We investigate the spatial characteristics of high-order harmonic radiation
generated in argon, and observe cross-like patterns in the far field. An
analytical model describing harmonics from an astigmatic driving beam reveals
that these patterns result from the order and generation position dependent
divergence of harmonics. Even small amounts of driving field astigmatism may
result in cross-like patterns, coming from the superposition of individual
harmonics with spatial profiles elongated in different directions. By
correcting the aberrations using a deformable mirror, we show that fine-tuning
the driving wavefront is essential for optimal spatial quality of the
harmonics