Outside the Optical: Other Kinds of Telescopes

<html> <head> <link rel="StyleSheet" href="/classes/classes.css" type="text/css"> <title> Outside the Optical: Other Kinds of Telescopes </title> </head> <body>  <a rel="license" href="http://creativecommons.org/licenses/by-nc-sa/2.0/"><img alt="Creative Commons License" border="0" src="http://creativecommons.org/images/public/somerights20.gif" /></a> Copyright © Michael Richmond. This work is licensed under a <a rel="license" href="http://creativecommons.org/licenses/by-nc-sa/2.0/">Creative Commons License</a>.   <h1> Outside the Optical: Other Kinds of Telescopes </h1> Astronomers started to investigate portions of the electromagnetic spectrum outside the optical in the 1930s. Advances in radar and rocket technology during World War II gave this new research a big push, and it has continued to grow ever since. Note that the optical region is just a teeny, tiny portion of the entire electromagnetic spectrum: <pre> Wavelength (m) Frequency (Hz) Energy (J) ------------------------------------------------------------------------------- -1 9 -24 Radio > 1 x 10 < 3 x 10 < 2 x 10 -3 -1 9 11 -24 -22 Microwave 1 x 10 - 1 x 10 3 x 10 - 3 x 10 2 x 10 - 2 x 10 -7 -3 11 14 -22 -19 Infrared 7 x 10 - 1 x 10 3 x 10 - 4 x 10 2 x 10 - 3 x 10 -7 -7 14 14 -19 -19 Optical 4 x 10 - 7 x 10 4 x 10 - 8 x 10 3 x 10 - 5 x 10 -8 -7 14 16 -19 -17 UV 1 x 10 - 4 x 10 8 x 10 - 3 x 10 5 x 10 - 2 x 10 -11 -8 16 19 -17 -14 X-ray 1 x 10 - 1 x 10 3 x 10 - 3 x 10 2 x 10 - 2 x 10 -11 19 -14 Gamma-ray < 1 x 10 > 3 x 10 > 2 x 10 ------------------------------------------------------------------------------- </pre> In musical terms, the optical region corresponds to a single key on the keyboard of light: <img src="./piano_spec.gif" width=777 height=242> Let's look at some representative telescopes for these other regions of the spectrum. Remember that the primary job of all telescopes is to gather light; if possible, it's good to focus the light, too. <h4> Radio </h4> <UL> <LI> The <a href="http://www.naic.edu/"> Arecibo Telescope </a> is the largest radio dish in the world, with a diameter of 305 meters (1000 feet)! <center> <img src="./parkr2.jpg" width=500 height=347> courtesy of the NAIC - Arecibo Observatory, a facility of the NSF and David Parker </center> Its primary mirror, made of wire mesh, reflects light upwards to the secondary and detectors, which hang high above: <center> <img src="./ingreg.jpg" width=482 height=694> courtesy of the NAIC - Arecibo Observatory, a facility of the NSF and Tony Acevedo </center> Now, despite its huge size, the Arecibo telescope can't take particularly sharp pictures of celestial objects. Astronomers describe the amount of fine detail visible in an image by the term resolution. Here's a low-resolution version of an image: <center> <img src="./trifid_blurry.jpg" width=640 height=480> </center> and here's a high-resolution version of the same image: <center> <img src="./trifid_sharp.jpg" width=640 height=480> </center> The best theoretical resolution of a telescope depends on its diameter and on the wavelength of the light it focuses: <pre> wavelength resolution goes like ---------- diameter </pre> The smaller this fraction, the smaller the details a telescope can distinguish. Even though the diameter of the Arecibo telescope is much larger than that of, say, the Keck optical telescope, the wavelength of its light is much, much, MUCH longer. As a result, its resolution is much worse (i.e. the fraction above is much larger), and its pictures are blurry. But there is a way for radio telescopes to take sharp pictures .... <LI> <a href="http://www.aoc.nrao.edu/vla/html/VLAhome.shtml"> The Very Large Array </a> of radio telescopes in Soccoro, New Mexico, has 27 antennas of 25 meters diameter each, arranged in patterns up to 22 miles across. <center> <a href="./twilight.jpg"> <img src="./twilight_small.jpg" width=423 height=202> </a> courtesy of Dave Finley and National Radio Astronomy Observatory and Associated Universities, Inc. </center> If one properly combines the signals from the individual dishes, one can achieve the same resolution as a single enormous telescope with the same diameter as the array. Radio interferometers can achieve the same resolution as optical telescope, as this image shows (green is optical, blue is radio): <center> <a href="./ngc4038_big.jpg"> <img src="./ngc4038_small.jpg" width=480 height=493> </a> </center> </UL> <h4> Infrared </h4> The