Transcript Chapter 6
Chapter 6: The Tools of the Astronomer Telescopes come in two general types Refractors use lenses to bend the light to a focus Reflectors use mirrors to reflect the light to a focus The most important property of any telescope is to gather large amounts of light and concentrate it to a focus. Light Gathering Power Area radius 2 Refraction is the bending of light when it goes from one medium to another “n” is the index of refraction. c n v The refracting telescope uses two lenses Since the eye already has a lens, the eyepiece is needed to bring the light rays back to parallel for the eye to see Large refractors can be very long and bulky The Largest Lens is 40” Lenses and refractors suffer from Chromatic Aberration This applies to camera lenses, your eye, telescopes and anything else that uses a lens to focus light Correcting for Chromatic aberration can be expensive The compound lens takes two lenses of different materials and combines them to correct for color distortion Color separation is useful in a prism so that we can obtain a spectrum of light Since it is meant to be separated we don’t call it an aberration. Instead, it is called dispersion A diffraction grating works on interference of light waves Diffraction is much more efficient at separating light into its colors than dispersion Unfortunately, diffraction also leads to problems Look closely enough and points aren’t just points but rings, too Refractors have many problems They suffer from chromatic aberration They are large and bulky and difficult to maneuver Even the best glass cuts off the IR and UV wavelengths Reflection is the bouncing of light off a surface i r Mirrors do not suffer from chromatic aberration and they do not cut off long or short wavelengths A concave mirror focuses light to a focal point Telescope mirrors are made so that the focus is a plane instead of a point There are several types of reflecting telescopes The resolution of a telescope depends on its size and the wavelength of the light Better resolution means smaller angle 2.06 10 5 D The atmosphere limits the resolving power of a ground-based telescope Adaptive Optics can clear up most of the distortions caused by the atmosphere The distortions are caused by differences in the air above the telescope Once a sight with good “seeing” is found everyone wants to use it Kitt Peak Arizona Many of the worlds largest optical telescopes are on top of Mauna Kea Hawaii The Largest Optical Telescope Keck I and II (for now) The 10 meter Keck Mirror How do we use telescopes? An early 19th century sketch of the head of a comet Early telescopic observations were done by eye using the “view and sketch” system By the early 1900’s photographic plates were the dominant scientific way to observe One of the earliest images of the Moon An early image of the Great Nebula in Orion Today scientific observations are done with a CCD Camera A CCD converts photons into electrons and then counts the electrons Each pixel acts like a light bucket, catching photons, converting them to electrons and storing them until they are read out by the electronics. Check out the CCD Simulator in the Animations section of the Telescope and Astronomical Instruments module on the ClassAction website Another common measuring device is the spectrograph Early spectrographs used prisms to separate the light into the colors Modern spectrometers use diffraction gratings The CCD takes a black & white picture of a spectrum Spectra can be displayed as a graph or rainbow of color Visible light is only a small part of the electromagnetic spectrum Observing In Radio Waves Radio waves have long wavelengths so they have poor resolution 2.06 10 To compensate for the long of radio waves, radio telescopes have very large diameters 5 D Under the dish at Arecibo The Very Large Array (VLA) The VLA can tie many radio dishes together in an interferometer which gives much higher resolution To view in IR you need to get above the atmosphere The Spitzer was placed in orbit to view IR Observing Neutrino’s opens up a new window on the universe Neutrino’s are very hard to detect since they don’t interact with normal matter very much Looking for gravity waves is another new technique LIGO uses an interferometer to detect the passing gravity waves and has two sites Some wavelengths require observing in space The Chandra Observatory sees in x-rays The GALEX mission observes the sky in ultraviolet WMAP looks in the microwave The sky looks different in each wavelength we observe The branch of astronomy called Cosmology is meeting up with high energy particle physics We also use computer models to simulate astronomical events Watch Galaxy Merger Simulation, Binary Pulsar Merger and Hypernova Swift 1 videos