How ALMA Works Vol.1: What is the Resolution of a Telescope?
──Let’s start from the most basic of the ALMA telescope. The first question is: what is a radio telescope?
Hiramatsu: ALMA is a radio telescope that receives radio waves coming from the Universe. When you look up at the night sky, you can see stars. The light emitted from stars travels a long way and reaches our eyes. Actually, the celestial bodies radiates not only visible light, but also other electromagnetic waves including radio waves, infrared, ultraviolet, and X-rays. The radio telescope is designed to receive radio waves in particular.
Among various types of radio telescopes, ALMA is a telescope that utilizes a mechanism called “radio interferometer”, which consists of multiple antennas to make a single giant telescope. We are not going into details of the radio interferometer at this point. It could be too confusing. Let’s begin with the basic mechanism of the radio telescope.
──It’s already difficult to get the meaning of “receiving radio waves”.
Hiramatsu: It’s totally understandable, since radio waves are invisible. To make it easy to understand, I will explain it taking the human eyes as an example.
The parabolic antenna is the most distinctive component of the radio telescope. The antenna is directed to a target object and collects radio waves coming from the pointed area. In comparison with the human eye, the antenna has similar functions to the eyeball that moves toward the direction where the one wants to see, and to the crystalline lens inside the eyeball that serves as a lens to collect light. The important point is that the antenna only collects radio waves and it does not recognize them.
──Could you explain more?
In the human eye, light is perceived by the retina on the back surface of the eyeball. The lens just collects light. In the radio telescope, what corresponds to the retina is the receiver, which actually receives radio waves. Radio waves from the Universe reach the Earth over a period of tens of thousands or hundreds of millions of years. People can deal with them only after they are converted to electrical signals by the receiver.
──I see. The radio telescope has two separate parts: one is to collect radio waves and another is to detect them.
Hiramatsu: Actually, it has one more important part. In the case of the human eye, an electrical signal generated in the retina is sent to the brain through nerves. When the brain reads and interprets the signal, we recognize that we are seeing something. The mechanism of the radio telescope is the same. The electrical signals generated in the receiver are transmitted to a computer through optical fibers.
──In summary, the antenna serves as the eyeball and the crystalline lens, the receiver as the retina, and the computer as the brain.
Hiramatsu: Right. Most animals have two eyes. Light is first received by the eyes, and the received signals are combined and processed by the brain. This is how animals see an object. ALMA uses 66 antennas to receive radio waves, and the signals received by respective antennas are processed by computers to produce “data” that brings value to astronomy.
──While people have two eyes, ALMA has 66. Does it mean ALMA has better vision?
Hiramatsu: If the telescope has “good vision”, it means two things. One is that it can see even a dark object. Another is that it can recognize even detailed parts of an object. The former ability is called “sensitivity”, and the latter is called “resolution”.
──We have a vision test. Does this have something to do with those abilities?
Hiramatsu: In the typical Japanese eye chart, the Landolt ring is used as an optotype. It looks like the letter C. A tested person sees a ring with a gap in various directions and tells where the position of a gap is. This test is to measure the resolution of your eyes, in other words, it assesses to what details you can see. If one has poor vision, the person may not be able to recognize the gap of a ring as the upper and the lower ends of C look like being connected. This is an analogy of a telescope that is only capable of capturing unclear, blurry images of a target object. A person with poor vision is similar to a low-resolution telescope.
Hiramatsu: Astronomers aspire to look into further details of an object and seek for darker objects to study. To satisfy those needs, we need a telescope having improved sensitivity and resolution. And then, the solution is to make a big telescope.
──Is it possible to improve both sensitivity and resolution by making a larger telescope?
Hiramatsu: Yes. What is important is the diameter of the mirror or the lens that collects light or radio waves. The larger the diameter, the more light can be collected. And, it consequently makes dark objects more visible. We are not going into the principles of the resolution as they are very complicate. Please just remember that the resolution can be improved by making a larger telescope for now.
Hiramatsu: We can achieve better performance by having a larger telescope, however we cannot extend the diameter unlimitedly. The world’s largest radio telescope is the 500-meter diameter telescope called FAST, which has been recently completed by China. Its antenna was constructed on the terrain of a natural basin. Despite such a scale of the telescope being complete, astronomers wish to have an even larger diameter. The problem is that such a gigantic structure is almost impossible to support, and the telescope needs to be moved freely to observe objects in various directions of the sky.
──How about floating a big telescope in space?
Hiramatsu: I think it’s a good idea. But, considering the size of the International Space Station which is approximately 100 meters long, we have to launch a number of rockets to build a much bigger telescope. It costs too much money. Just an example, the Hubble Space Telescope is 2.4 meters in diameter. It was launched 30 years ago. But, even at that time, it was not a large-scale telescope. In spite of that, its total cost from the development to launch amounted to five billion dollars, and its operation cost for the subsequent 20 years added up to 10 billion dollars (about one trillion yen at current exchange rate). Transporting materials to space and put them into operations is extremely challenging not only in terms of money spent, but also in terms of technologies required.
──I see. Then, building it on the ground seems more feasible, but it is also difficult to build such a giant ground-based telescope because of gravity. It’s quite a dilemma, isn’t it?
Hiramatsu: To solve that dilemma, astronomers came up with an idea of making a single giant telescope by combining multiple telescopes. This technique is called radio interferometry.
Since the mechanism of the radio interferometer is a little bit complicated, it will be explained next time.