Integration of SLITRONOMY into LENSTRONOMY. The global architecture of LENSTRONOMY is left unchanged. When the ‘SLIT_STARLETS’ profile is used (for modelling source or lens surface brightness), the image likelihood is computed through the sparse solver of SLITRONOMY, instead of the usual solver. Changes in LENSTRONOMY are indicated in red: a new light profile ‘SLIT_STARLETS’ has been introduced; linear amplitudes (originally defined for analytical profiles) are replaced by a sparse optimisation of individual coefficients in starlet space; source grid coordinate offsets are added as non-linear parameters, jointly optimised with, for example, lens model parameters. The integration of point source modelling is also illustrated. See Sect. 4 for details.