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Using the 20m Radio Telescope: A Manual for Teachers

Russell Kohrs1, Glen Langston2, Sue Ann Heatherly3

rkohrs@rockingham.k12.va.us glangsto@nrao.edu sheather@nrao.edu

1 Broadway High School, Broadway, VA; NRAO RET Summer 2012 2 Astronomer, National Radio Astronomy Observatory, Green Bank, WV 3 Director of Educational Programs, National Radio Astronomy Observatory, Green Bank, WV

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Table of Contents Chapter 1: Introduction to the 20m Radio Telescope and Radio Astronomy ........................................................3

Chapter 2: How Signals From Distant Radio Sources are Processed by the 20m Radio Telescope ..................5

Chapter 3: Procedure for Accessing and Observing ...............................................................................................8

Chapter 4: How to use the 20m Green Bank/Skynet Radio Telescope User Interface........................................10

Appendix A: Contact Information ............................................................................................................................13

Appendix B: Converting Data to Temperature or Flux Density.............................................................................14

Appendix C: The History of the 20m Green Bank/Skynet Radio Telescope ........................................................15

Appendix D: Important Definitions ..........................................................................................................................16

Works Cited.................................................................................................................................................................23

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Chapter 1: Introduction to the 20m Radio Telescope and Radio Astronomy This handbook is for great students observing the sky with the 20m telescope at Green Bank remotely.

Teachers and students can remotely control this six story tall telescope to conduct real data collection, analysis, and

even publication in a robust inquiry-based environment. Astronomical research is, in many ways, more complex and

rewarding than ever. This manual will, hopefully, simplify the complexity inherent in the physics and information

technology of this environment. Making the telescope accessible for anyone, even those who are less familiar with

the science and technology of radio astronomy.

The National Radio Astronomy Observatory (NRAO) at Green Bank, WV has teamed up with the Skynet

project at the University of North Carolina, Chapel Hill (UNC) in an effort to provide astronomical outreach to high

schools and colleges across the nation. This partnership will provide unprecedented access to radio and optical data

collection systems for teachers and professors interested in providing opportunities for their students to conducted

inquiry-based research projects with data that they collect. The types of data available include optical images that

can be obtained from several visual wavelength telescopes already operated by SKYNET as well as radio telescope

data from the newly recommissioned 20m radio telescope.

SKYNET is a network of robotic telescopes distributed around the world and has been operated by students,

faculty, and staff at the UNC since 2006. The program is adding more telescopes to its network regularly, all of which

are accessible and controllable by the general public, teachers, and college professors. SKYNET allows its users to

obtain free “telescope time” for research and viewing interests (SKYNET).

The NRAO has been one of the premier radio astronomy research sites in the world for decades. The NRAO

has also had robust educational outreach programs for many years. In this tradition, its partnership with SKYNET to

refurbish what will be the network’s first radio telescope is not at all unusual. Located in the National Radio Quiet

Zone, the site is an ideal laboratory for radio astronomy, allowing the faintest of extraterrestrial signals to be collected

and processed. Operated by the National Science Foundation in conjunction with Associated Universities, Inc., it is

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also home to the gigantic, 45 story tall, Robert C. Byrd Green Bank Telescope (GBT), the largest fully-steerable radio

telescope on the planet.

Radio astronomy provides an excellent inter-disciplinary opportunity to bolster STEM education in our

schools (Figure 1). Here at the NRAO and SKYNET, professional astronomers, computer scientists, and engineers

work directly with technologists of various stripes such as electrical technicians, drafters, and machinists to produce

world-renowned equipment capable of answering some of our deepest questions about the Earth and its space

environment.

20m NRAO/SKYNET Radio Telescope, Green Bank, WV

16” RC Optical Systems Open-Truss Optical Telescopes (Italy, England, New Mexico, Appalachian State, Morehead College, Hampden-Sydney College, CTIO Observatory, Chile)

Figure 1: A graphical illustration of the collaboration between SKYNET, the NRAO, and your classroom. Optical SKYNET consists of a network of 10 16” optical telescopes and one 20m radio telescope.

