Successful application of PSF-R techniques to the case of the globular cluster NGC 6121 (M 4) D. Massari, A. Marasco, O. Beltramo-Martin, J. Milli, G. Fiorentino, E. Tolstoy and F. Kerber A&A, 634 (2020) L5 Published online: 31 January 2020 DOI: 10.1051/0004-6361/201937359