is nice to

breathe and shields us from many forms of nasty radiation and meteors…

• It only allows us to use ground based telescopes in the visible, radio/microwave, and some infrared parts of the spectrum.

• For the other wavelengths, we must go into space…

Space Based Telescopes• Earth’s atmosphere

absorbs some radiation and distorts light that does get through.

• Telescopes in orbit avoid this and are limited only by their size.

• Some have proposed lunar observatories!

Multi-wavelength CrabThe Crab Nebula in Radio

The Crab Nebula in Infrared

The Crab Nebula in Visible

Visible Again

The Crab Nebula in Ultraviolet

The Crab Nebula in X-Ray