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Magnitude System
• Brighter –lower number
http://www.astronomynotes.com/starprop/appmag.gif
4 Vestabrightest asteroid
Magnitude difference Relative intensity0 11 2.512 6.313 15.84 39.85 10010 104
15 106
Initially
• Everybody observed with their eyes
Figure 7.1
Figure 7.2a
Parallel light Lens
Figure 7.2b
Why are Telescopes better than your eyes?
• They can observe light in different wavelength regions (eyes can only see visible light)
• They can collect more light than eyes• They can be built to compensate for the distorting
effects of the atmosphere
Figure 7.6
Refracting telescope
Reflecting Telescope
Reflecting Telescopes
Resulting image inverted
All large modern telescopes are reflectors
• Since light passes through the lens of a refracting telescope,
• You need to make the lens from clear, high-quality glass with precisely shaped surfaces
It is
• Its easier to make a high-quality mirror than a lens
Also,
• Large lenses are extremely heavy
Also
• Lens focuses red and blue light slightly differently• Called chromatic aberration
http://en.wikipedia.org/wiki/File:Lens6a.svg
• Light changes speed as it moves from one medium to another
• The degree of bending of the light's path depends on the angle that the incident beam of light makes with the surface, and on the ratio between the refractive indices of the two media.
• The refractive index of many materials (such as glass) varies with the wavelength of the light used
Also
• Light can be absorbed by the glass as it passes through the glass
• Minor problem for visible, but severe for ultraviolet and infrared light
Size of a telescope
• Diameter of its primary mirror or lens• Light collecting area is proportional to the
diameter squared since• Collecting area = π r2
• E.g., 8-meter telescope
• Telescope that took image b is twice as big as telescope that took image a
• Larger the telescope, more detail can be seen
a b
• Telescope on Mauna Kea (14,000 feet high)• Telescope is Japanese Subaru 8-m telescope
Atmosphere
• Atmosphere can absorb light• Atmosphere can scatter light• Atmosphere can distort light (twinkling)
Twinkling
• Twinkling of stars is caused by moving air currents in the atmosphere.
• The beam of light from a star passes through many regions of moving air while on its way to an observer’s eye or telescope.
• Each atmospheric region distorts the light slightly for a fraction of a second.
Advantages of space-based telescopes
• It can be open 24 hours, 7 days of week• Do not have to worry about distorting effects of
atmosphere • There is no extra background of light due to
scattering of light in the Earth’s atmosphere• Observe in more wavelength regions
Figure 7.20
http://www.scienzagiovane.unibo.it/English/radio-window/images/radiazioni-em.jpg
• Infrared light absorbed by molecules
http://www.ucar.edu/learn/1_3_1.htm
Not all light from a star reaches Earth
Light in space can be affected by dust
http://www.ipac.caltech.edu/2mass/outreach/survey.html
http://en.wikipedia.org/wiki/File:Rayleigh_sunlight_scattering.png
It does not help
• That you are closer to the stars
To measure light
• In the past, they used photographic plates• Now they use CCDs (charge-coupled devices)• CCD are electronic detectors• CCDs are chips of silicons
Figure 7.5
CCDs• CCDs convert light into electrons
William Boyle George Smith
Shared the 2009Physics Nobel Prizefor their discovery
How do they work?• The CCD is made up of pixels.• As the light falls on each pixel, the photons become electrons
due to the photoelectric effect. The photoelectric effect happens when photons of light hit the silicon of the pixel and knock electrons out of place.
• These electrons are then stored.• Essentially, the charge in each row is moved from
one site to the next, a step at a time. This has been likened to a “bucket row” or human chain, passing buckets of water down a line.
• As these buckets of electrons reach the end of the line they are dumped out and measured, and this analog measurement is then turned into a digital value.
• Thus, a digital grid is made which describes the image.
Color separation for digital cameras• Colored filters
CCDs
• CCDs can collect 90% of photons that strike them• Photographic plates can only collect 10% of the
photons• CCDs are split into squares called pixels• Data is in electronic form
Hubble Telescope
• Can observe in visible, infrared, and ultraviolet wavelength regions
• Named after Edwin Hubble, the father of modern cosmology
Hubble (launched in 1990)
Telescope is the size of a school bus
2.4 m mirror
Initially• Hubble’s primary mirror was polished to the
wrong shape • Was too flat at the edges• Was barely 2.3 micrometers out from the required
shape (1/50 the width of a human hair)• Images were not focused as well as they could be• Later shuttle mission fixed this problem by
installing a number of small mirrors
http://dayton.hq.nasa.gov/IMAGES/SMALL/GPN-2002-000064.jpg
Jupiter
Hubble replacement
• The first major components of the new James Webb Space Telescope are now being assembled.
• While Hubble is the size of a bus, the new James Webb will be the size of a jetliner.
• Will launch in 2014• James Webb is a former NASA administrator
during the Apollo program
• http://video.pbs.org/video/1456686369/
Any Questions?