Dust cell geometry. Top: the LoS towards each of the N stars is divided up into a number of dust cells. The centre of each cell is shown as a small black square. (Here we show cells of constant size.) The total number of dust cells (towards all stars) is J, and the length of dust cell j towards star n is denoted g_n,j. Bottom: these cells are represented by a sparse matrix G of size N × J. Each row has non-zero elements just for the cells along the LoS to that star.