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Chapter 2: How Signals From Distant Radio Sources are Processed by the 20m Radio Telescope When students and the public think of astronomical data what usually comes to mind are those beautiful

images obtained from the Hubble Space Telescope (HST) or from some other source. While these are certainly

good examples of important astronomical data, astronomers are quite adept at using the rest of the electromagnetic

(EM) spectrum at their disposal to gather data from distant objects in our universe. Many other observatories, both

land and space-based, have been designed to gather information from astronomical objects using gamma rays, X-

Rays, Ultraviolet, Infrared, microwave and radio waves. Unfortunately, several of these regions of the spectrum are

relatively inaccessible without space-based equipment, as the Earth’s atmosphere absorbs gamma, x-ray, much of

the ultraviolet and infrared, and most of the microwave radiation. Consequently, ground-based observing, through

networks like SKYNET/NRAO are limited to the visible and radio portions of the EM spectrum. Interestingly, while

visible light images are useful and beautiful to our eyes, as our eyes are sensitive to visible light, this is a very small

portion of the EM spectrum, from wavelengths of 400 to 700nm, and so only a limited amount of information can be

gathered here. The radio portion of the EM spectrum, by contrast, has wavelengths of light that stretch from 10cm all

the way beyond 100km.

The physics of light can be described in terms of particles AND waves. This fact makes light a very unique

phenomenon in the universe, as it has properties of energy and matter. In terms of waves of energy, the EM

spectrum, in its entirety, is a way in which to conceptualize light based upon the length of its waves, from crest to

crest. All portions of the EM spectrum are forms of light, but the light varies by the length of its wave, or the distance

between subsequent crests of the wave. As particles of matter, light is able to interact with other matter, reflecting off

of surfaces, refracting through substances, and passing through others. It is important to understand these different

properties of light when conducting radio astronomy, as both are very important in the collection and interpretation of

data.

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The light that we want to study in radio astronomy is produced in distant objects through the behavior of

excited electrons absorbing or emitting these photons. When photons of light interact with atoms, these photons can

be absorbed, creating a type of spectrum that we call an absorption spectrum, or they can be emitted, producing an

emission spectrum. Every atom has a unique absorption and emission spectrum, based upon the number and

arrangement of its electrons and so every atom also has a particular signature. Consequently, it is possible to detect

certain atoms in distant objects such as nebulae, stars, and galaxies by using radio and visible spectral data to tease

these signatures out of a complete spectrum.

On a quantum level the 20m is designed to filter, or polarize, light photons according to the spin direction, or

polarization, of those photons. When the atoms being observed emitted these photons, they were emitted by

electrons in one of two “spin states”, “spin up” or “spin down” (Nave, 2012), according to the Pauli Exclusion

Principle. Consequently, the data received by the telescope is conveyed to the observer as left and right polarization.

In theory, the data from these two polarization states should display equal values on average. However, it is

challenging to adjust the gain on the equipment for each polarization to just the right position to make this a reality.

Consequently, one is sometimes higher or lower than the other.

Extraterrestrial signals collected by the 20m radio telescope are reflected off of the dish, then off of the

secondary, or subreflector, through the feedhorn and into a receiver cooled to very low temperatures with liquid

helium. The purpose of the cryogenics is to quell the noise created by the vibration of particles as a result of the heat

of the equipment and ambient air temperature. The “noise” from these vibrations can easily drown out the faint

signals received from space. At this point, the signal is analog. It is amplified and converted to a digital signal at the

back end of the telescope which is located in a room adjacent to the control room in Green Bank, WV. From here, a

computer called “Cyborg”, in communication with a roachboard and a computer called “beans”, conduct all of the

signal processing using proprietary computer scripts written in C++ and Python, two computer language. These data

are then retrievable on the 20m website, www.gb.nrao.edu/20m/peak/log.htm, and, if taken through SKYNET, at

http://salsa.skynet.unc.edu/s. Figure 2 illustrates this general process for a typical radio telescope system.

