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MOSAIC is the ELT wide-field spectrograph capable of observing over 200 targets simultaneously within an AO-corrected 7.4 arcminute diameter field of view (FoV) across a wavelength range of 400-1800nm. The requirements for the AO correction are specified in terms of enclosed energy (EE) improvement across the field, with an improvement of 1.4 in H-band EE is expected within a 600 milliarcsecond aperture when AO correction is applied in poor seeing conditions. Ground-layer adaptive optics (GLAO) is used to provide this correction. Due to the partial AO correction provided by GLAO and the size of the aperture, the sum of individual wavefront error terms cannot be used to provide an estimate of system performance or used to define performance requirements relating to image quality or stability for instrument subsystems. In this paper we describe the method that will be used to determine the MOSAIC image quality budget, using the results of Monte-Carlo simulations of GLAO performance at the ELT to investigate the limitations of the proposed approach which show that a convolution of the AO corrected PSF with a high-order PSF representing instrumental and non-AO effects is sufficient to determine encircled energy for the MOSAIC instrument.
