Types of Telescopes The purpose of a telescope is to collect as much light as possible and focus it into a small area where it can be viewed with an eye or a camera. This can be done in 2 ways: • By using a lens to bend, orrefractthe light • By using a mirror toreflectthe light The size of a telescope is characterized by the diameter of its lens or mirror: a “36-inch telescope” has a lens (or mirror) that is 36 inches in diameter.
Refracting Telescopes Refracting telescopes use lenses to collect light over a large area and bring it to a focus.
Refracting Telescopes The first telescopes were refractors, but they have limitations: • A larger lens collects more light and allows one to see fainter objects. However, it is difficult to physically support a big lens because it must be held at the edges. • Lenses focus different wavelengths of light at different locations. As a result, images cannot appear in focus at all wavelengths simultaneously.
Reflecting Telescopes It is much easier to build and support a large mirror than a large lens. Mirrors also bring all wavelengths to a common focus, unlike refractors. As a result, all modern telescopes used by professional astronomers are reflectors.
Magnification (make things look bigger) easy to make a telescope with good magnification Collection of large amounts of light (see fainter things) most important feature of a telescope Sharp images (see more detail and structure) Map large areas of sky (search for particular objects) Detect light across the electromagnetic spectrum (see new phenomena) Sophisticated analysis of light (e.g., spectroscopy) Record images (e.g., photographs, digital pictures) Functions of Telescopes
The pupil of the eye expands at night in order to collect more light. Similarly, astronomers build telescopes with larger mirrors so that they can gather more light and see fainter objects in the sky. Night Day The largest telescopes can collect more than a million times as much light as the human eye.
The largest telescope mirrors have diameters of 10 meters. Rather than use single mirrors that are even larger, future telescopes will contain arrays of several mirrors. current future
Smaller mirrors are easier and cheaper to build, so large telescopes also use segmented mirrors.
The next generation of telescopes will have multiple mirrors that are equivalent to a single 30-50 meter mirror.
In addition to collecting more light and seeing fainter objects, larger mirrors also provide sharper images.
Twinkle, twinkle, little star Although large mirrors have the potential to produce sharp images, turbulence in the Earth’s atmosphere causes stars to appear blurry.
Sharper images from space The mirror of the Hubble Space Telescope is 2.4 meters in diameter, smaller than the largest ground-based telescopes. But it is above the atmosphere, so it produces very sharp images. ground-based telescope Hubble
Some telescopes on the ground now have the ability to partially correct for the blurring by the atmosphere in certain circumstances. normal blurry image of a star corrected for blurring
Mapping the Sky Hubble provides very sharp images of tiny patches of sky. In comparison, some telescopes are designed to photograph large areas of the sky. The Large Synoptic Survey Telescope will map half the sky 1000 times from 2016-2026. The primary goals are the discovery of all large near-Earth asteroids (>100 m) and measurements of dark energy.
Telescopes allow us to collect light across the entire electromagnetic spectrum
Gamma-ray and x-ray telescopes must be in space since those wavelengths don’t penetrate the atmosphere. Optical, infrared, and radio telescopes can be operate from the ground, although the atmosphere makes optical and infrared images blurry. Atmospheric Windows
Where to Put a Telescope on the Ground For an optical observatory, you need • Dark Skies • Stable air for relatively sharp images • Good weather • High altitude to get above as much of the atmosphere as possible. For a radio observatory, you need • An area far from cell phones and TV/radio stations
Optical (= Visible) & Infrared Observatories Hawaii Chile Texas Chile Arizona
Radio Observatories Aricebo Very Large Array (VLA)
Square Kilometer Array The Square Kilometer Array will be an array of thousands of radio dishes with a total collecting area of 1 km2 and a baseline of 3000 km. It will be built in either Australia or Africa and will be 100 times more sensitive than the VLA.
Space Observatories Hubble (visible) Chandra (x-ray) Compton (gamma ray) Spitzer (infrared) Galex (UV) WMAP (radio)
Space Observatories • Chandra X-ray Observatory • X-ray wavelengths • Can see things at very hot temperatures
Space Observatories • Spitzer Space Telescope • Infrared wavelengths • Can see things at very low temperatures