Light & Telescopes(Chapter 5)
All of what we know and understand about the stars is the result of observation and analysis of light.
Student Learning Objectives
• Describe the properties of light
• Identify types and properties of telescopes
What are the properties of light?
Electromagnetic waves carry information as well as energy.
Light speed is constant in a vacuum (in space)
• 3 x 108 m/s• 186,000 miles/sec• 670 million mph
The energy we are now observing from objects in space was generated at some time in the past.
Sun 8 min.Proxima Centauri 4.2 LY
Sirius 8.6 LYAndromeda
Galaxy2.5 x 106 LY
Practice:
In a science fiction movie, when a spaceship explodes, we see and hear the explosion instantly. What are the mistakes in this scenario?
Wave PropertiesElectromagnetic waves have a wavelength (l)
and frequency (n).
c = ln
Electromagnetic radiation exhibits wave-particle duality.
Photons No mass Pure energy
Each photon of light carries energy.
The more photons, the higher the intensity.
E = hc l
E = hn
Electromagnetic Radiation
http://www.chromoscope.net/
l n EGamma Rays
X-RaysUltraviolet
VisibleInfrared
Radio
http://spaceplace.nasa.gov/ir-photo-album/en/
We interpret particular wavelengths of visible light as particular colors!
Red Yellow Blue7000 Å 5500 Å 4000 ÅCooler Hotter
Less Energy More Energy
ROY G BV
Some of the Information in The Light
• Energy output• Temperature
• Chemical composition• Distance
• Relative Motion
Atmospheric Windows
Practice
1) What is the speed of a radio wave?
2) Which wavelength is longer?
a. 95.1 MHz
b. 1220 MHz
More Practice
3) Which star would be hotter, a blue star or a red star?
4) What affects the intensity of the light we receive from stars?
What are the main features of optical telescopes?
Reflection is a bounce (mirrors)
Refraction is a change in velocity (lenses)
Reflecting Telescope
Reflecting telescopes utilize a mirror.
Refracting telescopes utilize a lens.
Refracting Telescope
Newtonian Reflector Mirror
The objective or primary lens/mirror of the telescope is the most important part.
Refractor Issues
All refractors produce chromatic aberration (“color deviation or distortion”).
Interesting Note: A very long focal length will minimize chromatic aberration. This is why the early refracting telescopes (like Galileo’s) were made very long.
Reflector Issues
Reflectors may produce a fuzzy image due to spherical aberration (curvature distortion).
Hubble Images
Advanced optical telescopes use floppy mirror segments controlled by a computer.
Why?
Practice
1) What is the main purpose of a telescope?
2) Where are the telescopes that astronomers use?
3) Are there any telescopes that can be used during the day?
Mauna Kea, Hawaii(Elevation 14,000 Feet)
What are the functions of telescopes?
Light Gathering Power
Ability of the telescope to collect photons.
LGP = (Diameter)2
Resolving Power
Ability of the telescope to “see” fine detail, and distinguish individual objects from each other
a = (2.06 x 105) l Diameter
Larger D and Smaller lsharper images
Image Credit: Richard Bloch
An image of two stars through a
telescope with low angular resolution
An image of two stars through a
telescope with high angular resolution.
Magnifying Power
Telescopes enlarge the image.
Atmosphere & Light affect observations.
Mars with naked eye
Practice
1) Which telescope would allow you to see fainter objects?
a. Small diameter telescope
b. Large diameter telescope
c. Either as long as there is good magnification
2) What are the advantages of space based telescopes?
What is a radio telescope?
A large parabolic dish gathers radio waves and reflects this energy to a central focus.
Star formation regionsSETI program
Interferometers
A single radio telescope has very poor angular resolution.
An interferometer is a set of radio telescopes connected together.
Constructive light Interference
NRAO
DetectorsPhotometers: count
photons
Measure Intensity
CCD - Charge-Coupled Device: produce electronic images with millions of light detectors (pixels on a small silicon wafer)
Measure Intensity per Pixel
Camera!
Spectrographs: use a grating to split light into individual wavelengths
White Light → Rainbow
Adaptive optics: is a computer program that corrects for bad seeing
Removes blurring caused by atmosphere
Sun's Visible Spectrum
Image Credit: Canada-France-Hawaii Telescope
Image Credit: Gemini Observatory
Practice
1) What do people use everyday that has a CCD camera?
2) Optics are lenses and mirrors. How do adaptive optics differ from regular optics?
3) What are two tools that astronomers can use to produce the clearest image possible?