  X-rays provide one of the few bands through which we can study the epoch of
reionization, when the first galaxies, black holes and stars were born. To
reach the sensitivity required to image these first discrete objects in the
universe needs a major advance in X-ray optics. Generation-X (Gen-X) is
currently the only X-ray astronomy mission concept that addresses this goal.
Gen-X aims to improve substantially on the Chandra angular resolution and to do
so with substantially larger effective area. These two goals can only be met if
a mirror technology can be developed that yields high angular resolution at
much lower mass/unit area than the Chandra optics, matching that of
Constellation-X (Con-X). We describe an approach to this goal based on active
X-ray optics that correct the mid-frequency departures from an ideal Wolter
optic on-orbit. We concentrate on the problems of sensing figure errors,
calculating the corrections required, and applying those corrections. The time
needed to make this in-flight calibration is reasonable. A laboratory version
of these optics has already been developed by others and is successfully
operating at synchrotron light sources. With only a moderate investment in
these optics the goals of Gen-X resolution can be realized.
