astronomy is remote sensing we cannot repeat (or change) the universe in a controlled environment....
TRANSCRIPT
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Astronomy is remote sensing
We cannot repeat (or change)the Universe in a controlledenvironment. We cannot makeplanets, stars, or galaxies. Wecannot make the vacuum of space, nor the shape of space-time around a black-hole.
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Early Observatories• The Incan Chankillo Observatory
~2400yrs old, “solar”
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Galileo’s Telescope
The Starry Messenger (1610)• SIDEREAL MESSENGER
unfolding great and very wonderful sightsand displaying to the gaze of everyone,but especially philosophers and astronomers,the things that were observed byGALILEO GALILEI,Florentine patricianand public mathematician of the University of Padua,with the help of a spyglass lately devised by him,about the face of the Moon, countless fixed stars,the Milky Way, nebulous stars,but especially aboutfour planetsflying around the star of Jupiter at unequal intervalsand periods with wonderful swiftness;which, unknown by anyone until this day,the first author detected recentlyand decided to nameMEDICEAN STARS
(trans. A. van Helden, p. [26])
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Radio Astronomy “Discovered” in 1931 Jansky couldn’t get rid of the noise
Jansky’s radio telescope Reber’s radio telescope 1937
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X-ray Astronomy
• “Discovered” in 1949• Moon mapped in 1962• Giacconi wins Nobel ‘92
prize for X-ray astronomy
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Microwave Astronomy “Discovered” in 1964
Penzias and Wilson Mather and Smoot
Discovered CMB Discovered CMB Anisotropies
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Why a Telescope?
We have two telescopes, our eyes:
Collect Light
Form an Image
Interpret the Image
To Brain(interpretation)
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The Role of Lenses
Lenses can bring light rays to a focus (increase intensity)
Light rays from different locations form an image (the dog)
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The Importance of Light-Gathering Power
• Can only see to 6th magnitude by eye• Limit due to not enough photons• To see fainter things, we need Bigger eyes
Bigger objective gathers more light:Brighter imagesFlux depends on area of objectiveF π (D/2)∝ 2
D= diameter
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Example of how more photons give a more detailed image:
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So you want a really big eye: a telescope with large collecting area
2 ways of collecting light: lenses or mirrors (or both)
Galileo’s telescope
It is a “refracting” telescope (collects light with lenses)
Galileo’s telescope lens: 2.6 cm
Naked eye (pupil): 0.5 cm
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Refractor
Yerkes 40-inch telescope; largest refractor in the world
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Reflecting Telescopes: Mirrors• Can make mirrors much larger than lenses• Avoids chromatic aberration
Arrangements:
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Keck telescopesSubmit of Mauna Kea, Hawaii
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Gemini North Telescope 8.1 mt
Gemini North/South (Hawaii/Chile)
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Magellan Observatory, Las Campanas, Chile
6.5 mt
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Very Large Telescope (ESO)
Each 8.2mt,equivalent area 16 mt
Secondary mirror adjusts itself to produce best images
http://www.eso.org/public/videos/vlttrailer2009/
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In the future: Giant Magellan Telescope
Seven 8.4 meter or 27-foot segments, forming a single optical surface with a collecting area of 24.5 meters, or 80 feet in diameter.
The GMT will have a resolving power 10 times greater than the Hubble Space Telescope.
Construction has started
http://www.gmto.org/index.html
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Next decade: European Extremely Large Telescope (E-ELT)
39 mt800 segments, each 1.4 metres wide, but only 50 mm thick
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Job #2: Angular Resolution
Resolution (detail you can image) is limited by the optics• smallest angle which can be seen
• (rad) = 1.22 / D• due to diffraction
HST image of a point source (structure due to telescope optics)
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Diffraction of Light: Interaction of light waves and edges
Here, the edges are due to the telescope size (primary mirror)
Most power in central maximumWidth of maximum λ/D∝For circular apertures, θ(rad) = 1.22λ/D
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(rad) = 1.22 / D p rad = 180 degrees
=> 1 rad = 180x60x60/p = 2.05e5 seconds of arc (“)
(“) = 1.22x2.05e5 / D = 2.5e5 / D
For optical light λ = 5.5e-7m = 5.5E-5 cm is
q(rad) = 1.22 x [5.5E-5 cm/D(cm)] = 6.7e-5 / D(cm)
q(“) = 2.0e5 x [5.5E-5 cm/D(cm)] = 13.8” / D(cm)
Diffraction limit
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Diffraction limit for optical light (λ = 5.5E-5 cm) is
q = 13.8”/D(cm)
q = 6.7e-5 / D(cm)
The eye has D = 0.5 cm
So the limiting angle is about 28” (1.3E-4 radians)
Example: Resolution Limit of Your Eye
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Atmospheric Turbulence Also Messes Up Images (on Earth)
Actual images larger than diffraction limit
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Low resolution:
1’’
A better location + a better telescope = BETTER DATA!Better resolution
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Atmospheric Blurring (“seeing”): 0.6” at an excellent site
Diffraction limit for optical light is 13.8”/D(cm)
0.6” = 13.8”/D so D = 13.8/0.6 = 23 cm (9”, not very big)
So all ground-based telescopes are “seeing” limited unless you can do something special (adaptive optics).
In space, you avoid this seeing problem and get the full diffraction limited resolution (0.05” for Hubble Space Telescope).
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A better location + a better telescope = BETTER DATA!Better resolution
SDSS HST
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Atacama Large Millimeter Array, ALMA
Atacama dessert, Chile, international collaboration
64 antennas, 12 mt each0.3 – 4 mm, resolution to 0.005”Movable, at largest 14 kmThe largest and most capable imaging array of telescopes in the world.
Three configurations
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Detectors in the optical: Eyes, Film, CCD
Rods: B&W detectors
Cones: Color detectors
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Rods: higher sensitivity, but not color (faint things all look white)
Cones: 3 color receptors (like filters): blue, green, red
Eyes as Detectors, cont.
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CCDs take “black & white” pictures: do not measure energy of photons
Detectors in the optical: Eye, Film, CCDs
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A CCD is an array of light-collecting buckets – signal proportional to intensity of light
“read out” the light buckets…record how many photons landed in each bucket
Charge-coupled device CCD
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Instruments collect photons
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blue light
red light
CCD
+ filters
So how do we see color?
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Red Green Blue
how do you get color pictures?
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Red Green Blue
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Red Green Blue
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Imaging: A Primary Use of Telescopes
– use a camera to take pictures (images)– Photometry measure total amount of light
from an object (apparent brightness) in a given wavelength band
Spectroscopy: The Other Primary Use of Telescopes
a spectrograph separates the light into its different wavelengths
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Spectroscopy
• The spectrograph reflects light off a grating: a finely ruled, smooth surface
• Light interferes with itself and disperses into colors
• This spectrum is recorded by a digital CCD detector
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Diffraction Grating: Dispersing the Light
Has the same effect as a prism, but technically better
Maximum at angle sinθ = m λ / dd = separation of slits or steps, m = order
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White Dwarf
Example of spectra
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Atmospheric Absorption of Light • Earth’s atmosphere absorbs most types of light• Only visible, radio, and certain IR and UV light make it
through to the ground• To observe the other wavelengths, we must put our
telescopes in space!
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What about the light we see?
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From image to catalog
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An example : Michael Richmond (Creative Commons License)
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Source Catalog Creation:in practice
Bertin; Holwerda
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Source finding is hard andcomplicated
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Defining parents and their children
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Choosing apertures
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Separating stars and galaxies
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What do we do with the data?