This document presents the imaging simulations performed at IRAM to study the importance of the ALMA Compact Array (ACA) on the wide field imaging performances of ALMA. The simulations are based on a fast, approximate but sufficiently accurate method. The image reconstruction is based on two new CLEAN-based deconvolution methods for mosaics. The effects of i) pointing errors, ii) amplitude calibration errors and iii) atmospheric phase noise have been systematically studied. The effect of a poor knowledge of the primary beams has also been studied. Finally we investigated the impact of variations of the deconvolution parameters. A large number of simulations have been performed, using several test images. This allowed us to disentangle artifacts due to particular random sampling of the errors from the genuine, average impact of these errors on all images. We find that adding ACA observations improves the image quality of ALMA in some types of images. More importantly, ACA provides robust results, when ALMA fails in presence of some type of errors, or on some images. One of the two new deconvolution techniques requires only a very moderate increase in computing time to include the ACA data.
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