“…Paraxial rays and aberrations are then computed in terms of x(t), y(t), h(t), and their time derivatives _ xðtÞ, _ yðtÞ, _ hðtÞ. For computing the aberrations, we have used the differential algebra (''DA'') method [15,16]. The coordinates x(t), y(t), h(t) and velocity components _ xðtÞ, _ yðtÞ, _ hðtÞ are represented by DA Quantities, called X(t), Y(t), H(t) and _ X ðtÞ, _ Y ðtÞ, _ H ðtÞ.…”