A gigantic radio telescope ALMA produces images of target objects by combining observation results obtained with 66 or more antennas spread over a wide area. Like ALMA, a radio telescope consisting of multiple antennas that receives radio waves from the same astronomical object is called "radio interferometer." Here, you can learn how the radio interferometer works and why so many antennas are needed for the observation with an interferometer.
Basics of Radio Telescopes
Angular resolution (capability to discern small objects) of a telescope is proportional to the diameter of the main reflector (objective lens). If we want to see the ring of Saturn more clearly, we need a lager-diameter telescope. As we use a greater telescope, such as the Hubble Space Telescope (optical telescope) with a 2.4-meter main reflector, we can see more detailed and clearer images of astronomical objects.
In case of a radio telescope too, it is also necessary to increase the diameter of the main reflector (antenna) to see distant objects more clearly. The problem here is that we have to prepare a gigantic telescope of 2.4m×1000=2400m in diameter if we want to achieve an angular resolution equivalent to that of the Hubble Space Telescope at submillimeter wave whose wavelength is about 1000 times longer than that of visible light since the resolution is determined by the wavelength divided by the diameter.
But actually it does not seem feasible to construct such a gigantic telescope on earth: the solution for this is the aperture synthesis technique using interferometry.
Observation data of a radio telescope shows the frequency and the signal strength of the radio waves received by the antenna, not a beautiful image of an astronomical object. Radio wave images like a thermogram that you are familiar with are produced by collecting and analyzing enormous volume of data.
How the Interferometer Works <1> Identifying the Source Position
Radio interferometer is a technique to achieve high resolution by making a single virtual telescope with a diameter equivalent to the distance between two antennas located in a distance. Using a radio interferometer with high resolution, we can identify the position of the source.
Assume that two antennas are located in a distance and directed to an astronomical "object X". You may think that two antennas directed to the same direction would receive the same observation data, but actually their received data are different. What is the difference between them?
Since the light and radio wave travel through space at the same speed, the radio waves from the "Object X" will reach the two antennas at the same time if they are located at the equal distance from the object. When the two antennas are separately located as shown in the left figure, "Antenna A" becomes farther from the object by the distance equal to the length of the "Side C", and receives the radio wave later than "Antenna B". In short, the radio wave emitted by the "Object X" at a certain moment reaches the two antennas with a little time difference.
The radio waves received by the two antennas are converted to digital signals, and the closest waveforms are found by superposing crests and troughs of the two waves. The wave is strengthened when superposing crests of the two waves, while the wave is weakened when superposing a crest and a trough of two waves. This is called interferometry.
When we obtain the time difference between the two antennas, we can calculate the length of the "Side C" accordingly. With the "Distance D", the "Angle E" to the Object "X" is also calculated.
At this point, we have only obtained an angle from one direction. By repeating the same calculations with various pairs of antennas located in different points and synthesizing the data, we can identify the position of the "Object X" more accurately.
How the Interferometer Works <2> Producing images with Aperture Synthesis
Suppose a digital camera with extremely high resolution that is only capable of taking a detailed image of a part of an object: for example, only pores of skin when photographing your face. Of course, such a digital camera is totally useless. Similarly, when using an interferometer consisting of just two antennas, we can obtain the resolution equivalent to a telescope that has a diameter equal to the distance between the antennas, but we cannot image the entire shape of the object.
Obviously, ALMA has adopted several techniques to solve this problem: one of them is observing the target object in various antenna configurations using the rotation of the earth.
Even with only two antennas, it is possible to obtain a resolution equivalent to that of a giant single antenna by synthesizing multiple observation results acquired in various configurations (distances and locations) of the two antennas; however, we have to change the antenna configuration after each observation in order to obtain a detailed image, and it takes much time and energy to conduct an observation.
A solution for this is the aperture synthesis technique using the rotation of the earth invented by Martin Ryle, for which he was awarded the Nobel Prize in Physics.
When seen from the "Object X", the earth appears to rotate three-dimensionally, which means 66 or more ALMA antennas on the earth are changing their positions along with the rotation of the earth. In this concept, we can regard the areas covered by the antenna pairs moving along with the rotation as a single giant virtual telescope with a higher resolution than that achieved by just placing 66 antennas.
Since observations with ALMA are conducted by spreading 66 or more antennas over the area up to 18.5 km, we can obtain a two-dimensional radio image within about 10 minutes. If taking further time (about 8 hours for an observation), we can obtain an image with extremely high resolution. The aperture synthesis technique using the rotation of the earth will improve the image quality dramatically and show us the unknown universe with surprising clarity.