Telescopes & Light

History Hans Lippershey Middleburg, Holland –invented the refractor telescope in 1608 Galileo –the first to use a telescope in astronomy. Galileo's designs used a combination of convex and concave lenses. Kepler –improved the design to have two convex lenses, which made the image upside-down. Kepler's design is still the major design of refractors today, with a few later improvements in the lenses and the glass to make them.

The Powers of a Telescope Light Gathering Power Light Gathering Power : Astronomers prefer *large* telescopes. A large telescope can intercept and focus more starlight than does a small telescope. A larger telescope will produce brighter images and will be able to detect fainter objects. Resolving Power Resolving Power : A large telescope also increases the sharpness of the image and the extent to which fine details can be distinguished. Magnification Magnification : The magnifying power is the ability of the telescope to make the image appear large in the field of view.

Size Does Matter D Light-Gathering Power: Depends on the surface area (A) of the primary lens and is proportional to the telescopes diameter.

The Powers of a Telescope Resolving Power: Wave nature of light: The telescope aperture produces fringe rings that set a limit to the resolution of the telescope.

The Powers of a Telescope A larger magnification does not improve the resolving power of the telescope! Magnifying Power The ability of the telescope to make the object’s optical image appear bigger while being observed

Types of Telescopes Refracting Telescopes Refracting Telescopes : Use lenses as the optics to focus and bend light. Galileo used a refracting telescope. The human-eye is partly a refracting telescope.

Refracting Telescope Objective Lens Eyepiece Lens Focal Length Objective Focal Length of Eyepiece Refracting Telescope : Lens focuses light onto the focal plane Focal length

Disadvantages Blue Focus Red Focus Chromatic Aberration Refracting telescopes suffer from 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.

Advantages to Refracting Telescopes Refractor telescopes are rugged (do not need to be realigned often). The glass surface inside the tube is sealed from the atmosphere so it rarely needs cleaning. Since the tube is closed off from the outside, air currents and effects due to changing temperatures are eliminated. –This means that the images are steadier and sharper than those from a reflector telescope of the same size.

Drawbacks to Refracting Telescopes 1.The lens is supported only around the edges. 2.Light of different wavelengths comes to a focus in different places. 3.It is difficult to make a glass lens with no imperfections inside the lens.

Types of Telescopes Reflecting Telescopes Telescopes : Use mirrors as the optics to focus and bounce light. The rear view mirror on your car is a simple reflecting telescope. Reflecting Telescope: Concave Mirror focuses light onto the focal plane Most modern telescopes are reflecting telescopes. Focal length

Reflecting Telescope

Advantages 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.

Newton’s Telescope: The first reflecting telescope

ANOTHER TYPE OF TELECOPE IS THE RADIO TELESCOPE -Receive radio waves emitted from objects in space, including from very distant stars and galaxies -It can receive information in any weather and during day or night. -Used in tracking and collecting data from satellites and space probessatellitesspace probes

Where to put a Telescope? Far away from civilization – to avoid light pollution

On high mountain-tops – to avoid atmospheric turbulence and other weather effects Where to put a Telescope?

On high mountain-tops – to avoid atmospheric turbulence and other weather effects

“Seeing” Weather conditions and turbulence in the atmosphere set further limits to the quality of astronomical images. Bad seeing Good seeing

Hubble Space Telescope

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

Radio Astronomy

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)!