Light and Telescopes - Revision history

WikiSysop: /* Telescopes */

2013-01-30T08:13:44Z

Telescopes

← Older revision		Revision as of 08:13, 30 January 2013
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	We have a ~~refracting~~ telescope set up in the lab pointed at an artificial double star illuminated by a bright lamp. It takes a few minutes for the lamp to reach full brightness, so if it is not on you may want to turn it on now and come back to this later. If it is on, then do not turn it off so that others can do the experiment quickly.		We have a Celestron Schmidt-Cassegrain telescope set up in the lab pointed at an artificial double star illuminated by a bright lamp. It takes a few minutes for the lamp to reach full brightness, so if it is not on you may want to turn it on now and come back to this later. If it is on, then do not turn it off so that others can do the experiment quickly.

	You may need help from the lab assistant since this telescope ~~sometimes gets jostled~~ and ~~defocused~~. ~~He will make sure~~ it ~~is properly aligned~~ and show you how to focus it. ~~There~~ is ~~rotating mask~~ in front of the telescope~~'s objective~~ lens ~~that allows you~~ to ~~change~~		You may need help from the lab assistant since this telescope has high magnification and pointing it at the "stars" takes care. If you have difficulty, we'll help you align it, and show you how to adjust the focus for the best image.
	~~how large~~ the opening is. ~~Look~~ at the double star ~~and~~ have another student change the size of the opening. You'll see it get fainter (that's expected since there is less opening to capture light), and it will also get fuzzier as you go to smaller diameter openings. At some point you will not be able to tell there are two stars, while for a larger opening they will be very clearly separated. Each star will look something like this:
			The telescope is 8 inches (about 20 centimeters) in diameter. In perfectly still air under
			ideal conditions, it could resolve less than 0.5 arcseconds, that is 0.00014 degree. You may
			notice that you can see details in the small punched holes that mimic a double star. The purpose of this part of the experiment is for you to see what happens to the image when the stars are viewed through smaller and smaller telescopes.


			Take one of the paper plates and cut a hole about 2 inches (5 cm) in diameter. It doesn't have to be perfectly round to work. Hold it in front of the telescope and make sure that the hole is over the clear glass "corrector" lens of the telescope. For this to work, the hole will have to be off center, and the paper should cover the entire opening. It may be easier if one student looks while another holds this "mask" in place. You'll notice that the image will be dimmer, and that it will also be less distinct. Make several masks of decreasing hole size, and then
			while one person is looking at the double star through the telescope, have another student change the size of the opening. You'll easily see it get fainter (that's expected since there is less opening to capture light), and it will also get fuzzier as you go to smaller diameter openings. At some point you will not be able to tell there are two stars, while for a larger opening they will be very clearly separated. Each star will look something like this:

	[[File:airy-pattern.png \| 400 px \| center]]		[[File:airy-pattern.png \| 400 px \| center]]

	The rings of light are the diffraction pattern of the telescope's objective lens. The bigger the lens, the smaller this pattern. When the patterns of two nearby stars overlap such that the center of one falls in the dark ring around the other, the two stars can only just barely be distinguished. This condition is the Rayleigh Criterion for telescope resolution. For visible light it turns out that a telescope with 10 cm aperture will just resolve 1 arcsecond - that's 1/3600th of a degree.		The rings of light are the diffraction pattern of the telescope's objective lens. The bigger the lens, the smaller this pattern. When the patterns of two nearby stars overlap such that the center of one falls in the dark ring around the other, the two stars can only just barely be distinguished. This condition is the ''Rayleigh Criterion'' for telescope resolution. For visible light it turns out that a telescope with 10 cm aperture will just resolve 1 arcsecond - that's 1/3600th of a degree.

	''7. What was the aperture that just resolved these artificial stars from one another?''		''7. What was the aperture that just resolved these artificial stars from one another?''

WikiSysop: /* Telescopes */

2011-10-08T18:41:32Z

Telescopes

← Older revision		Revision as of 18:41, 8 October 2011
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	We have a refracting telescope set up in the lab pointed at an artificial double star illuminated by a bright lamp. It takes a few minutes for the lamp to reach full brightness, so if it is not on you may want to turn it on now and come back to this later. If it is on, then do not turn it off so that others can do the experiment quickly.		We have a refracting telescope set up in the lab pointed at an artificial double star illuminated by a bright lamp. It takes a few minutes for the lamp to reach full brightness, so if it is not on you may want to turn it on now and come back to this later. If it is on, then do not turn it off so that others can do the experiment quickly.

