03 lecture outline rev - university of new mexicophysics.unm.edu/.../03_lecture_outline_rev.pdf ·...
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AstronomyA BEGINNER’S GUIDETO THE UNIVERSE
EIGHTH EDITION
CHAPTER 3
TelescopesLecture Presentation
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3.0 Imaging the universe• Our original observations of the universe depended on our eyes!
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What other way(s) do you now use to obtain and store images?
3.0 Imaging the universe• Our original observations of the universe depended on our eyes!
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What other way(s) do you now use to obtain and store images? While your eyes are indispensible, what are the advantages of modern optical instruments?
3.1 Optical Telescopes• In astronomy we call the light collecting systems telescopes!
• But just like our eyes, the goal of telescopes is to gather light and bring it to a focus.
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3.1 Optical Telescopes• In astronomy we call the light collecting systems telescopes!
• But just like our eyes, the goal of telescopes is to gather light and bring it to a focus.
• Images can be formed through reflection orrefraction.
• Reflecting mirror is shown here:
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3.1 Optical Telescopes• Refracting lens is shown here:
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3.1 Optical Telescopes
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Light from slightly different directionsis focused to slightly different positions.
3.1 Optical Telescopes
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So where would you put the CCD (or film) to record the image of the comet?
3.1 Optical Telescopes• Typical amateur reflecting and refracting telescopes used by amateurs. Come to Friday night at our campus observatory!
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3.1 Optical Telescopes
(http://www.astronomy.ohio-state.edu/~pogge/Ast161/Unit4/Images/refractor.gif )
If/when you go the our campus observatory on Friday nights, ask the telescope operator to change the eyepiece to experience how that changes the angular size of what you see!
3.1 Optical Telescopes• Modern research telescopes are all reflectors. Figure shows details of the 10m diameter Keck telescopes. Do you see the person kneeling in c)?
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3.1 Optical Telescopes• For a variety of reasons there are many versions of reflecting telescopes: in all cases the “telescope primary” (the element that “collects” the light) is a reflecting mirror:
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3.1 Optical Telescopes• Image acquisition: Charge-coupled devices (CCDs) are electronic devices that can be quickly read out and reset. Q. How many mega-pixels are in your cell phone camera? iPhone 11 has 12 million!
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3.2 Telescope Size• Light-gathering power: Improves our ability to see the faintest parts of this galaxy
• Brightness is proportional to square of radius of mirror … so bigger is very important.
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3.2 Telescope Size• Light-gathering power: Improves our ability to see the faintest parts of this galaxy
• Brightness is proportional to square of radius of mirror … so bigger is very important.
• In the figure, part (b) was taken with a telescope twice the size of (a). So which image (a) or (b) shows the most detail?
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3.2 Telescope Size• Resolving power: this measures how well a telescope can separate two objects that are in almost the same direction in the sky. This is also related to how sharp an image looks.
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3.2 Telescope Size• Resolving power: thimeasures how well a telescope can separate two objects that are in almost the same direction in the sky. This is also related to how sharp an image looks.
• The figure shows the same two sources viewed by 3 telescopes. You can not tell from the top image that there are 2 sources! Which telescope has the best resolving power?
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Clearly two images!
Two images but not distinct
Two light sources are viewed by three telescopeseach with different angular resolution.
The telescope with the best resolving power is the one where the two sources are most distinct which is image (c).
3.2 Telescope Size
3.2 Telescope Size• Telescope resolution… because light is a wave!
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Diffraction is an intrinsic property of waves (see Chapt 2), and limits telescope resolution depending on light wavelength and telescope size (mirror diameter).
For the best resolving power we want the telescopeangular resolution to be as small as possible.
3.2 Telescope Size• Telescope resolution… because light is a wave!
For the same wavelength the larger the opening [or the larger the telescope mirror] (top image) the less the diffraction.
Most diffraction
Least diffraction
3.2 Telescope Size
angular resolution: (a) 10′ angular resolution: (b) 1′
angular resolution: (c) 5” angular resolution: (d) 1”
Image (d) with the smallest angular resolution has the bestresolving power (and the image looks sharpest!)
3.2 Telescope Size
3.3 High-Resolution Astronomy•
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Atmospheric blurring due to air movements is called “seeing”. Twinkle twinkle little star is an example of “seeing”!
