Table 2.

The 1st–10th terms of the two-dimensional Legendre polynomial.

Term	Expression	Aberration
1	1	Piston
2	$P_{X1}{P}_{Y0}$	X-tilt
3	$P_{X0}{P}_{Y1}$	Y-tilt
4	$P_{X2}{P}_{Y0}$	X-defocus
5	$P_{X1}{P}_{Y1}$
6	$P_{X0}{P}_{Y2}$	Y-defocus
7	$P_{X3}{P}_{Y0}$
8	$P_{X2}{P}_{Y1}$
9	$P_{X1}{P}_{Y2}$
10	$P_{X0}{P}_{Y3}$