Radio Telescopes

109-115 Nestled in the hills south of Penticton, British Columbia and amid the wine growing region of the Okanagan Valley lies the Dominion Radio Astrophysical Observatory (DRAO). This is a world-class research facility that gives Canadian Astronomers a glimpse of the universe in the radio region of the electromagnetic spectrum. This facility, and ones like it around the world operate "24-7" because, unlike their optical counterparts, radio telescopes can operate night or day and even through inclement weather.

Figure 5.24 Part of the Dominion Radio Astrophysical Observatory's synthesis array.

Radio Telescopes - "Pros and Cons"

A radio telescopes collects and focuses radio waves to allow astronomers to "see" the universe in an entirely different way than that afforded by visible light.

Figure 5.25 shows the puzzling galaxy Centaurus A. This is a rollover image showing both the optical view (yellowish image) and a radio view (greenish). The object looks entirely different in either view but both views - and those from other parts of the electromagnetic spectrum are needed to understand what is happening in this galaxy. Radio telescopes have extended our view of the universe and provide us with a more complete view of the heavens.

Figure 5.25 Rollover image showing the peculiar galaxy Centaurus A in both optical and radio wavelengths.

Advantages of Radio Telescopes

There are at least five very important advantages that radio telescopes have over their optical counterparts:

1. 90% of the atoms in the universe are Hydrogen atoms and much of this is too cold to emit any energy in the visible wavelength region. However, and a topic we will discuss in Unit 5, cold hydrogen gas is continuously emitting radio waves in the 21 cm wavelength region and hence radio telescopes can peer into vast regions of space hidden from the "visible" world.
2. Because radio waves are long wavelength they are less affected by dust in the galaxy. This means we can see through dusty regions that are obscured from view. Radio telescopes can look inside collapsing clouds forming new stars or the center of our Milky Way galaxy.
3. Many different molecules emit characteristic radio wavelengths. Radio telescopes enable us to study the chemical nature of space around.
4. Magnetic fields can deflect moving charged particles and cause them to emit electromagnetic radiation that most often is in the radio region. Radio telescope give us "eyes' with which we can study the magnetic properties of our galaxy as well as distant galaxies.
5. Radio telescopes can operate night or day and are much less weather dependent than optical telescopes. This means that from a "science per dollar" perspective radio telescopes can be very efficient.

Disadvantages of Radio Telescopes

Radio telescopes are not with their drawbacks - there are three very significant problems:

1. Although radio telescopes operate day and night they are "bathed" in a steady barrage of radio waves generated on Earth by modern society. Everything from talk radio to television re-runs to electrical noise generated by motors and car ignitions creates a background of radio interference. To counteract that radio astronomers have lobbied regulatory agencies to prohibit societal use of certain radio wavelength regions and put radio telescopes in remote areas away from human radio interference.
2. Radio waves are long wavelength, low frequency forms of electromagnetic radiation. This means that a radio wavelength region photon carries very little energy (orders of magnitude less than its optical counterpart). For this reason very large collecting arrays are needed to amplify radio signals. The Dominion Radio Astrophysical Observatory telescope in Penticton is a dish 26 am in diameter. The world's largest steerable dish is the 100 m telescope in Greenbank, West Virginia. Radio telescopes are very large, massive structures that must be able to move with clockwork precision.
3. Even with the large dish sizes of radio telescopes, the diffraction effects considered in Chapter 5.2 severely limit the resolution of radio telescopes. This is because of the very long wavelengths of radio waves. The Penticton telescope, for example, working at 21 cm has a resolution of about 0.5 degrees which is the about the same as the apparent diameter of the moon. By itself, this telescope will not be able to resolve details finer than that.

How to "See" with a Radio Telescope

Unlike optical telescopes, you cannot see directly with a radio telescope. Instead, a radio telescope scans a portion of the sky and maps the radio signal strength as a function of position. In this way a contour map can be assembled. A good analogy of this is provided in Figure 5.26 in which ticket prices are shown as contours around an NHL hockey rink. The green, outer ring shows the lowest price tickets which, by analogy could correspond to the weakest radio signal. The bright yellow are the most expensive seats and these could correspond to regions of highest signal strength. Figure 5.27 shows a radio image of an explosive event on the surface of one member of the binary the star system RS Ophiuchi obtained February 2006. Red represent the brightest (most energy emitted) region while blue is the faintest.