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Figure 2: A generalized schematic of a radio telescope system. Radio dishes must be large in order to collect the weak extraterrestrial signals at long wavelengths. The 20m radio telescope has a resolution of 6 arcminutes at 10GHz. This resolution changes with different receiver bands. (Brittanica.com)

Data can be collected via the 20m using a range of frequencies in X-band, from 8GHz to 10GHz, and range

of frequencies in the L-band, which is convenient for the observation of hydrogen gas. Though X-band signals are

not as numerous or strong, there are fewer sources of interference at those frequencies. L-band is more problematic

in terms of RFI, though there are more objects that emit at those frequencies and their signals are stronger. While

both frequency bands will be available, they will alternate bi-annually, as only one receiver can be mounted at a time.

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Chapter 3: Procedure for Accessing and Observing

There are two modes for using the 20m to conduct observations. In either case, it is necessary to first get in

touch with those listed in appendix A as contacts at the NRAO at Green Bank, WV. They are the people who

facilitate the manual use of the telescope, as described below, and are also the group under which you will be listed

as a participant with SKYNET. Follow the applicable instructions below:

For Manual Use

After you have made contact with the observatory to set up an observation time, you should spend some

time reading both this manual as well as the abbreviated version of this manual (which goes into more depth in terms

of the types of scans that can be done) so that your time on the “Armorhead VNC” (yes, the computers at Green

Bank all have names) session is as fruitful as possible. In addition, you will need two methods of communication.

First, you will need to download and install a free program called, “Teamviewer”

(http://www.teamviewer.com/en/download/index.aspx). This program allows you to control the computer called

“Armorhead” remotely as long as there is someone at the observatory that is hosting the session and on hand to help

in case of problems. They will give you a set of codes to join the session. Second, you should have a phone, Skype,

Google Phone, or some other device for audio communication available to communicate with your astronomer host at

the observatory. Using the telescope in manual mode is something that should be done for short lessons or to

introduce students to how the technology actually works. For larger projects, SKYNET should be used.

For SKYNET use

This method of using the 20m is quite simple. SKYNET is a wholly online interface which, while you are not

controlling the telescope directly, allows you to take as much data as you want during any time of the day by simply

adding your observations to a queue. When the telescope is available, it will begin collecting your data. In some

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cases, it may even be possible to watch it do this online, to some extent, by checking “SKYNET live” or the 20m

status page. Using SKYNET is the preferred method of controlling the telescope for everyday use.

After making contact with Green Bank, they will contact SKYNET to get a login for you, as the teacher. Most

likely, you will be placed under the “Green Bank” group. This is the only way to obtain access to SKYNET, as the

telescopes on this network are expensive, high-quality instruments. As an added perk, you will also have access to

their optical telescopes! Once you have a login, the skynet page, www.skynet.unc.edu, is very straightforward and

contains a wealth of guidance for beginning your first observations.

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Chapter 4: How to use the 20m Green Bank/Skynet Radio Telescope User Interface

A. Antenna Status: This box tells you the current activity and position of the telescope. i. Status – This box provides information on what the telescope is actually doing at the moment. Currently, it is idle. ii. Last Status – This box provides information on the time that the status was last updated. iii. Time Left – Here you can see how many seconds are left in the current scan. iv. Az/El – Gives you the position of the telescope in altitude and compass azimuth (both are in degrees) v. RA/Dec – This lists the current Right Ascension and Declination of the telescope B. Cyborg Status: Cyborg is the name of the computer that is acquiring and processing the incoming data. i. State – Like status in “A”, this is the current state of the computer. When not receiving data, it is idle. ii. Last Status – Similar to box A again, this displays the time of the last status update. iii. Acquiring Data – In this box, it will say either “True” or “False”. This simply tells you whether or not data is being acquired. iv. Filename – This is the name of the file that is being written in which your data can be found on Skynet