	You may need help from the lab assistant since this telescope sometimes gets jostled and defocused. He will make sure it is properly aligned and show you how to focus it. There is ~~an iris diaphragm~~ in front of the telescope's objective lens that allows you to change		You may need help from the lab assistant since this telescope sometimes gets jostled and defocused. He will make sure it is properly aligned and show you how to focus it. There is rotating mask in front of the telescope's objective lens that allows you to change
	how large the opening is. Look at the double star and have another student ~~slowly close~~ the ~~iris~~. You'll see it get fainter (that's expected since there is less opening to capture light), and it will also get fuzzier. At some point you will not be able to tell there are two stars, while for a larger opening they will be very clearly separated. Each star will look something like this:		how large the opening is. Look at the double star and have another student change the size of the opening. You'll see it get fainter (that's expected since there is less opening to capture light), and it will also get fuzzier as you go to smaller diameter openings. At some point you will not be able to tell there are two stars, while for a larger opening they will be very clearly separated. Each star will look something like this:

	[[File:airy-pattern.png \| 400 px \| center]]		[[File:airy-pattern.png \| 400 px \| center]]

WikiSysop: /* Some Properties of your Eyes */

2011-09-12T07:24:49Z

Some Properties of your Eyes

← Older revision		Revision as of 07:24, 12 September 2011
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	Close your left eye and focus with your right eye on the cross. ~~Move~~ slowly away from the screen. At some point the black spot should disappear. Increase the distance more and the spot will reappear.		Close your left eye and focus with your right eye on the cross. Starting only a few centimeters (say 2 inches) from the screen, move slowly away from the screen. At some point the black spot should disappear. Increase the distance more and the spot will reappear. If you don't see this at first, make sure you are looking with your right eye and that you are focused on the cross. It's the dot that will simply become invisible at just the right distance.

	Measure the distance at which the spot vanished, and the separation of the cross and spot on the screen.		Measure the distance at which the spot vanished, and the separation of the cross and spot on the screen.

WikiSysop: /* Experimenting with a Lens */

2011-09-12T07:21:23Z

Experimenting with a Lens

← Older revision		Revision as of 07:21, 12 September 2011
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	To see the nature of an image detected by the retina, let's begin by experimenting with a lens. For the lab will have a mounted lens you may use like a magnifying glass You will also need a small ruler and there should be one on the table with the lens. Although you can use a ruler with English units, scientific measurements today are always made with metric units. You can convert English to metric easily since one inch is 2.54 centimeters or 25.4 millimeters. There are 100 centimeters in a meter, and 10 millimeters in one centimeter.		To see the nature of an image detected by the retina, let's begin by experimenting with a lens. For the lab we will have a mounted lens you may use like a magnifying glass You will also need a small ruler and there should be one on the table with the lens. Although you can use a ruler with English units, scientific measurements today are always made with metric units. You can convert English to metric easily since one inch is 2.54 centimeters or 25.4 millimeters. There are 100 centimeters in a meter, and 10 millimeters in one centimeter.

	''1. Describe the lens you are using. Give its diameter, how its surfaces are curved, and other information that someone would need to reproduce your observations.''		''1. Describe the lens you are using. Give its diameter, how its surfaces are curved, and other information that someone would need to reproduce your observations.''

WikiSysop: /* Some Properties of your Eyes */

2011-09-12T07:03:56Z

Some Properties of your Eyes

← Older revision		Revision as of 07:03, 12 September 2011
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	Observe the printing on this page from different distances. If you wear glasses, try with them on and off. As you bring the page closer to your eye, you accommodate to the difference by changing the lens. While this action is mostly involuntary, you can feel the strain of looking at something that is closer than a foot or so away.		Observe the printing on this page from different distances. If you wear glasses, try with them on and off. As you bring the page closer to your eye, you accommodate to the difference by changing the lens. While this action is mostly involuntary, you can feel the strain of looking at something that is closer than a foot or so away.
	''
	12. What is the closest distance for which you can see the page clearly? Does it help to see the page more clearly up close with glasses on?''
			''12. What is the closest distance for which you can see the page clearly? Does it help to see the page more clearly up close with glasses on?''

