Real-time optical path difference compensation at the Plateau de Calern I2T interferometer

Abstract

Abstract. The fringe tracker system of the ASSI (Active Stabilization in Stellar Interferometry) beam combining table at the I2T interferometer is described and its performance evaluated. A new real-time algorithm for the optical path difference (OPD) measurement is derived and validated. It is based on a sinusoidal phase modulation whose amplitude is optimized. It also allows automatic fringe detection at the beginning of an observation when scanning the OPD. The fringe tracker servo-loop bandwidth is adjusted by a numerical gain and ranges between 20 and 50 Hz in the reported experiments. On stars, fringe-locked sequences are limited to 20 s due to fringe jumps. However, the fringe tracker is able to recover the coherence area after a few seconds. Such a fringe tracker operation can last more than one hour. A fringe tracking accuracy of 85 nm is achieved for visibility ranging between 7 and 24%, a turbulence coherence time of approximately 9 ms at 0.85 µm, a Fried parameter of around 14 cm at 0.5 µm and an average light level of 100 000 photoevents/s, (typically visual magnitude 2 in the conditions of the experiment). Visibility losses are evaluated and are found to be mainly due to turbulent wavefront fluctuations on the two telescopes and to the static aberrations of the optical train. The measurements of OPD and angle of arrival are reduced to derive turbulence parameters: the coherence time, the average wind speed, the Fried parameter and the outer scale. Our estimations for the outer scale range between 20 and 120 m, with an average value of the order of 40 m. Both OPD and angle of arrival data, obtained on 15 m baseline and a 26 cm telescope diameter respectively, are fully compatible with the same modified Kolmogorov spectrum of the turbulence, taking into account a finite outer scale.

Multiple aperture optical telescopes: some key issues for earth observation from a GEO orbit

“…Measurement of tip/tilt, or of higher-order aberration modes, is now a well established technique for monolithic telescopes, even on very extended objects such as the Earth seen from space [16]. Piston measurement has also been widely studied for metrology sensors, and piston compensation of distant telescopes has been demonstrated with non-cooperative sources on the ground [17,18]. But for most of these devices based on a pupil-plane combination, the contrast of interference fringes strongly decreases as the object extension increases, which makes them useless on very extended scenes.…”

Constraining the Adaptive Optics Point‐Spread Function in Crowded Fields: Measuring Photometric Aperture Corrections

“…To construct the AO corrected atmospheric MTF, we adopt a modified Kolmogorov atmospheric phase power spectrum with a finite outer scale, (Strohbehn 1968). Measurements suggest that the outer scale of turbulence, L 0 ≃ 10-100 m (Sorrente et al 2001;Linfield et al 2001). The form of Eq.…”

“…A5) (Strohbehn 1968). Measurements suggest that the outer scale of turbulence, L 0 ≃ 10-100 m (Sorrente et al 2001;Linfield et al 2001). The form of Eq.…”

Upgrading the GRAVITY fringe tracker for GRAVITY+

“…In modern instruments, this is the role of a specific subsystem: the fringe tracker. The crucial role of fringe-tracking in optical interferometry has justified the extensive development efforts made over the past decades (Shao et al 1988;Sorrente et al 2001;Delplancke et al 2006;Le Bouquin et al 2008;Cassaing et al 2008;Houairi et al 2008;Colavita et al 2010;Lozi et al 2011;Menu et al 2012;Choquet et al 2014;Lacour et al 2019). Various techniques have been proposed, A184, page 1 of 16 based either on hardware or software solutions, but they all share a common objective: to measure and stabilise the fringes, thereby maximising the signal-to-noise ratio (S/N) of interferometric measurements.…”