PowerPoint Presentation

Astrophysics from SpaceLecture 2: Beating the atmosphere

Prof. Dr. M. Baes (UGent)Prof. Dr. C. Waelkens (KUL)

Academic year 2015-2016

The Earth atmosphere

Lord Rayleigh (1842-1919)

The Earth atmosphere

Relative concentrations of permanent gases in the atmosphere is relatively constant.

Non-permanent gases: H2O and O3Water vapor in the atmosphere

H2O (and CO2 to some extent) are responsible for most of the extinction in the optical and NIR regimeGood news: H2O is low-altitude phenomenon

Mauna Kea Hawaii4

La Silla Chile5

Paranal Chile6

Optical observatoriesMost large ground-based optical/NIR observatories are located on mountain tops 7Diffraction of light

Diffraction is a natural consequence of the wave nature of light.

Airy diffraction pattern

Diffraction of lightAiry diffraction: point sources are converted to a disc (Airy disc) with size min ~ 1.22 /DExample: VLT with D = 8.2 moptical radiation at = 550 nmmin = 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 opticsElimination 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 arcsecParanal: 0.6 arcsecDome C: 0.3 arcsecAtmospheric 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 imagingUse very fast, short-exposure images and combine them at the end of the observationspeckle imaging: combine all imageslucky imaging: select only the best images

Traditional imageExposure time 300sSpeckle imageExposure time 300sLucky imageExposure time 15sMore Lucky imaging examples

Cats Eye nebulaGlobular cluster M13Adaptive opticsMeasure the distortions in the wavefront Compensate for them using deformable mirror or liquid crystal array

Adaptive opticsAdvantage 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 opticsStill many problemsreliable LGS AO systems are still rareatmospheric conditions must be optimal for AOturbulence cells are typically smaller than the telescope -> multi-conjugate AO

The solution: space telescopes