Astronomy and Astrophysics – Astrophysics
Scientific paper
Jun 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995a%26as..111..569f&link_type=abstract
Astronomy and Astrophysics Supplement, v.111, p.569
Astronomy and Astrophysics
Astrophysics
41
Atmospheric Effects, Techniques: Miscellaneous, Telescopes
Scientific paper
The tilt of a wavefront cannot be derived from an artificial star, thus preventing integration with adaptive optics. We propose the polychromatic artificial star concept to get the tilt without the need for a natural guide star, i.e.: everywhere in the sky. It is based on the variation of the air index with wavelength λ. We show that a Zernike mode can be known for any λ from differential measurements with respect to λ. We apply this property to image centroids. We propose to create the polychromatic artificial star from mesospheric sodium. Two lasers are required, tuned at 589nm (3S-4D) and at 569nm (3P- 4D). We develop the atomic pumping model using rate equations, taking into account the sequential population from 3S to 4D levels via the 3P level. We include the resonant two-photon absorption populating the 4D level directly from the 3S level. The spectrum of the radiative cascade from the 4D level spans from 0.33 to 2.3μm which fits very well the requirements of the polychromatic artificial star. We determine the returned flux required to measure the differential tilt as a function of the Fried parameter r_0_, of the wavelengths of the spot, and of the wavelength of the astronomical observation. The required returned flux is found not to be dependent on the telescope diameter D, as long as D/r_0_>1. We deduce a minimum duration for the laser emission and the corresponding required laser power. Existing lasers as those used in the isotopic separation program match the specifications. Finally we discuss future improvements of the concept, such as i) coupling the laser emission with an adaptive optics in order to form a diffraction limited spot, ii) emitting synchronous macro-pulses of picosecond pulses, and iii) processing of images at several wavelengths. They should enable lowering the laser power by at least one order of magnitude, thereby making the use of this technique in astronomical sites much more feasible.
Biraben François
Foy Renaud
Grynberg Gilbert
McCullough Peter R.
Migus Arnold
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