For eternity, we have looked to the heavens and wondered what secrets they hide. Ever since Galileo Galilei turned a primitive telescope to the skies in 1609, continuous innovation of this instrument has shed light on the farthest reaches of our solar system and beyond. Among many groundbreaking discoveries with optical telescopes, young innovators started experimenting with different types of light, and ultimately created the radio telescope. Late last year, one of the largest radio telescopes in the world, the Arecibo telescope, collapsed. As I read deeper into the topic, I could not have been any more surprised by how different radio telescopes are from optical telescopes. In this post, we will take a close look at the key differences between optical and radio telescopes.
Optical telescopes use a combination of lenses and mirrors to capture and magnify incoming visible light. The magnifying power of an optical telescope depends on the diameter of the primary mirror (primary light-gathering surface). After light reflects off the primary mirror and through a series of smaller mirrors, a powerful optical sensor with millions of pixels will capture incoming light and convert it into an image. Radio telescopes work on completely different principles.
The name radio telescope is a bit of a misnomer. While the radio part is correct, the telescope part is like calling a microphone a sonic camera. A radio telescope has 4 parts: the dish, the antenna, a receiver, and an analyzer. The parabolic dish captures and reflects radio waves (light with a long wavelength that is invisible to humans) to a single focal point. The antenna is positioned at this focal point and collects radio waves. The receiver reads the radio waves and converts them to electrical signals. This signal is processed by a powerful analyzer, to extract useful information and compose a pixel. Fun fact: this is actually the same technology that your cell phone works on, just a lot more sophisticated! Now you know a little bit about how you message your friends and family every day.
You may have noticed that I used the word pixel instead of image when describing a radio telescope’s output. A radio telescope can only produce a small fraction of an image each scan. In fact, radio telescopes rarely produce a full image. So by now you must be wondering: why on Earth would anyone use a radio telescope over an optical telescope? There are several key reasons why.
Radio telescopes have a much greater range than optical telescopes. Optical telescopes rely on massive mirrors and lenses to capture an image. Even the slightest defect in these components will lead to the entire telescope being useless. A radio telescope, on the other hand, can be built out of almost any reflective metal. Due to this, a radio telescope’s dish can be far bigger than the mirrors of an optical telescope. In addition to the bigger dish, the receiver of a radio telescope is millions of times more sensitive than those of an optical telescope. This leads to a wide range; one that is greater than the range of even the largest of optical telescopes!
Along with greater range, radio waves pass through cosmic dust while optical telescopes require near-perfect conditions to function. This is because visible light has a wavelength of 450- 750 nm (nanometers). Dust particles, both atmospheric and cosmic, have similar dimensions as to the wavelength of visible light. This limits an optical telescope’s visibility. Radio waves have a wavelength that is between one millimeter and several hundred meters. At this scale, dust particles are practically transparent to radio telescopes.
In a radio telescope, all of the information from a scan is recorded on a single pixel. But what’s the point of having such a low-resolution image? How can you see anything? You can’t. To get higher resolution images, you would have to take multiple scans and compile them (put them all together). Yes, this is time-consuming and produces a lower resolution image than one from an optical telescope. But! It is possible to see celestial bodies that would be too dim to notice with an optical telescope. For example, radio telescopes have allowed us to see into the center of galaxies. Remember that big news article about our first photo of a black hole two years ago? Well, it was radio telescopes that helped capture that amazing photo you see below.
The first image of a black hole, taken by the EHT collaboration (source)
In recent history, both optical and radio telescopes have improved greatly. While radio telescopes have certain benefits over optical telescopes, both have their individual uses. In the future, both radio telescopes and optical telescopes will expand our view of the universe, just as their predecessors did before them.
Post By: Armaan G.
Photo Credits: CSIRO Science Image, link
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