Astrophysics from Space
Lecture 2: Beating the atmosphere
Prof. Dr. M. Baes (UGent)
Prof. Dr. C. Waelkens (KUL)
Academic year 2014-2015
The Earth atmosphere
Lord Rayleigh (1842-1919)
The Earth atmosphere
Relative concentrations of
permanent gases in the atmosphere
is relatively constant.
Non-permanent gases: H20 and O3
Water vapor in the atmosphere
H20 (and CO2 to some extent) are
responsible for most of the extinction in
the optical and NIR regime
Good news: H20 is low-altitude phenomenon
Mauna Kea – Hawaii
La Silla – Chile
Paranal – Chile
Optical observatories
Most large ground-based
optical/NIR observatories are
located on mountain tops
Diffraction of light
Diffraction is a natural consequence
of the wave nature of light.
Airy diffraction pattern
Diffraction of light
Airy diffraction: point sources are converted to a disc (Airy
disc) with size θmin ~ 1.22 λ/D
VLT with D = 8.2 m
optical radiation at λ = 550 nm
θmin = 0.017 arcsec
Unfortunately, it is currently impossible to
realize diffraction limited observations in the
optical, due to optical aberrations in the
telescope and atmospheric turbulence.
Active optics
Elimination of the aberrations in the telescope optics by
(continuous) correction of the telescope shape.
Result: seeing-limited observations
Atmospheric turbulence
Atmospheric turbulence spreads out optical radiation to a much
bigger disk (seeing disc) than the diffraction limit.
The seeing depends critically on the site.
Typical seeing values:
Belgium: few arcsec
La Palma: 1 arcsec
Paranal: 0.6 arcsec
Dome C: 0.3 arcsec
Atmospheric turbulence
In seeing-limited observing, the 8 m VLT telescope has the
same resolution as an amateur 30 cm telescope
Beating the atmosphere
Seeing is the result of “speckles” moving on the plane of the
sky at atmospheric time scales 10 ms to 100 ms
Speckle imaging and Lucky imaging
Use very fast, short-exposure images and combine them at
the end of the observation
• speckle imaging: combine all images
• lucky imaging: select only the “best” images
Traditional image
Exposure time 300s
Speckle image
Exposure time 300s
Lucky image
Exposure time 15s
More Lucky imaging examples
Cat’s Eye nebula
Globular cluster M13
Adaptive optics
1. Measure the distortions in the wavefront
2. Compensate for them using deformable mirror or liquid
crystal array
Adaptive optics
Advantage of AO: one can keep integrating
(so not limited to bright sources as speckle/Lucky imaging)
Problem: bright reference star necessary: NGS/LGS
Adaptive optics
Adaptive optics
Still many problems
• reliable LGS AO systems
are still rare…
• atmospheric conditions
must be optimal for AO
• turbulence cells are
typically smaller than the
-> multi-conjugate AO
The solution: space telescopes

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