telescopes. light hitting a telescope mirror huge mirror near a star * * * small mirror far from 2...
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Telescopes
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Light Hitting a Telescope Mirror
huge mirror near a star*
*
*small mirror far from 2 stars
In the second case (reality), light rays from any single point of light are essentially parallel. But the parallel rays from the second star come in at a different angle.
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Light rays from a distant source, parallel to the "mirror axis" all meet at one point, the focus.
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Image Formation
"focal plane"
Light rays from a distant, extended source are all focused in the same plane, the "focal plane" creating an image of the source.
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Optical Telescopes - Refracting vs. Reflecting
Refracting telescope
Focuses light with a lens (like a camera).
<-- object (point of light) image at focus
Problems:
- Lens can only be supported around edge.
- "Chromatic aberration".
- Some light absorbed in glass (especially UV, infrared).
- Air bubbles and imperfections affect image quality.
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Reflecting telescope
Focuses light with a curved mirror.
<-- object image
- Can make bigger mirrors since they are supported from behind.
- No chromatic aberration.
- Reflects all radiation with little loss by absorption.
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Refracting Telescope Reflecting Telescope
Yerkes 40-inch (about 1 m). Largest refractor.
Cerro-Tololo 4 -m reflector.
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Chromatic Aberration
Lens - different colors focus at different places.
white light
Mirror - reflection angle doesn't depend on color.
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Reflecting telescope focus options
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Kitt Peak (Arizona) 4-m telescope and spectrograph at Cassegrain focus.
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*
Larger mirror captures more light from star. Can look at fainter objects with it.
Light gathering power area of mirror.
Mirror size
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Hence the drive to building large telescopes...
Keck 10-m optical telescope
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Image of Andromeda galaxy with optical telescope.
Image with telescope of twice the diameter, same exposure time.
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Instruments and Detectors
Imaging (recording pictures)
Photographic plate CCD ("charge-coupled device")
Spectroscopy
Prism Diffraction Grating } spread out light
into a spectrum
Record spectrum on photographic plate or CCD.
1000's
1000'sCCD
Size about a few cm. 1000's of "pixels" on a side.
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Resolving Power of a Mirror
(how much detail can you see?)
fuzziness you would see with your eye.
detail you can see with a telescope.
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"Angular resolution" is the smallest angle by which two objects can be separated and still be distinguished.
For the eye, this is 1' (1/60th of a degree). Looking at the Moon, you can distinguish features separated by > 100 km.
angular resolution wavelengthmirror diameter
For a 2.5-m telescope observing light at 5000 Angstroms (greenish), resolution = 0.05".
But, blurring by atmosphere limits resolution to about 1" for light. This is called seeing (radio waves, for example, don't get blurred).
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Seeing
*
dome
Air density varies => bends light. No longer parallel
Parallel rays enter atmosphere
CCD
No blurring case. Rays brought to same focus.
* Sharp image on CCD.
Blurring. Rays not parallel. Can't be brought into focus.
Blurred image.
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Example: the Moon observed with a 2.5 m telescope
1" => 2 km
0.05" => 100 m
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North America at night
So where would you put a telescope?
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Kitt Peak National Observatory, near Tucson
Mauna Kea Observatory, Hawaii
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Radio Telescopes
Large metal dish acts as a mirror for radio waves. Radio receiver at prime focus.
Surface accuracy not so important, so easy to make large one.
But angular resolution is poor. Remember:
angular resolution wavelengthmirror diameter
D larger than optical case, but wavelength much larger (cm's to m's), e.g. for wavelength = 1 cm, diameter = 100 m, resolution = 20".
Jodrell Bank 76-m (England)
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Parkes 64-m (Australia) Effelsberg 100-m (Germany)
Green Bank 100-m telescope (WV) Arecibo 300-m telescope (Puerto Rico)
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Interferometry
A technique to get improved angular resolution using an array of telescopes. Most common in radio, but also limited optical interferometry.
D
Consider two dishes with separation D vs. one dish of diameter D.By combining the radio waves from the two dishes, the achieved angular resolution is the same as the large dish.
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Example: wavelength = 1 cm, separation = 2 km, resolution = 1"
Very Large Array (NM). Maximum separation 30 km
Very Long Baseline Array. Maximum separation 1000's of km
VLA and optical image of Centaurus A
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Astronomy at Other Wavelengths
Telescopes also observe infrared, UV, X-rays and gamma rays.These mostly done from space because of Earth's atmosphere.
Chandra X-ray satellite
X-ray image of Center of Milky Way Galaxy
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