Experimental validation of a technique to measure tilt from a laser guide star

Abstract

We have experimentally demonstrated for what is believed to be the first time a method for sensing wave-front tilt with a laser guide star (LGS). The tilt components of wave fronts were measured synchronously from the LGS by use of a telescope with a 0.75-m effective aperture and from the star Polaris by use of a 1.5-m telescope. The Rayleigh guide star was formed at an altitude of 6 km and at a corresponding range of 10.5 km by projection of a focused beam at Polaris from the full aperture at the 1.5-m telescope. Both telescope mounts were unpowered and bolted in place, allowing us to reduce substantially the telescope vibration. The maximum value of the measured cross-correlation coefficient between the tilt for Polaris and the LGS is 0.71. The variations of the measured cross-correlation coefficient in the range from 0.22 to 0.71 are caused by turbulence at altitudes above 6 km, which was not sampled by the laser beacon but affected tilt for Polaris, the cone effect for turbulence below 6 km, residual mount jitter of the telescopes, and variations of the signal/noise ratio. The results support our concept of sensing atmospheric tilt by observing a LGS with an auxiliary telescope and indicate that this method is a possible solution for the tip-tilt problem.

Fundamental limits on isoplanatic correction with multiconjugate adaptive optics

“…Indeed, a similar analysis demonstrates that the global image motion of a scene, arising from the tilt modes over the metapupil, is range dependent. Although this is unimportant for current astronomical MCAO system designs using LGSs, because the lasers are not used to sense global tilt, it will be a concern in any system designed to recover tilt from laser signals in the future [19][20][21][22] and for any nonastronomical system viewing targets at finite distance. If image motion information is sensed from beacons at two distinct ranges, then tip-tilt driving signals can be computed for two DMs that would correct the motion independently of range.…”

Polychromatic guide star: feasibility study

“…Then, the resulting theoretical ratio of the return flux at the LLNL versus that at the CEA is 11. the return flux at the CEA is expected to be lower by a factor of 3.6.…”

“…The Strehi ratio due to the tilt writes as26 Stilt = 1 2' (11) 1 + (h + u) () where ph and ob stand for the photon noise standard deviation and for the bandwidth error respectively, where .A is the observation wavelength, and where D is the diameter of the telescope under consideration. The Strehi ratio due to the tilt writes as26 Stilt = 1 2' (11) 1 + (h + u) () where ph and ob stand for the photon noise standard deviation and for the bandwidth error respectively, where .A is the observation wavelength, and where D is the diameter of the telescope under consideration.…”

Performance analysis of polychromatic laser guide stars used for wavefront tilt sensing

“…The tilt problem is caused by the inverse return property of light, which makes it impossible to measure the overall wavefront tilt with a single on‐axis monochromatic LGS (Pilkington 1987; Séchaud et al 1988). A number of different solutions to the tilt problem have been proposed and some of them have been pursued experimentally (Esposito et al 2000; Foy et al 2000b; Belenkii et al 1999; Belenkii 2000). In this paper, we examine in detail the use of a polychromatic laser guide star (PLGS) as a possibility to solve the tilt problem (Foy et al 1995).…”