MOSAIC GLAO performance and system architecture: AO for the entire ELT focal plane
Résumé
MOSAIC is a wide-field s pectrograph, c ombining multiple-object s pectroscopy a nd i ntegral fi eld un its, to cover the ELT focal plane with a field-of-view o f 7.8 a rcmin f rom t he b lue t o t he n ear-infrared, 3 90-1800nm. I n the current Phase B design, AO is GLAO supported by four LGS in a fixed a sterism a nd w ith m ultiple NGS. Although the GLAO correction is modest compared to other ELT instrumentation, the use of the integrated M4/M5 correction elements and the existing LGS allows for an efficient de sign wh ich is ou tlined. MOSAIC GLAO will use the ELT PFS guide-probes to compensate for high-frequency tip/tilt errors, greatly relaxing the requirements on the instrumental NGS sensors. The Phase A architecture used the same pick-off mirrors as the IFU instruments to feed the NGS-WFS from anywhere in the focal plane, which was mandatory for the proposed MOAO design. The reduced performance requirements at Phase B allows us to take advantage, instead, of the four 2 arcmin diameter field-of-view t hrough t he L GS c utouts, a rranged i n a s quare p attern at an off-axis d istance o f 3.75 a rcmin. I n e ach L GS c utout, a w ide-field-imager is im plemented-alongside on e LGS WFS-to acquire multiple NGS that supports both slow tip/tilt measurements, isolating instrument-Nasmyth flexure, s olving f or t he a strometric d istortion e xpected f rom e rrors i n t he E LT o ptical p ath, a nd s upporting the alignment of MOS apertures with the field. T he l atter i s a k ey r equirement f or M OSAIC, l eading t o 40mas accuracy in MOS aperture positioning and 40mas rotation displacement at the edge of the scientific field.
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