Astronomy (ASTR-101) Notes
written by Bianca Agatha
These are just notes for me to memories and keep up with stuff.
Last Updated
05/31/21
Chapters
5
Reads
653
Lesson Two
Chapter 2
I* A telescope is an optical instrument that magnifies a distant object and makes it appear brighter. They are astronomy’s most important tool and were used to discover Uranus, Neptune, and Pluto. Muggle astronomers have discovered, and continue to discover, new asteroids, comets, stars and galaxies (collections of stars), moons, and even planets orbiting other stars. By examining the colour of light from distant galaxies with the 100-inch-wide Hooker telescope, then the biggest one in existence, an American astronomer Edwin Hubble was able to conclude that the universe is expanding, initiating the branch of astronomy called cosmology, which studies the origin and evolution of the universe. More information about Hubble can be found here: https://en.wikipedia.org/wiki/Edwin_Hubble
* The earliest telescopes had two lenses at opposite ends of a tube. At the far end of the tube is the light-gathering lens, called the objective lens. This lens is convex – that is, thicker in the middle than at the edges – like a magnifying glass and is called a positive lens. Distant objects seen through a magnifying glass on its own appear blurry. To make them appear sharp, you need another lens at the near end of the tube, called the eyepiece. In the earliest telescopes, the eyepiece was concave – that is, thinner in the middle than at the edges – and was called a negative lens. This was the design of the first Muggle telescope, invented by Hans Lippershey, a Dutch eyeglass maker, in 1608. It made distant objects look three times as big as with the naked eye. If you want to build a telescope of this kind, instructions can be found here: http://galileo.rice.edu/lib/student_work/astronomy96/mtelescope.html
* Two years later, Galileo Galilei, a famous astronomer, improved on that design. He found that by making the objective lens less curved, he could improve the magnifying power from three to 20, making it a more useful astronomical tool. He used it to discover the four largest moons of Jupiter, which are therefore called the Galilean moons. He also discovered that Venus has phases like the Moon, confirming Copernicus’s belief that the Earth revolves around the sun rather than the other way around. Any telescope whose eyepiece is a negative lens is now called a Galilean telescope.
* One problem with the Galilean telescope is that it has a very narrow field of view, so you can only see a very small part of the sky with it. Johannes Kepler found that if the eyepiece is also a positive lens, you can see much more of the sky. Everything you look at appears upside down, but astronomers don’t care much about that because they can easily adjust to it; this type of telescope is called an astronomical telescope. People who want to look at things on Earth, don’t want either of those disadvantages, so they use what is called a terrestrial telescope. In that sort of telescope, the eyepiece has two positive lenses; the one nearest your eye turns what you see right side up again.
* In the newer models there are two tubes instead of one, a wider tube containing the objective lens, and a narrower tube, which can be slid in or out, containing the eyepiece. In the terrestrial telescope, the magnifying power can be adjusted too - the farther apart the two lenses of the eyepiece are, the greater the magnifying power. With the astronomical type, you have to change the eyepiece in order to change the power, but astronomers are prepared to do that rather than have an extra lens, which absorbs a bit of the precious light they need for their observations.
* Telescopes that use only lenses are called refracting telescopes, as lenses refract light. Refracting telescopes of any design have a problem: the objects seen at the edge of the field of view appear to have colour fringes because of the way light bends differently along the edges of the glass. Modern refracting telescopes use several lenses in the eyepiece to solve that problem, but in 1688, Isaac Newton solved it by using mirrors instead of lenses – he invented the reflecting telescope, so called because mirrors reflect light – so telescopes that use his design are called Newtonian telescopes.
* The amount by which a telescope magnifies distant objects is called the telescope’s power. The weaker the objective lens or mirror is and the stronger the eyepiece is, the more powerful the telescope will be. Aside from magnifying things, astronomers want to see things that are too dim to be seen with the naked eye, and the bigger the objective lens or mirror is, the more light it will gather. Suppose you double the diameter (the width) of the objective lens or mirror. Will it gather twice as much light? Nope! The amount of light it gathers depends on its area, not its diameter. You’re making the lens twice as wide and twice as long: 2 x 2 = 4, so it will gather four times as much light. If you triple the diameter: 3 x 3 = 9.
* If you take any number and multiply it by itself, you get the square of that number. The square of 1 is 1, the square of 2 is 4, the square of 3 is 9, and so on. The square of a number is represented by a superscript 2.
* Another advantage of making the objective lens or mirror bigger is that it improves the resolution of the telescope – that is, how close together two points of light can appear to be and still be seen as two distinct points instead of one. If one of the two points of light is between you and the other one, they can be trillions of miles apart and still appear to be close together, whereas if you are between them, they could be close to you and still appear far apart. The observed closeness of two points of light is measured as an angle, not a straight-line distance.
* The ancient Greeks divided the circle into 360 degrees. If one star is on the eastern horizon and another one is on the western horizon, they are half a circle – 180 degrees – apart. If they are 1/180th of that distance apart, then they are one degree apart, and you would only have to move your eyes a little bit to move from the first to the second. Now, someone with average vision can, at best, distinguish two points of light about 1/20th of a degree apart with the naked eye. Rather than writing many tiny fractions of a degree to describe the observed closeness between two stars, they use even smaller units known as arcminutes and arcseconds. A degree is divided into 60 arcminutes and an arcminute is divided into 60 arcseconds.
