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Telescopes Types of Telescopes A telescope collects light over a large area and focuses it in a small area where it can be viewed with an eye or a camera. This can be done in 2 ways: 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. Comparison of refractors and reflectors Functions of Telescopes • 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 rare objects) • Detect light across the electromagnetic spectrum (see new phenomena) • Sophisticated analysis of light (e.g., spectroscopy) • Record images (e.g., photographs, digital pictures) 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. Day Night The largest telescopes can collect more than a million times as much light as the human eye. 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. The mirrors in the largest optical/infrared telescopes have diameters of 10 meters. Rather than use single mirrors that are even larger, future telescopes will contain 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. Functions of Telescopes • 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 rare objects) • Detect light across the electromagnetic spectrum (see new phenomena) • Sophisticated analysis of light (e.g., spectroscopy) • Record images (e.g., photographs, digital pictures) 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 ground-based telescope Hubble ground-based telescope Hubble Some telescopes on the ground now have the ability to partially correct for the blurring by the atmosphere. normal blurry image of a star corrected for blurring normal blurry image of Neptune corrected for blurring Mapping the Sky Hubble provides very sharp images of tiny patches of sky. In comparison, some telescopes are designed to photograph the entire sky, making it possible to search for new rare objects. Wide-field Infrared Survey Explorer (WISE) Wide-field Infrared Survey Explorer (WISE) Wide-field Infrared Survey Explorer (WISE) Functions of Telescopes • 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 rare objects) • Detect light across the electromagnetic spectrum (see new phenomena) • Sophisticated analysis of light (e.g., spectroscopy) • Record images (e.g., photographs, digital pictures) Telescopes allow us to collect light across the entire electromagnetic spectrum We see different phenomena at different wavelengths Atmospheric Windows Gamma-ray and x-ray telescopes must be in space since those wavelengths don’t penetrate the atmosphere. Optical, infrared, and radio telescopes can operate from the ground, although the atmosphere makes optical and infrared images blurry. Where to Put a Telescope on the Ground For an optical observatory, you need • Dark skies (avoid city lights) • Clear skies/few clouds • Stable air for sharper images • 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 Arizona Chile Radio Observatories Aricebo Very Large Array (VLA) Space Observatories Hubble (visible) Compton (gamma ray) Galex (UV) Chandra (x-ray) Spitzer (infrared) WMAP (radio) Chandra X-ray Observatory Because it is observes X-rays, Chandra detects objects that have very high temperatures. Orbit of Chandra Spitzer Space Telescope Because it observes infrared light, Spitzer detects objects that have lower temperatures. Orbit of Spitzer James Webb Space Telescope James Webb Space Telescope Long Fairing 17m Upper stage H155 Core stage P230 Solid Propellant booster Stowed Configuration Orbit of JWST Orbit of JWST Astronomy Picture of the Day Slides beyond this point contain extra material that you might find interesting but is not covered on homeworks and exams 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 Australia and Africa and will be 100 times more sensitive than the VLA. Square Kilometer Array How to get access to a telescope • Identify an important question you want to answer • Identify the type of data needed to answer the question • Identify a telescope that can collect those data (and that you are eligible to use) • For most telescopes, proposals to use them are solicited every 6-12 months • Submit a proposal describing the problem you want to address and the amount of time you need with the telescope • There’s a lot of competition for telescope time (only 1/8 Hubble proposals are approved) Are the colors in astronomical images real? Are the colors in astronomical images real? Different colors perceived by the eye correspond to different wavelengths of visible light. Are the colors in astronomical images real? A given image taken with a camera on a telescope shows light at a specific wavelength, so it is monochrome and does not contain colors. However, astronomers sometimes give monochrome images false color to make them more visually appealing to the public. If one has images at multiple wavelengths across the visible spectrum, then one can add them together to make a true color image (i.e., an image with colors similar to what your eyes would see). If the images have different wavelengths but don’t span the visible spectrum, one can still add them together to make a color image, but it probably won’t have the same colors as your eyes would see. If images are at wavelengths outside the visible, it isn’t possible to make a true color image. Instead, the shorter wavelengths are usually colored blue and the longer wavelengths are colored red to mimic how our eyes perceive the visible spectrum.