	By the way, when you use a telescope you should focus your eye for infinity. The telescope accepts light from a distant star over its full diameter, and funnels that light into a smaller beam that fits into your iris. The eye focuses this light on the retina, just as if it came directly from the star.		By the way, when you use a telescope you should focus your eye for infinity. The telescope accepts light from a distant star over its full diameter, and funnels that light into a smaller beam that fits into your iris. The eye focuses this light on the retina, just as if it came directly from the star.
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	The area of a circle is		The area of a circle is

	Area = Pi x R2 = Pi x D2 / 4		Area = Pi x R x R = Pi x D x D / 4

	where "Pi" is 3.14, R is the radius of the circle or D is its diameter. For example, the area of a circle 2 mm in diameter is 3.14 square millimeters. A 100 mm diameter telescope (4 inches) is a typical small telescope used by amateur astronomers. With an appropriate choice of magnification, all of the the light that is collected by the lens leaves the eyepiece and enters the pupil of a dark adapted eye.		where "Pi" is 3.14, R is the radius of the circle or D is its diameter. For example, the area of a circle 2 mm in diameter is 3.14 square millimeters. A 100 mm diameter telescope (4 inches) is a typical small telescope used by amateur astronomers. With an appropriate choice of magnification, all of the the light that is collected by the lens leaves the eyepiece and enters the pupil of a dark adapted eye.
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	''15. Describe how you could or could not "see in the dark", and what happened to your night vision after being exposed to bright light. After the exposure to bright light, how long it take for your vision to adapt to darkness?''		''15. Describe how you could or could not "see in the dark", and what happened to your night vision after being exposed to bright light. After the exposure to bright light, how long it take for your vision to adapt to darkness?''


	== The Sky at Night ==		== The Sky at Night ==

	Wait for a night when it is at least partly clear so that you can see some sky. Find a safe, convenient location, and take a few minutes to look up. Think about what you see, given the experiments you have done here.		Wait for a night when it is at least partly clear so that you can see some sky. Find a safe, convenient location, and take a few minutes to look up. Think about what you see, given the experiments you have done here.

WikiSysop at 07:01, 12 September 2011

2011-09-12T07:01:40Z

Show changes

WikiSysop: /* Some Properties of your Eyes */

2011-09-12T06:59:02Z

Some Properties of your Eyes

← Older revision		Revision as of 06:59, 12 September 2011
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	Close your left eye and focus with your right eye on the cross. Move slowly away from the screen. At some point the black spot should disappear. Increase the distance more and the spot will reappear.		Close your left eye and focus with your right eye on the cross. Move slowly away from the screen. At some point the black spot should disappear. Increase the distance more and the spot will reappear.

	~~7. What happened when you tried this?~~

	Measure the distance at which the spot vanished, and the separation of the cross and spot on the screen.		Measure the distance at which the spot vanished, and the separation of the cross and spot on the screen.
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	15. Describe how you could or could not "see in the dark", and what happened to your night vision after being exposed to bright light. After the exposure to bright light, how long it take for your vision to adapt to darkness?		15. Describe how you could or could not "see in the dark", and what happened to your night vision after being exposed to bright light. After the exposure to bright light, how long it take for your vision to adapt to darkness?


	== The Sky at Night ==		== The Sky at Night ==

	Wait for a night when it is at least partly clear so that you can see some sky. Find a safe, convenient location, and take a few minutes to look up. Think about what you see, given the experiments you have done here.		Wait for a night when it is at least partly clear so that you can see some sky. Find a safe, convenient location, and take a few minutes to look up. Think about what you see, given the experiments you have done here.

