1 Radio Astronomy and the NRAO Phil Jewell Assistant Director for Green Bank Operations National Radio Astronomy Observatory VIP Visit to Green Bank 8 May 2004

2 Presentation Overview l What is Radio Astronomy? l What is the National Radio Astronomy Observatory? l What are the differences between single dishes and Interferometers? l What makes Green Bank and the GBT special?

3 What is Radio Astronomy? l Radio waves are part of the electromagnetic spectrum, along with the infrared, visible light, ultraviolet, x-rays, and gamma rays. l Radio astronomy studies cosmic electromagnetic emission from about 10 MHz to 1000 GHz (wavelengths from about 30 meters to 0.3 mm.) From Living with a Star – NASA/UCB

4 What can we do with Radio Astronomy? l Radio waves have lower energy than light waves, x-rays, etc. Whereas light waves come from very hot objects such as stars, radio waves usually come from cooler objects. Astronomers use the whole electromagnetic spectrum to probe the universe. l Because radio waves have long wavelengths, they pass through dust that obscure light waves. This allows radio astronomers to probe to the heart of star forming regions, to disks around the nuclei of galaxies, and possibly to the event horizons of black holes.

5 What sorts of things do Radio Astronomers Observe? l Solar System objects 6The Sun, planets, asteroids, and comets 6From their own emission, or in some cases, from radar echoes l The structure of the Milky Way and other galaxies. 6Can trace the gas and dust through the emission of atomic and molecular spectral lines l The star formation process 6Can use molecular spectral line and dust emission to trace the collapse of interstellar clouds l Neutron stars 6Extremely dense, collapsed stars that are rotating as fast as 1000 times a second. 6An incredible laboratory for basic physics that cannot be replicated on Earth.

6 What sorts of things do Radio Astronomers Observe? – cont’d l Black hole physics l Incredible energy engines in the nuclei of galaxies and quasars l The origin and structure of the universe and galaxies within it l Astrochemistry 6Almost 130 molecules – some quite complex -- are known in the interstellar medium 6Some are of biological significance and may be a key to the formation of life on the early Earth

7 What is the National Radio Astronomy Observatory? l Since 1956, the NRAO has been the premier radio astronomy observatory in the U.S. l It’s primary mission is to operate and develop unique radio astronomical telescopes and related instruments for use by researchers at universities and institutes in the U.S. and around the world. l The NRAO is headquartered in Charlottesville, Virginia and presently operates three main instruments: 6The Robert C. Byrd Green Bank Telescope (GBT) –Green Bank, West Virginia 6The Very Large Array (VLA) –Near Socorro, New Mexico 6The Very Long Baseline Array (VLBA) –10 telescopes in an array from St. Croix, US Virgin Islands to Mauna Kea, Hawaii l The NRAO is also in charge of the North American contribution to the construction and operation of the Atacama Large Millimeter Array (ALMA), an international project being built in Chile. l The ALMA North American Science Center will be in Charlottesville.

8 The Robert C. Byrd Green Bank Telescope

9 The Very Large Array 27, 25-meter diameter telescopes located on the Plains of San Augustin near Socorro, New Mexico. EVLA expansion project underway which will give x10 improvement in sensitivity.

10 The Very Long Baseline Array (VLBA) Mauna Kea Hawaii Owens Valley California Brewster Washington North Liberty Iowa Hancock New Hampshire Kitt Peak Arizona Pie Town New Mexico Fort Davis Texas Los Alamos New Mexico St. Croix Virgin Islands 10, 25-meter diameter telescopes for ultra-high angular resolution

11 Atacama Large Millimeter Array 64, 12-meter diameter dishes for millimeter and sub-millimeter- wave imaging, presently under construction in Chile.

12 NRAO Headquarters Facilities Charlottesville, Virginia HQ + NA ALMA Science Center (Edgemont Road on UVa Campus) NRAO Technology Center (Ivy Road)

13 Interferometers and Single Dishes l Radio Astronomy telescopes come in two basic flavors, single dishes and and arrays of dishes, known as interferometers

14 Interferometers l Interferometers – arrays of antennas that work together -- provide 6very high angular resolution 6imaging through earth rotation synthesis l Their angular resolution is set by the farthest distance between antennas l Modern interferometers have extraordinary high resolution imaging capability. Examples: 6NRAO Very Large Array -> EVLA 6NRAO Very Long Baseline Array 6Atacama Large Millimeter Array (ALMA)

15 Single Dishes l Single dishes are limited in their angular resolution by the size of the dish, but … l They are very sensitive to large scale emission that may be missed by interferometers, and l They have very high sensitivity to weak, extended emission l In addition, they allow 6Easy use of innovative instrumentation –Comparatively easy to build an instrument for one dish rather than 27 or 64 –So… single dishes and interferometers provide complementary information and capabilities, which is why the NRAO has both types of telescope.

16 Single Dishes and Interferometers -- Sensitivity to Structures on Different Angular Scales Example: 6The large scale structure of the GBT image of the Omega Nebula (M17) would not be detected by most interferometers. 6And conversely, an interferometer might be able to image a tiny site of star formation in this cloud that would be seen only as a point by a single dish.

17 Why is there an observatory in Green Bank? l The National Radio Astronomy Observatory was founded in 1956 l Green Bank was the first site of the Observatory and served as its first headquarters until 1967 when it was moved to Charlottesville. l Green Bank was chosen for its sheltered location and natural protection from radio frequency interference, yet proximity to the population centers on the east coast. l In the late 1950s, the region around Green Bank was given special protection by the Federal Communications Commission and became the National Radio Quiet Zone.

18 National Radio Quiet Zone Owing to the sensitivity of radio astronomy observations, the NRQZ is critical to the continued success of radio astronomy in Green Bank, and the GBT in particular.

The story of the GBT – The 300 Foot

The 300 Foot in collapse – Nov. 1988

21 What makes the GBT special? l Size l Unblocked main aperture l Precision Control System 6Active Surface 6Sensing systems l Frequency coverage l National Radio Quiet Zone location

22 GBT Size l Largest fully-steerable telescope in the world l At 16.7 Million Pounds (7600 metric tons), probably the largest moving structure on land. l Despite size and mass, built to extremely high precision l Why is the GBT so big? Sensitivity

23 Conventional optics with symmetric (blocked) feed supports Effelsberg 100 m TelescopeNRAO 140 Foot Telescope

24 Unblocked aperture 100 x 110 m section of a parent parabola 208 m in diameter Cantilevered feed arm is at focus of the parent parabola

25 GBT Pointing and Surface Compensation Systems l To overcome distortions in its reflecting surface that result from gravity and thermal changes, the GBT is equipped with a fully active surface (motor actuated, computer controlled) l Various temperature and position sensing systems and computer models will be used to point the telescope and command the surface actuators l Ultimately, this will allow the GBT to work to ~115 GHz or 2.6 mm wavelength

26 GBT active surface system l Surface has 2004 panels 6average panel rms: 68  m l 2209 precision actuators

27 One of 2209 actuators. l Actuators are located under each set of surface panel corners Actuator Control Room 26,508 control and supply wires terminated in this room Surface Panel Actuators

28 Discrete HI Clouds in the Galactic Halo Artist’s rendition of the Milky Way with actual GBT data in the inset – Lockman (2002).

29 Continuum Images of the Rosette Nebula Ghigo & Maddalena (2003)

30 GBT / Arecibo radar image of the Moon B. Campbell et al. (2004)

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