  The shortest light pulses produced to date are of the order of a few tens of
attoseconds, with central frequencies in the extreme ultraviolet range and
bandwidths exceeding tens of eV. They are often produced as a train of pulses
separated by half the driving laser period, leading in the frequency domain to
a spectrum of high, odd-order harmonics. As light pulses become shorter and
more spectrally wide, the widely-used approximation consisting in writing the
optical waveform as a product of temporal and spatial amplitudes does not apply
anymore. Here, we investigate the interplay of temporal and spatial properties
of attosecond pulses. We show that the divergence and focus position of the
generated harmonics often strongly depend on their frequency, leading to strong
chromatic aberrations of the broadband attosecond pulses. Our argumentation
uses a simple analytical model based on Gaussian optics, numerical propagation
calculations and experimental harmonic divergence measurements. This effect
needs to be considered for future applications requiring high quality focusing
while retaining the broadband/ultrashort characteristics of the radiation.