* If you double the diameter of the objective lens or mirror, you double the resolution – that is, you can resolve two stars that appear twice as close together. If the diameter of a lens or a mirror is about 12 centimeters, the resolution is about one arcsecond. Telescopes are getting bigger and bigger; the biggest one so far is the Keck telescope, 10 meters in diameter. Can it resolve two stars that are 0.012 arcseconds apart? Not without a very expensive trick. The problem is that movement of the air makes the stars appear to move around (and twinkle too), making it hard to achieve a resolution much better than one arcsecond no matter how big the telescope is. Large modern telescopes solve this problem by using what is called adaptive optics, in which the mirror deforms hundreds of times per second to compensate for the apparent movement of the stars. But there is another solution to this problem: putting your telescope above the atmosphere by launching it into orbit around the Earth, which brings us to our next tool used by Muggle astronomers.
* The earliest telescopes had two lenses at opposite ends of a tube. At the far end of the tube is the light-gathering lens, called the objective lens. This lens is convex – that is, thicker in the middle than at the edges – like a magnifying glass and is called a positive lens. Distant objects seen through a magnifying glass on its own appear blurry. To make them appear sharp, you need another lens at the near end of the tube, called the eyepiece. In the earliest telescopes, the eyepiece was concave – that is, thinner in the middle than at the edges – and was called a negative lens. This was the design of the first Muggle telescope, invented by Hans Lippershey, a Dutch eyeglass maker, in 1608. It made distant objects look three times as big as with the naked eye. If you want to build a telescope of this kind, instructions can be found here: http://galileo.rice.edu/lib/student_work/astronomy96/mtelescope.html
* Two years later, Galileo Galilei, a famous astronomer, improved on that design. He found that by making the objective lens less curved, he could improve the magnifying power from three to 20, making it a more useful astronomical tool. He used it to discover the four largest moons of Jupiter, which are therefore called the Galilean moons. He also discovered that Venus has phases like the Moon, confirming Copernicus’s belief that the Earth revolves around the sun rather than the other way around. Any telescope whose eyepiece is a negative lens is now called a Galilean telescope.
* One problem with the Galilean telescope is that it has a very narrow field of view, so you can only see a very small part of the sky with it. Johannes Kepler found that if the eyepiece is also a positive lens, you can see much more of the sky. Everything you look at appears upside down, but astronomers don’t care much about that because they can easily adjust to it; this type of telescope is called an astronomical telescope. People who want to look at things on Earth, don’t want either of those disadvantages, so they use what is called a terrestrial telescope. In that sort of telescope, the eyepiece has two positive lenses; the one nearest your eye turns what you see right side up again.
* In the newer models there are two tubes instead of one, a wider tube containing the objective lens, and a narrower tube, which can be slid in or out, containing the eyepiece. In the terrestrial telescope, the magnifying power can be adjusted too - the farther apart the two lenses of the eyepiece are, the greater the magnifying power. With the astronomical type, you have to change the eyepiece in order to change the power, but astronomers are prepared to do that rather than have an extra lens, which absorbs a bit of the precious light they need for their observations.
* Telescopes that use only lenses are called refracting telescopes, as lenses refract light. Refracting telescopes of any design have a problem: the objects seen at the edge of the field of view appear to have colour fringes because of the way light bends differently along the edges of the glass. Modern refracting telescopes use several lenses in the eyepiece to solve that problem, but in 1688, Isaac Newton solved it by using mirrors instead of lenses – he invented the reflecting telescope, so called because mirrors reflect light – so telescopes that use his design are called Newtonian telescopes.
* The amount by which a telescope magnifies distant objects is called the telescope’s power. The weaker the objective lens or mirror is and the stronger the eyepiece is, the more powerful the telescope will be. Aside from magnifying things, astronomers want to see things that are too dim to be seen with the naked eye, and the bigger the objective lens or mirror is, the more light it will gather. Suppose you double the diameter (the width) of the objective lens or mirror. Will it gather twice as much light? Nope! The amount of light it gathers depends on its area, not its diameter. You’re making the lens twice as wide and twice as long: 2 x 2 = 4, so it will gather four times as much light. If you triple the diameter: 3 x 3 = 9.
* If you take any number and multiply it by itself, you get the square of that number. The square of 1 is 1, the square of 2 is 4, the square of 3 is 9, and so on. The square of a number is represented by a superscript 2.
* Another advantage of making the objective lens or mirror bigger is that it improves the resolution of the telescope – that is, how close together two points of light can appear to be and still be seen as two distinct points instead of one. If one of the two points of light is between you and the other one, they can be trillions of miles apart and still appear to be close together, whereas if you are between them, they could be close to you and still appear far apart. The observed closeness of two points of light is measured as an angle, not a straight-line distance.
* The ancient Greeks divided the circle into 360 degrees. If one star is on the eastern horizon and another one is on the western horizon, they are half a circle – 180 degrees – apart. If they are 1/180th of that distance apart, then they are one degree apart, and you would only have to move your eyes a little bit to move from the first to the second. Now, someone with average vision can, at best, distinguish two points of light about 1/20th of a degree apart with the naked eye. Rather than writing many tiny fractions of a degree to describe the observed closeness between two stars, they use even smaller units known as arcminutes and arcseconds. A degree is divided into 60 arcminutes and an arcminute is divided into 60 arcseconds.
* If you double the diameter of the objective lens or mirror, you double the resolution – that is, you can resolve two stars that appear twice as close together. If the diameter of a lens or a mirror is about 12 centimeters, the resolution is about one arcsecond. Telescopes are getting bigger and bigger; the biggest one so far is the Keck telescope, 10 meters in diameter. Can it resolve two stars that are 0.012 arcseconds apart? Not without a very expensive trick. The problem is that movement of the air makes the stars appear to move around (and twinkle too), making it hard to achieve a resolution much better than one arcsecond no matter how big the telescope is. Large modern telescopes solve this problem by using what is called adaptive optics, in which the mirror deforms hundreds of times per second to compensate for the apparent movement of the stars. But there is another solution to this problem: putting your telescope above the atmosphere by launching it into orbit around the Earth, which brings us to our next tool used by Muggle astronomers.
