Integrating AO in a performance budget: toward a global system engineering vision

International audienceEAGLE (Extremely large Adaptive telescope for GaLaxy Evolution) is one of the eight E-ELT instrument concepts that was developed as part of the Phase A E-ELT instrument studies. EAGLE is a near-infrared wide field multi object spectrograph1. It includes its own multi-object adaptive optics system (MOAO) and its subsystems are cooled down so as to ensure that the instrument can both achieve the desired spatial resolution and to be sure that the instrument is background limited, as required in the primary science case, to deliver the performance in the K-band. In this paper we discuss the performance matrix developed to allow us to partition and allocate the important characteristics to the various subsystems as well as to describe the process to verify that the current concept design will deliver the required performance. Due to the integrated nature of the instrument, a large number of AO parameters have to be controlled. The performance matrix also has to deal with the added complexity of active optical elements such as the science channel deformable mirrors (DMs). This paper also defines a method of how to convert the ensquared energy (EE) and signal-to-noise ratio (SNR) required by the science cases into the "as designed" wavefront error and the overall residue wavefront error. To ensure successful integration and verification of the next generation instruments for ELT it is of the utmost importance to have method to control and manage the instrument's critical performance characteristics at very early design steps