Experimental validation of LGS wavefront sensing in presence of low-order modes induced by Sodium profile variations


Laura Schreiber (1)(2) Matteo Lombini (3) Emiliano Diolaiti (3) Giovanni Bregoli (3) Giuseppe Cosentino (1) Italo Foppiani (3)


(1) Università di Bologna – Dipartimento di Astronomia (2) INAF – Osservatorio Astronomico di Padova (3) INAF – Osservatorio Astronomico di Bologna


The current baseline for the Multi-Conjugate adaptive optics module (MAORY) for the E-ELT is based on 6 Sodium Laser Guide Stars (LGS) projected from the edge of the telescope pupil. LGS wavefront sensing is performed by 6 Shack-Hartmann WFS of order 84x84 operated at 500Hz. The slope measurement accuracy is affected by the perspective elongation effect: the loss of performance may be mitigated through advanced centroiding algorithms, that however require a ‘template’ of the LGS spot in each subaperture. While the template is being used, some sodium profile variation unavoidably occurs. This mismatch between the template and the actual spot translates into time varying low order aberrations that must be monitored by a Reference Wavefront Sensor (WFS) based on Natural Guide Stars. The dimensioning of this WFS in term of number of required subapertures and operating frequency is a key aspect with potentially strong impact on the final sky coverage of MAORY. The LGS WFS laboratory prototype available at INAF – Osservatorio Astronomico di Bologna has been enhanced in order to simulate automatically variable and repeatable sodium profiles and ‘quantify’ the order and power of non-atmospherical aberrations induced by the sodium variation itself and by its effect on advanced centroid algorithms. This paper shows the results of the tests performed with this improved prototype.