Astro 201: Sept. 16, 2010 New: Copies of Lecture Notes and HW are now on d2l, and should be faster to download. HW #3 on line, due Tuesday Midterm #1:

Astro 201: Sept. 16, 2010

• New: Copies of Lecture Notes and HW are now on d2l, and should be faster to download.

• HW #3 on line, due Tuesday• Midterm #1: Tuesday, Sept. 28 – more info

later• Today:

– IR camera demo and lab – write-up due in one week

– Telescopes

LIGHTand

Telescopes

Spectrum of the Sun

X-ray spectrum

SummaryAstronomers take “images” of objects

Astronomers also take “spectra” of objects

Temperature

Type of atoms (hydrogen, helium, iron, etc)

how fast the object is moving, at least radially

Sometimes astronomers take images though filters which isolate specific wavelengths (rough) spectral pictures

IR Light

The 10 micron camera contains a detector which is sensitive to infrared light.

All objects radiate "black body" radiation, or "Planck radiation", by virtue of their temperature.

• For objects near room temperature the radiation peaks in the infrared. Your eyes, which are sensitive to optical light, cannot see this radiation unless the object is VERY hot.

• The visible or optical light you see is reflected optical light from the sun or lamps.

• Hotter things are brighter in the IR camera than cooler things.

• Some materials are opaque to IR light, but transparent to visible light.

• Some materials are transparent to IR light, but opaque to visible light.

• IR light can be reflected by a mirror, just like optical light.

Everyday Uses of IR light

• One everyday use of IR light is in remote control devices

• IR cameras are used on ships and in buildings to look for hot spots in electrical wiring

• night-time spotting of people (who are warmer than their surroundings)

• for seeing "through" smoke in a fire

Astronomers use IR lightTo measure temperatures; also to look "through" dust

The Trapezium in Orion: Stars are forming out of gas and dust

Optical or Visible Wavelengths IR wavelengths

Why use telescopes?Why use telescopes?

(1)(1) Light Light Gathering PowerGathering Power:

A large telescope can intercept and focus more light than does a small telescope.

A larger telescope will produce brighter images and will be able to detect fainter objects.

(2)(2) Resolving Resolving PowerPower: A large telescope also increases the sharpness of

the image and the extent to which fine details can be distinguished.

(3) Detect types of light besides optical: radio, X-ray, ultraviolet, infraredput the telescope in space, above the atmosphere which

absorbs many wavelengths

Optical Light Telescopes:

Refracting (use a lens)Reflecting (use a mirror)

REFRACTING TELESCOPE: Examples Galileo’s telescope, our eyes

A CONVEX lens (thick in the middle) focuses light to a point.

Light gatheredFrom a largeArea is Concentrated Can see fainterObjects than you Can with your eye

Refracting TelescopeRefracting Telescope

Objective LensEyepiece Lens

Focal Length Objective

Focal Length of Eyepiece

Refracting Telescope: Lens focuses light onto

the focal plane

Focal length

Light from a large area is concentrated in a small area.

focus

Reflecting TelescopesReflecting Telescopes: Use mirrors as the optics

A mirror shaped like a PARABOLA focuses light to a point.

Newton’s Telescope: The first reflecting telescope

PrimaryMirror

SecondaryMirror

The world’s biggest telescopes are reflectors, not refractors.

What’s wrong with lenses?

(1)Lenses absorb light.

(2)Lenses sag.

(3)Lenses have chromatic aberration: colors don’t focus at the same point.

Blue Focus

Red Focus

Chromatic AberrationChromatic Aberration. As light passes through a lens, just as a prism will disperse light, the lens will focus bluer wavelengths differently than the redder wavelengths.

World’s largest refracting telescope:

Yerkes Observatory, D = 1 meter, completed

In 1898.

Reflecting telescopes do not suffer from Chromatic Aberration. All wavelengths will reflect off the mirror in the same way.

Reflecting telescopes can be made very large because the mirrored surfaces have plenty of support. Thus, reflecting telescopes can greatly increase in light gathering and resolving power.

Reflecting telescopes are often cheaper ($$$) to make than similarly sized refracting telescopes.

Amount of light collected per second is is proportional to the AREA of the lens or mirror.

2D4

Area

D = diameter of lens/mirror

A bigger lens or mirror is able to resolve finer structures in the image

low resolution

high resolution

Two stars are “RESOLVED” if they are seen as separate points.

Smallest angle resolved is proportional to 1/D where D = the diameter of the mirror

MAGNIFICATION is not as important: Big, blurry image is less useful than small, sharp image.

Large Binocular Telescope:

Mt. Graham, near Safford AZ.

Two mirrors, each 8.4m in diameter

A MODERN REFLECTING TELESCOPE:

Where to put a Telescope?

Far away from civilization – to avoid light pollution

““Seeing”Seeing”

Weather conditions and turbulence in the

atmosphere set limits to the quality of astronomical

images from ground-based observatories

Bad seeing Good seeing

Mountain topobservatories are puton peaks where theAtmospheric turbulenceis minimal

= twinkling

Laminar vs. Turbulent Fluid Flow

Air becomes turbulent when it encountersa barrier – e.g. a mountaintop

bad seeing

Laminar flow

Turbulent Flow

The Hubble Space Telescope is 600 kilometers above the Earth’s surface.

Hubble Space Telescope has great angular resolution; it’s above the turbulent atmosphere.

Light-gathering ability? Not as great; it’s only D = 2.4 meters in diameter.

Problem: It costs a lot of money to put a telescope in space!

Problem #2: It’s really hard to repair telescopes in space – only Hubble was designed to be repairable

X-Ray AstronomyX-Ray Astronomy

X-rays are completely absorbed in the atmosphere.

X-ray astronomy has to be done from satellites.

NASA’s Chandra

X-ray Observatory

Gamma-Ray AstronomyGamma-rays: most energetic electromagnetic radiation; traces the most violent processes in the Universe

The Compton Gamma-Ray Observatory

Infrared AstronomyAlthough short wavelength IR gets through the atmosphere, longer wavelength IR does not.In space, can cool the telescopes so it’s not a source of high background

Spitzer Space Telescope

Next Huge NASA mission, afterHubble Space Telescope ends:

James Web Space Telescope (JWST)

Radio telescopes detect radio frequency radiation which is invisible to your eyes.

Parabolic “dish” of a radio telescope acts as a mirror, reflecting radio waves to the focus.

Radio telescopes can be hugebecause they don’t haveto be as smooth asoptical telescopes: thewavelength of radio light isseveral cm’s and mirrors only have to be smooth to about 1/20 of a wavelengthto focus the light well

Surface of mirror

Arecibo Radio Observatory in Puerto Rico

Radio InterferometryThe Very Large Array (VLA): 27 dishes are combined to simulate a large dish of 36 km

in diameter.

Even larger arrays consist of dishes spread out over the entire U.S. (VLBA = Very Long Baseline Array) or even the whole Earth (VLBI = Very

Long Baseline Interferometry)