electromagnetic radiation and telescopes (how we get information about the cosmos and how we gather...
TRANSCRIPT
Electromagnetic Radiation and Telescopes
(How we get information about the cosmos and how we gather the information)
a) bend around corners and edges.
b) separate into its component colors.
c) bend through a lens.
d) disperse within a prism.
e) reflect off a mirror.
Diffraction is the tendency of light to
Clicker Question
a) bend around corners and edges.
b) separate into its component colors.
c) bend through a lens.
d) disperse within a prism.
e) reflect off a mirror.
Diffraction is the tendency of light to
Clicker Question
Diffraction affects all telescopes and limits the sharpness of all images.
Telescopes
Light Hitting a Telescope Mirror
huge mirror near a star*
* small mirror far from a star
In the second case (reality), light rays from any single point of light are essentially parallel.
Light rays from a distant source, parallel to the "mirror axis“, all meet at one point, the focus.
CCD**
Reflecting and refracting telescopes
Optical Telescopes
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.
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.
Refracting Telescope Reflecting Telescope
Yerkes 40-inch (about 1 m). Largest refractor.
Cerro-Tololo 4 -m reflector.
Chromatic Aberration
Lens - different colors focus at different places.
white light
Mirror - reflection angle doesn't depend on color.
Types of reflecting telescopes
Clicker Question
Modern telescopes use mirrors rather than lenses for all of these reasons EXCEPT
a) light passing through lenses can be absorbed or scattered.
b) large lenses can be very heavy.
c) large lenses are more difficult to make.
d) mirrors can be computer controlled to improve resolution.
e) reflecting telescopes aren’t affected by the atmosphere as much.
Clicker Question
Modern telescopes use mirrors rather than lenses for all of these reasons EXCEPT
a) light passing through lenses can be absorbed or scattered.
b) large lenses can be very heavy.
c) large lenses are more difficult to make.
d) mirrors can be computer controlled to improve resolution.
e) reflecting telescopes aren’t affected by the atmosphere as much.
Reflecting instruments like the KECK telescopes can be made larger, and more capable, than refractors.
Keck 10-m optical telescope
Mirror sizeMirror with larger area captures more light from a star. Can look at fainter objects with it.
30-100 m optical telescopes now being considered!
Image of Andromeda galaxy with optical telescope.
Image with telescope of twice the diameter, same exposure time.
The Two Main Types of Observation
Imaging (recording pictures)
Spectroscopy (making a spectrum) usually using a diffraction grating
In both cases, image or spectrum usually recorded on a CCD ("charge-coupled device")
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.
(a) 10′; (b) 1′; (c) 5″; (d) 1″
"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). Or 100 km atdistance of the Moon.
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 of optical telescopes to about 1". This is called seeing.
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.
Example: the Moon observed with a 2.5 m telescope
1" => 2 km
0.05" => 100 m
Ground-based telescope image, 1” resolution
Hubble SpaceTelescope image,0.05” resolution
North America at night
So where would you put a telescope?
Kitt Peak National Observatory, near Tucson
Mauna Kea Observatory, Hawaii
Clicker Question
Resolution is improved by using
a) larger telescopes & longer wavelengths.
b) infrared light.
c) larger telescopes & shorter wavelengths.
d) lower frequency light.
e) visible light.
Clicker Question
Resolution is improved by using
a) larger telescopes & longer wavelengths.
b) infrared light.
c) larger telescopes & shorter wavelengths.
d) lower frequency light.
e) visible light.
Diffraction limits resolution; larger telescopes and shorter-wave light produces sharper images.
a) they don’t require chemical development.
b) digital data is easily stored & transmitted.
c) CCDs are more light sensitive than film.
d) CCD images can be developed faster.
e) All of the above are true.
An advantage of CCDs over photographic film is
Clicker Question
a) they don’t require chemical development.
b) digital data is easily stored & transmitted.
c) CCDs are more light sensitive than film.
d) CCD images can be developed faster.
e) All of the above are true.
An advantage of CCDs over photographic film is
Clicker Question
a) the quality of the telescope’s optics.
b) the transparency of a telescope’s lens.
c) the sharpness of vision of your eyes.
d) the image quality due to air stability.
e) the sky’s clarity & absence of clouds.