A

B

C

D

E

F H

G

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C. Weather: Here, the interface is collecting real-time weather data from a weather station on site in Green Bank, WV. i. Is Good – You will either see a “True” or “False” here, in answer to the question, “Is the weather ‘good’ for observing”. If it is not, the telescope will not be usable until it is. ii. Time – This displays the current date and time at the Green Bank site. iii. Temperature, Humidity, Pressure, and Wind – These boxes simply display the current conditions at Green Bank. D. Antenna Control: It is from this box that the user tells the telescope what type of scan should be conducted as well as when and how to stow. i. Slew – Pushing this button and then clicking anywhere on the sky display will tell the telescope to go to that point. ii. Track – After choosing an object to observe with the cursor, choosing this option will cause the telescope to follow that object for a specified time. iii. RA/Long Map – This option creates a “map” of your object by tracing out sequential paths above and below the object that are parallel to Declination while creating a map that is parallel to Right Ascension. iv. Dec/Lat Map – This is like the RA/Long Map, but the paths of the scan are reversed. v. Daisy – In this scan, the telescope crosses the center of the object 10 times while also scanning around the object in a pattern that looks like a daisy. vi. Abort – This command stops any scan that is currently in progress. vii. Stow Zenith – This command tells the telescope to park itself pointing straight up toward the zenith. This is typically done at the end of any scan. viii. Stow East and Stow West – These stow positions are useful in certain weather conditions. Stow zenith unless you have reason to do otherwise. E. Sky Viewer Settings: In this box, you can toggle the various objects and coordinate systems that are available in the display. At the bottom, it also tells you the current position of your cursor in both Right Ascension/Declination and Elevation/Azimuth. i. Record Path – This command tells the viewer to drag a red line behind the telescope so that you can see where you have been recently. ii. Show Path – This command allows you to turn that path off or on. Sometimes, the path can Interfere with your ability to see an object that you might want to observe. iii. Show Az/El Grid – This grid is fixed relative to the ground position of the telescope so that the Elevation when the telescope is stowed at the zenith is 90°. Azimuth is measured in 360° around the telescopes axis. iv. Show RA/Dec Grid – This creates a Right Ascension/Declination grid. Right Ascension is measured in 24 hours from the Vernal Equinox and Declination is measured in degrees, positive and negative, from the celestial equator. This grid shows you the actual position of objects relative to the sky. v. Show Galactic Plane – Here, you can turn on and off the galactic plane line. vi. Bright Source Catalog – Checking this box will populate the display with radio-bright objects, man of which are contained in the Third Cambridge Catalogue of Radio Sources. The objects move in real- time. vii. Messier Catalog – This will display the population of the 110 Messier Objects that are visible in the sky at Green Bank. The objects move in real-time.

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viii. Solar System Catalog – This will show the major solar system objects visible in real-time from Green Bank. Take care when observing the Sun. ix. Snap to Catalog – When checked, this box forces the selection of any bright objects near the location that the user has chosen. x. Zoom To Fit – Using your cursor, it is possible to zoom in on the display by drawing a box starting with the upper left-hand corner. When you are done zooming, clicking this button restores the display. F. Weather Override – Only the astronomer hosting your session or the telescope operator on duty should ever check this button. G. The display has already been mentioned quite a bit. However, one other thing to notice about it is the scale bar in the upper left hand corner. This gives you an idea, in degrees, arcminutes, and arcseconds (depending on the zoom level), how large the area is that you are viewing. It is often very helpful in setting the parameters, such as the radius, of daisy scans or map scans. H. Finally! This is the most exciting part of the entire interface. It is where you can see your raw data coming in real-time. There are two axes. The X-axis displays time in seconds while the Y-axis displays the intensity of the signal in a unit called “counts”, which is the recorded number of photons that have been collected by either the left or right polarization filters. To convert counts to Kelvins (temperature) or even to Janskys (flux density), refer to Appendix B.

