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Astrophysics from Space Lecture 2: Beating the atmosphere. Prof. Dr. M. Baes (UGent) Prof. Dr. C. Waelkens (KUL) Academic year 2013-2014. The Earth atmosphere. Lord Rayleigh (1842-1919). The Earth atmosphere.

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Astrophysics from Space

Lecture 2: Beating the atmosphere

Prof. Dr. M. Baes (UGent)

Prof. Dr. C. Waelkens (KUL)

Academic year 2013-2014

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

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 = 0.014 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 arcsecLa 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 imageExposure 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

Still many problems

- Reliable LGS AO systems are still rare…

- Atmospheric conditions must be optimal for AO

- Turbulence cells are typically smaller than telescope

-> multi-conjugate AO