3.3 High-Resolution Astronomy• To reduce (or eliminate) the “seeing” issue:– Put telescopes on mountaintops, especially in deserts.
– Put telescopes in space.
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3.3 High-Resolution Astronomy• To reduce (or eliminate) the “seeing” issue:– Use adaptive optics—control mirrors by bending them slightly, often 1000 times/second, to correct for atmospheric distortion. Image a) is without and b) is with adaptive optics!
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Adaptive optics: Track atmospheric changes (e.g. with laser) then adjust mirrors in real time … very clever and it works!
3.3 High-Resolution Astronomy
3.3 High-Resolution AstronomyOriginally designed by Air Force to monitor space satellites.
3.3 High-Resolution AstronomyAdaptive optics: now used on many ground based telescopes to improve image resolution.
These images show two more examples of the improvements possible with adaptive optics: (left) image(s) are without adaptive optics and (right) image(s) are with adaptive optics.
3.4 Radio Astronomy• Radio telescopes:– Similar to optical reflecting telescopes– Less sensitive to imperfections (due to longer wavelengths) thus can be made very large
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3.4 Radio Astronomy
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•• 2nd largest radio telescope: 300m dish at Arecibo, Puerto Rico. Largest, 500m, is in Pingtang county, China.
3.4 Radio Astronomy
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Can observe 24 hours a day.Clouds, rain, and snow are not an issue.
Observations in Radio and Optical often find totally different information!
3.4 Radio Astronomy• Big problem: longer wavelength means poorer resolution. This is now solved by interferometry.
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54 minutes west of Socorro, NM
3.4 Radio Astronomy
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Extreme distance
• Interferometry:– Combines information from several widely separated radio telescopes as if it came from a single dish.
– Resolution will be that of a dish whose diameter = extreme distance between dishes.
3.4 Radio Astronomy• Interferometry requires preserving the phase relationship between waves over the distance between individual telescopes. See Chapt 2 for a short discussion on wave “interference”.
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Signals cancel
Signals add
3.4 Radio Astronomy• Using radio interferometry, radio telescopes can get radio images whose resolution can be close to that of optical telescopes:
• a) is radio (R) image • b) is optical (V) image of same (probably colliding) galaxies.
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What about telescopes at other wavelengths?Infrared (I) telescopes can often image where visible (V) radiation is blocked and can generally use optical telescope mirrors and lenses.
3.5 Space-Based Astronomy
The a) and c) (left) images are in the visible (V);; the b) and d) (right) images are in the infrared (I). It is hard to believe these are the “same” photos taken at different wavelengths!
3.5 Space-Based Astronomies• Infrared telescopes can also be in space or flown on balloons.
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Ultraviolet (U) observing must be done in space, as the atmosphere absorbs almost all ultraviolet rays.
3.5 Space-Based Astronomy
3.5 Space-Based Astronomies• X-ray (X) image of supernova remnant Cassiopeia A
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3.5 Space-Based Astronomies• Gamma (G) rays are the most high-energy radiation we can detect.
• These are even harder to image …this supernova remnant would be nearly invisible without the Fermi satellite and its gamma-ray detector. (for astronomers but too detailed for us)
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3.5 Space-Based Astronomies• Much can be learned from observing the same astronomical object at many wavelengths.
• Here is the Milky Way seen at 5 different parts of the EM spectrum: R, I, V, X andG. Looks like 5 different objects!
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Summary of Chapter 3• Refracting telescopes make images with a lens.• Reflecting telescopes make images with a mirror.• Modern research telescopes are all reflectors.• CCDs are used for data collection.• Data can be formed into images, analyzed spectroscopically, or used to measure intensity.
• Large telescopes gather much more light, allowing study of very faint sources.
• Large telescopes also have better resolution.
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Summary of Chapter 3 (con’t)• Resolution of ground-based optical telescopes is limited by atmospheric effects.
• Resolution of radio or space-based telescopes is limited by diffraction.
• Active and adaptive optics can minimize atmospheric effects.
• Radio telescopes need large collection area;; diffraction is limited.
• Interferometry can greatly improve resolution.
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Summary of Chapter 3 (con’t) • Infrared and ultraviolet telescopes are similar to optical.
• Ultraviolet telescopes must be above the atmosphere.
• X-rays can be focused, but very differently from visible light.
• Gamma rays can be detected. This must be done from space.
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