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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. Telescopio Nazionale Galileo (3.5m). Hubble Space Telescope (2.4m). 2.5m Mount Wilson telescope.
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Role of atmosphere in astronomical observations
Definition of angular resolution
Optical, radio, X-ray telescopes
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?
By using adaptive optics
By using interferometry
By putting a telescope in space (HST)
Adaptive optics use wavefront sensors to adapt a deformable mirror to variations of the local seeing
Adaptive optics at
Comparison of NTT/SOFI and VLT/NACO images
Angular size of each image: 16.5” x 11.5”
With an ARRAY of telescopes I can combine the wave trains
(aperture synthesis) and improve the resolution
Very Large Telescope:
Four 8.2m telescopes
Resolution = /B
B = Baseline
By image synthesis
Comparison of ground-based optical
and HST Images of the Homunculus
Nebula around Eta Carinae. The
Larger axis of the nebula is about 30
Natural concavities favour
Screening from human-related
Resolution: ~10 arcmin
Croce del Nord (Medicina)
Very Large Array (New Mexico)
Very Long Baseline Interferometry (ang. Res.: milliarcsecond)
European VLBI Network (EVN)
Very Long Baseline Array (VLBA)
HALCA: Japanese 8m
Antenna orbiting the
Ang. Res: milli-to-microarcsec
Of the active
(z = 0.033)
Chandra XMM-Newton ASCA
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