Physics – Optics
Scientific paper
May 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001sf2a.conf..623v&link_type=abstract
Abstracts from SF2A-2001: Semaine de l'Astrophysique Francaise, meeting held in Lyon, France, May 28-June 1st, 2001, Eds.: F. Co
Physics
Optics
Scientific paper
Multi--conjugate adaptive optics systems use several reference guide stars to correct the image degradation in a chosen Field Of View (FOV). When the scientific FOV is far away from the reference field, the wavefront sensors, namely a Shack-Hartmann one, can experience in closed--loop non--flat wavefronts and the non--linear response due the variation of the size of the spot can propagates up to the achieved correction. The random variation of the signal results in a residual error which limits the correction reachable with such a system. We estimate the lower limit for such an effect when each reference spot centroid is obtained by a 4--quadrants sensor. We show that the atmospheric defocus term inside a single sub-aperture introduces a non-negligeable error on the centroiding measurement when r0 > 10 is reached. Therefore, even if this effect could be neglected for 8 meter telescope class in the infrared, it becomes relevant in case of visible AO, especially for the ELT case. The range of applicability of this result and the countermeasures to cancel or minimize this problem are slightly sketched out.
Diolati E.
Ragazzoni Roberto
Tordi Massimiliano
Viard Elise
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