Seeing in astronomy is a measurement of
Clicker Question
Seeing in astronomy is a measurement of
Clicker Question
“Good Seeing” occurs when the atmosphere is clear and the air is still.
Turbulent air produces “poor seeing,” and fuzzier images.
a) the quality of the telescope’s optics.
b) the transparency of a telescope’s lens.
c) the sharpness of vision of your eyes.
d) the image quality due to air stability.
e) the sky’s clarity & absence of clouds.
Point images of a star
Smeared overall image of star
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".
Andromeda radiomap with Effelsberg telescope
Effelsberg 100-m (Germany)
Andromeda galaxy –optical
Longer wavelength means poorer angular resolution.Advantages of radio astronomy:• Can observe 24 hours a day.• Clouds, rain, and snow don’t interfere.• Observations at an entirely different frequency; get totally different information.
Radio Astronomy
Parkes 64-m (Australia)
Green Bank 100-m telescope (WV) Arecibo 300-m telescope (Puerto Rico)
Jodrell Bank 76-m (England)
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.
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
Radio dishes are large in order to
Clicker Question
a) improve angular resolution.
b) give greater magnification.
c) increase the range of waves they can collect.
d) detect shorter waves than optical telescopes for superior resolution.
Radio dishes are large in order to
Clicker Question
Resolution is worse with long-wave light, so radio telescopes must be large
to compensate.
a) improve angular resolution.
b) give greater magnification.
c) increase the range of waves they can collect.
d) detect shorter waves than optical telescopes for superior resolution.
Clicker Question
Radio telescopes are useful because
a) observations can be made day & night.
b) we can see objects that don’t emit visible light.
c) radio waves are not blocked by interstellar dust.
d) they can be linked to form interferometers.
e) All of the above are true.
Clicker Question
Radio telescopes are useful because
a) observations can be made day & night.
b) we can see objects that don’t emit visible light.
c) radio waves are not blocked by interstellar dust.
d) they can be linked to form interferometers.
e) All of the above are true.
The Very Large Array links separate radio telescopes to create
much better resolution.
Clicker Question:
When multiple radio telescopes are used for interferometry, resolving power is most improved by increasing:
A: the distance between telescopes;
B: the number of telescopes in a given area;
C: the diameter of each telescope;
D: the power supplied to each telescope
Astronomy at Other WavelengthsTelescopes also observe infrared, UV, X-rays and gamma rays.Mostly done from space because of Earth's atmosphere.
Infrared allows you to see radiation fromwarm dust ininterstellar gas.
Spitzer SpaceTelescope - infrared
Infrared also allows you to see through dust. Dust is good atblocking visible light but infrared gets through better.
Trifid nebula in visible light Trifid nebula with Spitzer
GLAST – Gamma Ray Large Area Space Telescope
Gamma rays are the most energeticphotons, tracing high-energy events inUniverse such as “Gamma-ray Bursters”.
Latest mission is GLAST – successfullylaunched this year.
X rays and gamma rays will not reflect off mirrors as other wavelengths do; need new techniques.
X rays will reflect at a very shallow angle, and can therefore be focused.
Gamma Rays cannot be focused at all; images are coarse
X-ray Optics
Hubble Space Telescope and its successor-to-be: the James WebbSpace Telescope
Advantage of space for optical astronomy: get above blurring atmosphere – much sharper images.
Center of M51: HST (left; 0.05” resolution) vs.ground-based (right; 1” resolution)
The James Web Space Telescope
Will have diameter 6.5 meters (vs. HST 2.5 meters) – much higher resolution and sensitivity. Will also observe infrared, whereasHubble is best at visible light. Expected launch 2013.
Mock-up of JWST
Clicker Question:
The biggest telescopes on Earth are:
A: Gamma-ray telescopes.
B: X-ray telescopes.
C: Optical telescopes
D: Radio telescopes
E: Infra-red telescopes
Much can be learned from observing the same astronomical object at many wavelengths. Here, the Milky Way.
• 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 image, 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.
Summary of Chapter 3
• 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.
Summary of Chapter 3, cont.
• Infrared and ultraviolet telescopes are similar to optical.
• Ultraviolet telescopes must be above atmosphere.
• X rays can be focused, but very differently than visible light.
• Gamma rays can be detected but not imaged.
Summary of Chapter 3, cont.