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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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Lecture 2: Beating the atmosphere
Prof. Dr. M. Baes (UGent)
Prof. Dr. C. Waelkens (KUL)
Academic year 2013-2014
Lord Rayleigh (1842-1919)
Relative concentrations of permanent gases in the atmosphere is relatively constant.
Non-permanent gases: H20 and O3
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
Most large ground-based optical/NIR observatories are located on mountain tops
Diffraction is a natural consequence of the wave nature of light.
Airy diffraction pattern
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.
Elimination of the aberrations in the telescope optics by (continuous) correction of the telescope shape.
Result: seeing-limited observations
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
In seeing-limited observing, the 8 m VLT telescope has the same resolution as an amateur 30 cm telescope
Seeing is the result of “speckles” moving on the plane of the sky at atmospheric time scales 10 ms to 100 ms
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
Exposure time 300s
Speckle imageExposure time 300s
Exposure time 15s
Cat’s Eye nebula
Globular cluster M13
1. Measure the distortions in the wavefront
2. Compensate for them using deformable mirror or liquid crystal array
Advantage of AO: one can keep integrating(so not limited to bright sources as speckle/Lucky imaging)
Problem: bright reference star necessary: NGS/LGS
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