WikiSysop: /* Telescopes */

2011-09-12T06:57:31Z

Telescopes

← Older revision		Revision as of 06:57, 12 September 2011
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	how large the opening is. Look at the double star and have another student slowly close the iris. You'll see it get fainter (that's expected since there is less opening to capture light), and it will also get fuzzier. At some point you will not be able to tell there are two stars, while for a larger opening they will be very clearly separated. Each star will look something like this:		how large the opening is. Look at the double star and have another student slowly close the iris. You'll see it get fainter (that's expected since there is less opening to capture light), and it will also get fuzzier. At some point you will not be able to tell there are two stars, while for a larger opening they will be very clearly separated. Each star will look something like this:

	[[File:airy-~~pattery~~.png \| 400 px \| center]]		[[File:airy-pattern.png \| 400 px \| center]]

	The rings of light are the diffraction pattern of the telescope's objective lens. The bigger the lens, the smaller this pattern. When the patterns of two nearby stars overlap such that the center of one falls in the dark ring around the other, the two stars can only just barely be distinguished. This condition is the Rayleigh Criterion for telescope resolution. For visible light it turns out that a telescope with 10 cm aperture will just resolve 1 arcsecond - that's 1/3600th of a degree.		The rings of light are the diffraction pattern of the telescope's objective lens. The bigger the lens, the smaller this pattern. When the patterns of two nearby stars overlap such that the center of one falls in the dark ring around the other, the two stars can only just barely be distinguished. This condition is the Rayleigh Criterion for telescope resolution. For visible light it turns out that a telescope with 10 cm aperture will just resolve 1 arcsecond - that's 1/3600th of a degree.

	What was the aperture that just resolved these artificial stars from one another?		7. What was the aperture that just resolved these artificial stars from one another?

	The stars are only 8 arcseconds apart, but when you see them through the telescope they appear to be separated by far more than this. In fact, if you walk over and look at the artificial stars you'll see they are tiny holes in a metal strip, and even up close you can hardly tell there are two of them. The telescope magnifies the view, making the stars appear to be farther apart in angle than they actually are.		The stars are only 8 arcseconds apart, but when you see them through the telescope they appear to be separated by far more than this. In fact, if you walk over and look at the artificial stars you'll see they are tiny holes in a metal strip, and even up close you can hardly tell there are two of them. The telescope magnifies the view, making the stars appear to be farther apart in angle than they actually are.

WikiSysop: /* Telescopes */

2011-09-12T06:56:11Z

Telescopes

← Older revision		Revision as of 06:56, 12 September 2011
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	'''Magnification'''		'''Magnification and Resolution '''

	~~'''Resolution'''~~

WikiSysop: /* Telescopes */

2011-09-12T06:55:42Z

Telescopes

← Older revision		Revision as of 06:55, 12 September 2011
Line 57:		Line 57:
	'''Magnification'''		'''Magnification'''

			'''Resolution'''


			We have a refracting telescope set up in the lab pointed at an artificial double star illuminated by a bright lamp. It takes a few minutes for the lamp to reach full brightness, so if it is not on you may want to turn it on now and come back to this later. If it is on, then do not turn it off so that others can do the experiment quickly.

			You may need help from the lab assistant since this telescope sometimes gets jostled and defocused. He will make sure it is properly aligned and show you how to focus it. There is an iris diaphragm in front of the telescope's objective lens that allows you to change
			how large the opening is. Look at the double star and have another student slowly close the iris. You'll see it get fainter (that's expected since there is less opening to capture light), and it will also get fuzzier. At some point you will not be able to tell there are two stars, while for a larger opening they will be very clearly separated. Each star will look something like this:

			[[File:airy-pattery.png \| 400 px \| center]]

			The rings of light are the diffraction pattern of the telescope's objective lens. The bigger the lens, the smaller this pattern. When the patterns of two nearby stars overlap such that the center of one falls in the dark ring around the other, the two stars can only just barely be distinguished. This condition is the Rayleigh Criterion for telescope resolution. For visible light it turns out that a telescope with 10 cm aperture will just resolve 1 arcsecond - that's 1/3600th of a degree.

			What was the aperture that just resolved these artificial stars from one another?

	'~~''Resolution'''~~		The stars are only 8 arcseconds apart, but when you see them through the telescope they appear to be separated by far more than this. In fact, if you walk over and look at the artificial stars you'll see they are tiny holes in a metal strip, and even up close you can hardly tell there are two of them. The telescope magnifies the view, making the stars appear to be farther apart in angle than they actually are.

	== Some Properties of your Eyes ==		== Some Properties of your Eyes ==