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Supporting material for Lecture 1:
Role of atmosphere in astronomical
observations
Definition of angular resolution
Optical, radio, X-ray telescopes
Atmospheric absorption
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2.5m Mount
Wilson telescope
Telescopio
Nazionale
Galileo
(3.5m)
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Hubble Space
Telescope
(2.4m)
Resolution or minimum resolvable distance is the minimum
distance between distinguishable objects in an image
Sources with angular size larger than the resolution are called
Extended, otherwise their are point-like, i.e., their angular
Extent does not exceed the point spread function
The resolution of ground-based optical images is limited by the
Atmospheric seeing (about 1-2 arcseconds on average).
How can one overcome the atmospheric limitation in optical?
1) By using adaptive optics
2) By using interferometry
3) By putting a telescope in space (HST)
Adaptive optics use wavefront sensors to adapt a
deformable mirror to variations of the local seeing
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Adaptive optics at
Gemini telescope
Luminous infrared galaxy IRAS 18293-3413 (79 Mpc)
Comparison of NTT/SOFI and VLT/NACO images
Angular size of each image: 16.5” x 11.5”
INTERFEROMETRY
With an ARRAY of telescopes I can combine the wave trains
(aperture synthesis) and improve the resolution
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Very Large Telescope:
Four 8.2m telescopes
Resolution = /B
B = Baseline
VLTI/AMBER discovers a companion to HD 87643
By image synthesis
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VLT NACO
Comparison of ground-based optical
and HST Images of the Homunculus
Nebula around Eta Carinae. The
Larger axis of the nebula is about 30
arcseconds
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HST-WFPC2
Radiotelescopes
Natural concavities favour
Screening from human-related
Radio-interference
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Resolution: ~10 arcmin
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Parabola in
Medicina, Bologna
(32m)
Arecibo (100m)
Croce del Nord (Medicina)
Radiointerferometry:
Aperture synthesis
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Angular resolution:
~1 arcsec
Very Large Array (New Mexico)
Very Long Baseline Interferometry (ang. Res.: milliarcsecond)
Very Long Baseline Array (VLBA)
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European VLBI Network (EVN)
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30000 km
Halca
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The VLBI Space
Observatory Program
(VSOP)
HALCA: Japanese 8m
Antenna orbiting the
Earth
Ang. Res: milli-to-microarcsec
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Relativistic jet
Of the active
galaxy Mkn501
(z = 0.033)
X-ray telescopes: grazing incidence
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X-ray observations of Eta Carinae
Chandra
XMM-Newton
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ASCA
X-ray: 0.1 - 100 keV
The history of X-ray astronomy started in the 1960s:
R. Giacconi 2003:
Nobel Lecture: The dawn of x-ray astronomy
Reviews of Modern Physics 75, 995
In 40 years the X-ray flux sensitivity has improved by 10
orders of magnitudes, comparable to the improvement in
optical instruments sensitivity in the last 400 years!
First high-energy baloon experiments: about e-08 erg/s/cm2
Chandra (launched 1999): e-18 erg/s/cm2