Magnitude differences for stars of all spectral types between NISP and external systems (2MASS, MKO, and VISTA), as a function of colour in an external system. The red lines show the linear fits given in Eqs. (E.14) through (E.22). We note that the two fits displayed in the third row are skewed by a number of very red sources that are not easily visible in these linear grey scale maps. This shows that a simple, ‘one-flts-alľ model is not always an appropriate way to transform between photometric systems.