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Telescopes Astronomy 1-1 Lecture 06-1 Functions Three basic functions. Magnification To increase the size of the object that is being examined Resolution The ability to discriminate fine detail Light Gathering Power The ability to collect light Astronomy 1-1 Lecture 06-2 Types of Optical Telescopes Two basic types Refractors These are telescopes that use a lens to bring the light to a focus Reflectors These are telescopes that use a mirror to bring the light to a focus Both type of telescopes can have problems associated with them Astronomy 1-1 Lecture 06-3 Refractors Light is manipulated by lenses to form an image Astronomy 1-1 Lecture 06-4 Refractor Problems Chromatic aberration •Lenses are used to bend the light to bring it to a focus •How much the light is bent is dependent upon the wavelength •So different colors are focused at different points •Problem can be corrected Light gathering power •For more light, need wider lenses •Cost •Weight Astronomy 1-1 Lecture 06-5 Reflectors Mirrors are used to manipulate the light Astronomy 1-1 Lecture 06-6 Reflector Problems - I Spherical Aberration Different parts of a spherical mirror have slightly different focal lengths This causes the image to be fuzzy because the light is focused over a range of positions The problem is corrected by varying the curvature of the mirror as function of position This makes the mirror parabolic Astronomy 1-1 Lecture 06-7 Spherical Aberration Astronomy 1-1 0.5 wavelength 0.25 wavelength 0.125 wavelength 0.1 wavelength Lecture 06-8 Reflector Problems - II Coma Parabolic mirrors will focus all of the incoming light at one point However light that is reflected off the sides will be distorted in shape A spot of light will be distorted into a tear drop shape High quality reflecting telescopes use a combination of a spherical mirror and correcting lenses at the front of the telescope Astronomy 1-1 Lecture 06-9 Light Gathering The larger the telescope - the better the light gathering Brightness is proportional to square of radius of mirror Astronomy 1-1 Lecture 06-10 Resolving Power The larger the telescope also improves the resolving power The resolving power is the ability to discern two closely spaced objects. Astronomy 1-1 Lecture 06-11 Resolving Power 10' 1' 5" 1" Astronomy 1-1 Lecture 06-12 Transparency of Earth's Atmosphere Earth's atmosphere is not transparent to all the radiation incident upon the Earth. Wavelengths in two primary regions make it through the atmosphere. The visible wavelengths, and Radio wavelengths. Earth based telescopes are therefore limited to observing in these two bands of radiation. To see in the other wavelengths requires space based observation platforms. Astronomy 1-1 Lecture 06-13 Transparency of Earth's Atmosphere Astronomy 1-1 Lecture 06-14 Detectors Image acquisition – no longer photographic Charge-coupled devices (CCDs) are electronic devices, can be quickly read out and reset Astronomy 1-1 Lecture 06-15 Images and Detectors Image processing by computers can sharpen images Astronomy 1-1 Lecture 06-16 High-Resolution Astronomy Air movements blur images Astronomy 1-1 Lecture 06-17 High-Resolution Astronomy Solutions: Put telescopes on mountaintops, especially in deserts Put telescopes in space Astronomy 1-1 Lecture 06-18 High-Resolution Astronomy Active optics: Control mirrors based on temperature and orientation Astronomy 1-1 Lecture 06-19 High-Resolution Astronomy Adaptive optics: Track atmospheric changes with laser; adjust mirrors in real time Astronomy 1-1 Lecture 06-20 High-Resolution Astronomy These images show the improvements possible with adaptive optics: Astronomy 1-1 Lecture 06-21 Radio Astronomy Radio telescopes Similar to optical reflecting telescopes Prime focus Less sensitive to imperfections (due to longer wavelength) Can be made very large Astronomy 1-1 Lecture 06-22 Radio Astronomy Astronomy 1-1 Lecture 06-23 Radio Astronomy Longer wavelength means poor angular resolution Advantages of radio astronomy 1. Can observe 24 hours a day 2. Clouds, rain, and snow don’t interfere 3. Observations at an entirely different frequency; get totally different information Astronomy 1-1 Lecture 06-24 Interferometry Combine information from several widely spread radio telescopes as if they came from a single dish Resolution will be that of dish whose diameter = largest separation between dishes Astronomy 1-1 Lecture 06-25 Interferometry Interferometry involves combining signals from two receivers; the amount of interference depends on the direction of the signal Astronomy 1-1 Lecture 06-26 Interferometry Can get radio images whose resolution is close to optical Interferometry can also be done with visible light but is much more difficult due to shorter wavelengths Astronomy 1-1 Lecture 06-27 Space-Based Astronomy Infrared radiation can image where visible radiation is blocked; generally can use optical telescope mirrors and lenses Astronomy 1-1 Lecture 06-28 Space-Based Astronomy Infrared telescopes can also be in space; the image on the left is from the Infrared Astronomy Satellite Astronomy 1-1 Lecture 06-29 Space-Based Astronomy The Spitzer Space Telescope, an infrared telescope, is in orbit around the Sun. These are some of its images. Astronomy 1-1 Lecture 06-30 Space-Based Astronomy Ultraviolet observing must be done in space, as the atmosphere absorbs almost all ultraviolet rays. Astronomy 1-1 Lecture 06-31 Space-Based Astronomy X-rays and gamma rays will not reflect off mirrors as other wavelengths do; need new techniques X-rays will reflect at a very shallow angle and can therefore be focused Astronomy 1-1 Lecture 06-32 Space-Based Astronomy X-ray image of supernova remnant Astronomy 1-1 Lecture 06-33 Space-Based Astronomy Gamma rays cannot be focused at all; images are therefore coarse Astronomy 1-1 Lecture 06-34 Full-Spectrum Coverage Much can be learned from observing the same astronomical object at many wavelengths. Here, the Milky Way: Astronomy 1-1 Lecture 06-35