Figure 5.26 Hockey rink analogy for how a radio contour image can be created by scanning a region of the sky.	Figure 5.27 Radio image of an explosive event on the star RS Ophiuchi (Courtesy NRAO)

As you will soon see, radio telescopes can be used in a very clever way to overcome their inherent "low resolution" and provide images many times sharper than the Hubble Space Telescope!

Very Long Baseline Interferometry

In 1968 Canadian astronomers combined the signals from the Penticton 26 m telescope with the now decommissioned 46 m radio telescope at Algonquin Park, Ontario. This was a historic first in astronomy for it effectively created a telescope with the resolution of a dish nearly 3000 km in diameter! At a frequency of 448 MHz this meant that the Penticton-Algonquin park telescope was able to resolve details down to about 0.05 seconds of arc - a resolution comparable to that of the Hubble Space Telescope.

Interferometry is the technique used by astronomers to combine signals from widely separated telescopes. It works because when electromagnetic waves from a distant source arrive at Earth they will encounter each telescope at slightly different times. When the signals from each telescope are added then the waves interfere with each other (hence the name interferometry) and by studying the interference patterns very precise resolution is achieved. Figure 5.29 illustrates this with an applet. As you drag the moveable telescope along the interferometer track the image of the supernova remnant 3C10 changes in "sharpness". Figure 5.30 shows the Very Large Array (VLA) operated by the National Radio Astronomical Observatories of the US and is situated in New Mexico. Figure 5.31 shows the supernova remnant 3C10 at fill resolution when the signals from the 27 dishes combine to create the equivalent of a dish 36 km in diameter. At this size the image has a resolution of about 1 second of arc - comparable to that achieved at major optical observatories.

Figure 5.29 Applet illustrating how varying the baseline between radio telescopes in an array can change resolution.

Figure 5.30 The Very Long Array (VLA) displaying the 27 dishes that can be moved along railway tracks which form a large Y-shaped array.	Figure 5.31 Tycho's supernova remnant as captured in the radio region by the VLA. Resolution is approximately 1 second of arc.

Currently the biggest operational array of radio telescopes is the Very Long Baseline Array which combines 10 radio telescopes across half of the globe from Hawaii and continental US to the Virgin Islands. Figure 5.32 shows a montage of the telescopes involved in this array.

Figure 5.31 THe VLBA or Very Long Baseline Array of 10, 25 m dishes.

With a baseline of more than 8000 km the telescope has a resolution comparable to your being able to stand in Montreal and read a newspaper in Vancouver!

Looking to the Future - Alma

High in the Chilean Andes, in the heart of the Atacama desert is an ambitious project that will bring unprecedented resolution and sensitivity to astronomy. This is an international project - the most expensive and sophisticated ever launched in ground-based astronomy and Canada is a key player in this.

Figure 5.32 shows an artist's conception of what the Atacama Large Millimeter/submillimeter Array (ALMA) will look like, the inset photo is of Professor Chris Wilson of McMaster University - Canadian Project Scientist for ALMA.When operational in 2013 ALMA will provide the highest resolution ground based images ever seen. It is also operating in the millimeter to sub-millimeter range which is the upper edge of the radio window.

Figure 5.32 The ALMA telescope with inset of Canadian Project Scientist, Professor Christine Wilson of McMaster University. (Artist's conception courtesy of NRAO)

The following is a video clip that explains in greater detail the scope and impact of ALMA.

Figure 5.33 Video Clip providing detail on ALMA and the science it will produce (clip courtesy of NRAO)

Practice

To understand how radio telescopes work and have extended our understanding of the cosmos

Chp 6.3

The James Clerk Maxwell Telescope (JCMT) is a radio telescope jointly operated by the UK, Canada and the Netherlands. It works in the microwave region and is situated on the top of Mauna Kea - next to the Canada France Hawaii telescope.