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Appendix A: Contact Information

NRAO Green Bank:

Glen Langston: Astronomer Email: glangsto@nrao.edu Phone: (304) 456-2224 Sue Ann Heatherly: Director of Educational Programs Email: sheather@nrao.edu Phone: (304) 456-2209 SKYNET – University of North Carolina, Chapel Hill Josh Haislip: Programmer, Designer, Webmaster Email: jhaislip@gmail.com

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Appendix B: Converting Data to Temperature or Flux Density

When analyzing data from the 20m, your may need to import ASCII data into Excel and convert the given left and right intensities (in counts) to kelvins or Janskys in order to conduct certain analyses. To convert to Kelvins: 1) The intensities in the ASCII file are total intensities, meaning the object intensity and the system intensity. You need to subtract the system values for the left and right polarizations (which can be found in the corner of the 2D plot) and then use the following conversion factors that can be found in an Excel table on the website. To convert to Janskys: Use the following equation (re-arrange as necessary):

€

S =2σTAAηA

Where: S = Flux Density (Janskys) σ = Stefan-Boltzman constant (1.38 x 10-23) TA = Average temperature (Left, Right polarization, in K) ηA = Telescope Efficiency (elevation dependent, available on website) A = Area of dish (available on website)

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Appendix C: The History of the 20m Green Bank/Skynet Radio Telescope The 20m telescope, named for the diameter of its dish, was constructed by Radiation Systems Inc. (RSI) and

completed in 1994. For most of its life, it was funded and run by the US Naval Observatory (USNO) as part of a

network of telescopes. This network, the National Earth Orientation Service (NEOS), is tasked with providing the

most up-to-date information on changes in the Earth’s axial rotation (US Naval Observatory, 2012). In order to do

this, five measurements are required, two angles which identify the direction of the Earth’s rotation axis within the

Earth, and angle describing the rotational motion of the Earth, and two angles which characterize the direction of

Earth’s rotation axis in space. With these data, the Earth’s orientation in space can be fully characterized. This

information is vital for navigators, geodesists, and anyone involved in accurate timekeeping. This information can

also be used by scientists attempting to determine accurate positions of satellites relative to the Earth’s surface,

which is of key importance for GPS determination, LANDSAT accuracy, and a host of other programs, both private

and governmental. In this capacity, the 20m was also a part of the USNO’s Navy Network (NAVNET) program in

conjunction with International Earth Rotation Service (IERS) and with NASA’s Space Geodesy Program (20 Meter

Geodetic Telescope)

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Appendix D: Important Definitions

Term Definition

Absorption Spectrum A continuous spectrum missing a series, or signature, of spectral line wavelengths unique to the atom being observed.

Arc Minute A measure of angular separation, - one sixtieth of a degree.

Azimuth Position of the telescope with respect to north. Measured in degrees, minutes, and seconds of arc

Back End The equipment, including computers, Analog to Digital converters, and tuners that aid in the processing of radio signals by the operator.

Bandwidth The difference between the highest and lowest frequencies to which a receiver is sensitive.

Beam Width The angle within which an antenna receives radio waves.

Cambridge Catalog of Radio Sources

An astronomical catalogue of celestial radio sources detected at 159 MHz and 178 MHz. It was published in 1959 by the Radio Astronomy Group of the University of Cambridge. References to entries in this catalogue in the scientific literature use the prefix 3C followed by the entry number, with a space, e.g. 3C 273.

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Continuous Spectrum A spectrum having no lines or bands, especially a spectrum of radiation distributed over an uninterrupted range of wavelengths.

Counts

A measure of the intensity of a signal created by photons bouncing off of the dish and into the receiver. This unit can be converted to a temperature (K) and subsequently to Janskys (intensity per unit area).

Cyclotron Radiation Electromagnetic radiation emitted when charged particles are moved within a magnetic field at non-relativistic speeds (i.e., not close to the speed of light).

Daisy Scan A type of scan which move the telescope in a "daisy" pattern, repeatedly over the object being observed so that both signal and noise can be effectively distinguished.