near-infrared region is similar to the optical in many ways. One can use telescopes much like optical telescopes -- with mirrors made of glass -- but one does need to switch to IR-sensitive detectors. The big problem is that the Earth's atmosphere blocks most of the infrared radiation, so one must get as high as one can.... <UL> <LI> <a href="http://sofia.arc.nasa.gov/"> The SOFIA telescope</a> has a 2.5-meter mirror <center> <img src="./sofia_mirror.jpg" width=640 height=430> </center> which will eventually ride in a 747 to avoid most of the Earth's atmosphere: <center> <img src="./sofia_plane.gif" width=709 height=406> </center> The SOFIA data will be processed here at RIT before being distributed to scientists around the world. </UL> <h4> X-ray </h4> X-rays are blocked completely by the Earth's atmosphere (and a good thing, too), so one must place X-ray telescopes in orbit. Another problem with X-rays is that they penetrate ordinary matter. That means that they tend to go through mirrors instead of bouncing off them. The only way to focus X-rays is to design a telescope so that the incoming light always grazes the mirrors at a small angle. <UL> <LI> One of the biggest X-ray telescopes currently operating is <A href="http://chandra.harvard.edu/">Chandra, </a> which used to be called AXAF. Joel Kastner, professor in the Imaging Sciences Department, worked on Chandra for many years, and is now analyzing some of its data. Here's a schematic of Chandra's mirror system, seen from the side: <center> <img src="./chandra_graze.gif" width=600 height=272> </center> So the mirrors are nested cylinders: <center> <img src="./chandra_diagram.gif" width=444 height=275> </center> The Chandra mirrors were aligned right here in Rochester, by Kodak. X-rays are produced by very energetic processes, which usually require high temperatures. Compare these two views of the Orion Nebula: first, optical: <center> <img src="orion_hst.gif" width=420 height=508> </center> And now in X-rays, as seen by Chandra: <center> <img src="chandra_m42.gif" width=509 height=512> </center> </UL> <h4> Gamma-ray </h4> Gamma rays are even more energetic than X-rays, and they, too, are blocked by the Earth's atmosphere. The conventional way to observe them is to place a telescope in orbit ... but it turns out that one can observe them indirectly from the ground, too. <UL> <LI> <a href="http://cossc.gsfc.nasa.gov/cgro/index.html"> The Compton Gamma Ray Observatory</a> orbited the Earth from April 5, 1991, until June 4, 2002, detecting high-energy photons with several different instruments. <center> <img src="cgro.gif" width=499 height=299> </center> It detected thousands of gamma ray bursts, but was only able to indicate the location of each one to about 5 degrees. <center> <img src="cgro_bursts.jpg" width=691 height=507> </center> <LI> <a href="http://egret.sao.arizona.edu/"> The Whipple Gamma Ray Observatory </a> has a 10-meter dish made of multiple mirrors to detect Cerenkov light from gamma-rays striking the Earth's atmosphere. <center> <img src="whipple.gif" width=478 height=438> </center> Even though the gamma rays themselves don't make it to the ground, remnants of their violent collisions with air molecules do. <center> <img src="air_shower.gif" width=637 height=465> </center> <LI> <a href="http://www.lanl.gov/milagro/"> The Milagro Gamma-ray Observatory</a> is a swimming pool lined with phototubes, and covered with an opaque sheet. <center> <a href="./milagro.gif"> <img src="milagro_small.jpg" width=400 height=319> </a> </center> <center> <a href="./milagro_tubes_big.jpg"> <img src="milagro_tubes_small.jpg" width=500 height=202> </a> </center> It detects the particles (not the photons) created by the collision of a gamma ray with the upper atmosphere. </UL> <hr> <h2> "Telescopes" for detecting particles, not electromagnetic radiation </h2> <h4> Neutrino </h4> Neutrinos are ghostly particles which almost never interact with matter. They can (and do) pass through an entire planet untouched. Every second, over ten trillion neutrinos produced in the core of the sun zip through your body! So, how can one detect these neutrinos? One needs to put a big chunk o' matter in their path, monitor the matter very carefully ... and be prepared to wait patiently. <UL> <LI> <a href="http://amanda.berkeley.edu/"> AMANDA </a> is a neutrino telescope which uses the Antarctic ice cap as its "primary detector". A very few neutrinos collide with the nucleus of