Light and Telescopes

What do you think? What is the main purpose of a telescope? Why do stars twinkle?

If you pass white light through a prism, it separates into its component colors. R.O.Y. G. B.I.V spectrum long wavelengthsshort wavelengths

If white light contains all colors, how fast does it move? 186,000 miles per second 300,000 kilometers per second 3 x 10 8 m/s first successfully determined by Danish astronomer Ole Roemer in 1675

But, what is light? In the 17th Century, Isaac Newton argued that light was composed of little particles while Christian Huygens suggested that light travels in the form of waves. In the 19th Century, Thomas Young demonstrated that light bends slightly around corners and acts like interfering waves.

Thomas Young’s interference experiment

Scottish physicist James Clerk Maxwell showed mathematically in the 1860s that light must be a combination of electric and magnetic fields.

It wasn’t until 1905 that our current understanding of the nature of light emerged. Einstein showed that light sometimes behaves as particles and sometimes as waves. Photon energy = Plank’s constant x speed of light / wavelength

Visible light is only one type of electromagnetic radiation emitted by stars Each type of EM radiation travels at exactly the same speed - the speed of light!

Not all EM radiation can penetrate Earth’s atmosphere.

What is Light and Why Would Astronomers Want to Study the Properties of Light? Sometimes we say light is made of waves Sometime we say light is made of particles called photons Moves very fast, at miles per second 300,000 km per second consider a prism...

If you pass white light through a prism, it separates into its component colors. R.O.Y. G. B.I.V spectrum long wavelengthsshort wavelengths

Analyzing The Properties of Light Visible Light is but one part of the entire electromagnetic (EM) spectrum. EM Spectrum includes all kinds of light radio waves (all light moves at the same speed - micro waves light speed) infrared light visible light ultra violet light (some light photons have x rays shorter wavelengths gamma rays and more energy than others)

Visible light is only one type of electromagnetic radiation emitted by stars Each type of EM radiation travels at exactly the same speed - the speed of light!

Not all EM radiation can penetrate Earth’s atmosphere.

Astronomers use different instruments to look at light of different wavelengths - sometimes, we even have to go above Earth’s atmosphere.

SOFIA - the Stratospheric Observatory for Infrared Astronomy

Observations at other wavelengths are revealing previously invisible sights UV Ordinary visible infrared Map of Orion region

Consider Orion in Different Wavelengths of Light!

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

TODAY’S Sun as seen in visible light from Earth and from space in X-rays by satellites

Observations at wavelengths other than visible light are revealing previously invisible sights Visible light imageradio wavelength image

High Energy Gamma Rays - Compton Gamma Ray Observatory (GRO) Satellite

The Sky’s emission of Gamma Rays

But, we receive GRBs from every direction !! The fact that GRBs come from every direction imply that GRBs don’t come from our galaxy, but from other galaxies spread in every direction!

Radio wavelength observations are possible from Earth’s surface

The Very Large Array (VLA) in New Mexico

Different types of EM radiation require different types of telescopes A refracting telescope uses a lens to concentrate incoming light A reflecting telescope uses mirrors to concentrate incoming starlight

Analyzing Spectra: The Properties of Light Visible Light is one part of the electromagnetic (EM) spectrum. EM radiation is described as a wave with wavelengths in the range 1x m to 100 m. Visible light is only a tiny small part of the entire electromagnetic spectrum. Little bits of light are called photons.

Dividing Light Into a Spectrum Astronomers separate out light into its individual components using a diffraction grating or using a prism - then they analyze each part independently!

Filter Detector 81 blue4600 A81

Filter Detector 85 blue4600 A81 green5300 A85

Filter Detector 83 blue4600 A81 green5300 A85 yellow5800 A83

Filter Detector 78 blue4600 A81 green5300 A85 yellow5800 A83 orange6100 A78

Filter Detector 70 blue4600 A81 green5300 A85 yellow5800 A83 orange6100 A78 red6600 A70 The spectrum is continuous. UVIR

Spectra Most light sources contain energy in lots of different wavelengths. We can measure the brightness in various wavelength bands--the result is called the spectrum. The spectrum (total character of light emitted) can tell us a lot about a source.

A refracting telescope uses a lens to concentrate incoming light Similar to a magnifying glass

A larger objective lens provides a brighter (not bigger) image

lenses reverse images

Three main functions of a telescope Brighten (called light gathering power) See fine detail (called resolution) and least important, Magnify magnification = (objective lens focal length / eyepiece lens focal length)

Functions of a Telescope To gather light. –want a large objective –range of few inches to 10 meters!! To resolve fine detail. –limited by size and atmospheric “seeing” To magnify –least important –about 50x per inch of aperture (rule of thumb)

Refracting telescopes have drawbacks Spherical aberration Chromatic aberration

Special achromatic compound lenses and lens coatings can often fix this aberration

Refracting telescopes have drawbacks Spherical aberration Chromatic aberration Sagging due to gravity distorting the lens Unwanted refractions opaque to certain wavelengths of light Yerkes Observatory - 40-inch Refracting Telescope: The Largest Refracting Telescope in the World

Reflecting telescopes use mirrors to concentrate incoming starlight

Newtonian Focus Prime Focus Cassegrain focus coude’ focus

Astronomer’s face two major obstacles in observing the stars Light Pollution from Cities Effects of Twinkling from Earth’s atmosphere

Tucson, Arizona in 1959 and 1980

Earth’s atmosphere hinders astronomical research Image of stars taken with a telescope on the Earth’s surface Same picture taken with Hubble Space Telescope high above Earth’s blurring atmosphere

Rapid changes in the density of Earth’s atmosphere cause passing starlight to quickly change direction, making stars appear to twinkle.

Advanced technology is spawning a new generation of equipment to view the universe CCDs (charge-coupled devices) Large telescopes on remote mountain tops –Mauna Kea in Hawaii –Cerro Pachon in Chile Adaptive Optics to counteract the blurring of Earth’s atmosphere Orbiting space observatories

A Charge-Coupled Device (CCD)

Ordinary Photographs vs. CCDs Film (negative) CCD (negative) CCD (positive) Same integration (I.e. exposure) time. Different quantum efficiency: Film 1% CCD 70%

Matching 10-m, multiple mirror Keck Telescopes in Hawaii with adaptive optics

High above Earth’s atmosphere, the Hubble Space Telescope provides stunning details about the universe

What did you think? What is the main purpose of a telescope? A telescope is designed to collect as much light as possible. It also improves resolution and magnifies images. Why do stars twinkle? Rapid changes in the density of Earth’s atmosphere cause passing starlight to change direction, making stars appear to twinkle.