Declination Celestial Latitude, referenced by the Earth's equator projected onto the sky. Measured in positive or negative degrees, minutes, and seconds of arc.

Dish The parabolically shaped portion of a radio telescope. Telescopes are often named by the size of their dish.

Doppler Shift

The change in length of a wave (light, sound, etc.) due to the relative motion of source and receiver. Things moving toward you have their wavelengths shortened. Things moving away have their emitted wavelengths lengthened.

Electromagnetic Spectrum

A collective term for radiation consisting of oscillating electric and magnetic fields. It includes from shortest to longest wavelengths (gamma rays, X-rays, ultraviolet, visible light, infrared, microwave, and radio waves).

Electron A negatively charged particle commonly found in the outer layers of atoms. The electron has only 0.0005 the mass of the proton.

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Electron Polarization

A property of electrons defined by the Pauli Exclusion Principle but simply described as up or down, or left and right. Radio telescopes are equipped with polarizing filters that distinguish between the two electron polarizations.

Electron Spin

A property of electrons defined by the Pauli Exclusion Principle but simply described as up or down. Electron spin is based upon rules governed by the size of the atom and, as such, by the number of electrons in the atoms outer shell.

Elevation Position of the telescope with respect to the horizon. Measured in degrees, minutes, and seconds of arc

Emission Spectrum A spectrum created by the emission of photons by excited, or hot, gases that results in a series of lines at wavelengths particular to each individual atom.

Frequency Number of wave vibrations per second; 1 Hertz is one cycle per second (e.g., 1420 MHz = 1,420,000,000 vibrations per second).

Front End A term used to describe the dish, receiver and any other equipment mounted on the antenna itself.

Gain The ratio of the maximum power that can be extracted from the receiving antenna to the power delivered to the back end of the system. It can be thought of as "volume".

Gigahertz A frequency of one billion cycles per second.

Globular Cluster A spherical star cluster containing 50,000 to 1 million stars; generally old and metal-poor, globular clusters may be the left over building blocks of galaxy formation.

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Hertz A measure of the number of oscillations per second. 1 Hz = 1 cycle or oscillation/second.

Interference

For radio telescopes, this typically means unwanted signals, noise, or static. It also describes the result of combining the signals that two telescopes receive when observing the same source, which results in a pattern of oscillating values or "fringes" that depends on the separation of the two telescopes.

Ionosphere The outer layers of the Earth's atmosphere (above 50 miles from the surface of the Earth), where many of the gas atoms are ionized by high-energy extraterrestrial radiation.

Jansky A unit of measurement of flux density equal to 10-26 Watts / meter2 / Hz, named after the radio astronomy pioneer Karl Jansky.

Kelvin

A scale that measures an object's temperature over absolute zero, the theoretical coolest temperature where all molecular and atomic motion ceases. On the Kelvin scale, the freezing point of water is 273 (273 K = 0o C = 32o F).

Light Waves Electromagnetic radiation at light frequencies (about 1015 cycles per second), or a wavelength between 400 and 700 billionths of a meter.

Map Scan

A type of scan that tells the antenna to move in parallel lines that cross the source a number of specified times to created a detailed "map" of the observed region. These can be done parallel to right ascension or parallel to declination.

Megahertz A frequency of one million cycles per second.

Messier Catalog

A catalog of bright sources compiled by Charles Messier in the 18th century that were detractors from his search for new comets. There are 110 such objects total and they include galaxies, star cluters, and nebulae.

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Microwaves A form of electromagnetic radiation that has a band of wavelengths stretching from 0.1m to 0.0001m and with frequencies that range from 1010Hz to over 1012Hz.

Nanometer A unit of length in the metric system, equal to one billionth of a meter.

Narrow Band

A somewhat subjective term used to describe a narrow range of radio frequencies. The FM band is considered to a be narrow band when discussed by radio astronomers, as it consists of a freqency range of only 20MHz.

