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

LIGHT and Telescopes

Spectrum of the Sun

X-ray spectrum

Summary Astronomers 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 light To measure temperatures; also to look "through" dust The Trapezium in Orion: Stars are forming out of gas and dust Optical or Visible WavelengthsIR wavelengths

Why use telescopes? (1)Light Gathering Power (1)Light Gathering 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)Resolving Power (2)Resolving Power : 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, infrared put 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 gathered From a large Area is Concentrated  Can see fainter Objects than you Can with your eye

Refracting Telescope Objective Lens Eyepiece 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 Telescopes Reflecting Telescopes : Use mirrors as the optics A mirror shaped like a PARABOLA focuses light to a point.

Newton’s Telescope: The first reflecting telescope Primary Mirror Secondary Mirror

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 Aberration Chromatic 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. 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” Weather conditions and turbulence in the atmosphere set limits to the quality of astronomical images from ground-based observatories Bad seeing Good seeing Mountain top observatories are put on peaks where the Atmospheric turbulence is minimal = twinkling

Laminar vs. Turbulent Fluid Flow Air becomes turbulent when it encounters a 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 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 Astronomy Gamma-rays: most energetic electromagnetic radiation; traces the most violent processes in the Universe The Compton Gamma-Ray Observatory

Infrared Astronomy Although 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, after Hubble 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 huge because they don’t have to be as smooth as optical telescopes: the wavelength of radio light is several cm’s and mirrors only have to be smooth to about 1/20 of a wavelength to focus the light well Surface of mirror

Arecibo Radio Observatory in Puerto Rico

Radio Interferometry The 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)