an atom in the ice, producing a brief flash of Cerenkov radiation. In order to detect those brief flashes, scientists have bored holes more than 1 mile deep in the ice and suspended phototubes in the holes. <center> <img src="amanda_unit.gif" width=397 height=597> </center> <center> <a href="./amanda_big.gif"> <img src="amanda_small.gif" width=360 height=450> </a> </center> Here's the record of an actual event (muon, not neutrino) recorded by Amanda (click on the figure to watch a short animation). <center> <a href="./amanda_event.gif"> <img src="amanda_event_small.jpg" width=414 height=462> </a> </center> <LI> <a href="http://www.sno.phy.queensu.ca/"> The Sudbury Neutrino Observatory </a> is a big plastic ball filled with very pure heavy water, lined with phototubes, placed in an old mine. <center> <img src="sno_schem.jpg" width=368 height=550> </center> <center> <a href="./sno_bot_big.jpg"> <img src="sno_bot_small.jpg" width=550 height=365> </a> </center> <center> <a href="./sno_filled_big.jpg"> <img src="sno_filled_small.jpg" width=550 height=549> </a> Photo courtesy of Ernest Orlando Lawrence Berkeley National Laboratory. </center> Scientists watch for simultaneous detections of flashes by many tubes all at once. A real neutrino event will appear as a ring on the side of the sphere opposite the place the neutrino entered: <center> <a href="./sno_event_big.gif"> <img src="sno_event.gif" width=307 height=298> </a> </center> </UL> <h4> Solar wind particles </h4> The <a href="http://www.genesismission.org/"> Genesis Space Mission </a> sent a spacecraft far from the Earth's magnetic field to collect particles of the solar wind. The basic idea was simple: expose plates of pure metal <center> <img src="./genesis_array.jpg" width=648 height=445> </center> to the Sun, and let solar wind particles smash into them and stick. <center> <a href="./genesis_collect_big.jpg"> <img src="./genesis_collect_small.jpg" width=750 height=559> </a> </center> Some scientists are especially interested in the number and type of oxygen atoms in the solar wind. Even though the Genesis spacecraft was designed to spend over two years collecting particles, they realized that it might not be long enough to build up a statistically significant sample of oxygen (which is much less common in the solar wind than hydrogen or helium). Therefore, they designed one special collector which could concentrate particles, putting many more than usual into a small spot on one collector. You can see this collector at the center of the spacecraft: <center> <img src="./concentrator.gif" width=750 height=559> </center> This concentrator is just a telescope -- but it uses electric fields (instead of polished glass) to reflect charged particles (instead of light rays). Here's a closeup view of it: <center> <img src="./figk2.gif" width=563 height=426> </center> When active, the back surface of the collector, which is shaped like a parabola, is given a large positive electric charge: <center> <img src="./add_charge.gif" width=563 height=426> </center> Here's a side view: <center> <img src="./figk1.gif" width=721 height=542> </center> If we apply a positive charge to the "solid electrode" at the base of the concentrator, then a positive ion which flies into the concentrator will "reflect" off that positive charge and end up on the target: <center> <a href="./ion_bounce_anim.gif"> <img src="./ion_bounce_1.gif" width=762 height=612> </a> </center> <hr> For more information: <UL> <LI> Bill Keel has put together a Web site about <a href="http://www.astr.ua.edu/keel/telescopes/"> The Telescope's He's Used (and what he's done with them). </a> <LI> Interested in interferometry? Play with <a href="http://wwwnar.atnf.csiro.au/astronomy/vri.html"> a toy radio interferometer</a> created by Nuria McKay, Derek McKay and Mark Wieringa. <LI> Watch the <a href="http://www.sofia.usra.edu/Gallery/aircraft/AC/images/2000AC0016.mpg"> SOFIA aircraft video </a> to see how the telescope will work in flight. </UL>  <a rel="license" href="http://creativecommons.org/licenses/by-nc-sa/2.0/"><img alt="Creative Commons License" border="0" src="http://creativecommons.org/images/public/somerights20.gif" /></a> Copyright © Michael Richmond. This work is licensed under a <a rel="license" href="http://creativecommons.org/licenses/by-nc-sa/2.0/">Creative Commons License</a>.   </body> </html>

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Outside the Optical: Other Kinds of Telescopes