Nebula A stellar nursery. A cloud of primarily hyrogen gas in which new stars are born. These are different than planetary nebulae, which are produced in the death throes of medium mass stars.

Noise The effects produced by random electrical fluctuations in radio receivers. These tend to conceal or distort the effects of true celestial radio power, which also has the character of noise.

Open Cluster An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. They are located within the disk of the galaxy.

Pauli Exclusion Principle

The quantum mechanical principle that no two identical fermions (particles with half-integer spin) may occupy the same quantum state simultaneously.

Photon A discrete packet of electromagnetic energy.

Planetary Nebula The gaseous outer layers of a star that have been ejected into space as the star collapses into a white dwarf. The ultraviolet radiation from the white dwarf causes the gas to fluoresce.

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Radio Waves

Electromagnetic radiation at radio frequencies (10,000 Hz to 300x109 Hz). Radio signals enter a receiver as photons of light emitted from astronomical, or local, sources and are recorded according to their intensity and polarization. This intensity is interpreted by the analog portion of the system as a voltage and can be converted to a digital signal by equipment at the back end of the system.

Radio Window The property of Earth's atmosphere that allows certain wavelengths of electromagnetic radiation in the radio range to pass through.

Receiver An electronic device that amplifies, detects, and gives a measure of the intensity of radio signals.

Reflector The dish of a radio telescope.

Resolution

The ability of a telescope to show detail. Also known as resolving power. One common way to describe the resolution of a telescope is to state the minimum angular separation at which a double star, whose two components are fairly bright and have very nearly the same brightness, can be distinguished as two separate stars.

Right Ascension Celestial Longitude, starting at the vernal equinox and running through the north and south celestial poles. Measured in hours, minutes, and seconds of arc.

Sagittarius A The powerful radio source located at the center of the Milky Way Galaxy.

Slew A term used to describe the moving of a radio telescope toward the location of observation.

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Spectral Line Mode An observation mode in which the telescope's spectrometer is set to detect certain defined individual spectra.

Spectral Lines

Light given off at a specific frequency by an atom or molecule. Every different type of atom or molecule gives off light at its own unique set of frequencies. Thus, astronomers can look for gas containing a particular atom or molecule by tuning the telescope to one of the gas's characteristic frequencies. For example, carbon monoxide (CO) has a spectral line at 115 Gigahertz (or a wavelength of 2.7 mm).

Spectrometer an instrument used for measuring spectral wavelengths

Spectrum A plot of the intensity of light at different frequencies. Or the distribution of wavelengths and frequencies.

Spectrum Mode An observation mode in which the telescope's spectrometer is set to detect a band of spectra.

Stow

The rest position that a radio telescope is placed in. This position is typically pointed at the zenith, or at a position at which the structure is balanced and there is the least possible mechanical strain under the force of gravity.

Synchrotron Radiation Radiation emitted by charged particles being accelerated in magnetic fields and moving at near the speed of light.

Track Scan A type of scan where the telescope points directly at an object and follows it for a specified length of time.

Wavelength The distance between two adjacent crests of a wave motion. For electromagnetic radiation, the product of frequency and wavelength is equal to the speed of light.

Wide Band A specific range of electromagnetic wavelengths or frequencies, as those used in radio broadcasting.

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Works Cited 20 Meter Geodetic Telescope. (n.d.). Retrieved 6 22, 2012 from National Radio Astronomy Observatory: https://science.nrao.edu/facilities/gbt/other-telescopes/20meter Nave, R. (2012, 7 5). Visualizing Electron Orbitals. Retrieved 7 5, 2012 from Georgia State University Department of Physics: hyperphysics.phy-astr.gsu.edu/hbase/chemical/eleorb.html SKYNET. (n.d.). Retrieved 6 22, 2012 from SKYNET: skynet.unc.edu US Naval Observatory. (2012, 1 5). Retrieved 6 22, 2012 from IERS Rapid Service/Prediction Center: http://maia.usno.navy.mil/