  We present a concept of the X-ray imaging system with high angular-resolution
and moderate sensitivity. In this concept, a two-dimensional detector, i.e.,
imager, is put at a slightly out-of-focused position of the focusing mirror,
rather than just at the mirror focus as in the standard optics, to capture the
miniature image of objects. In addition, a set of multi-grid masks (or a
modulation collimator) is installed in front of the telescope. We find that the
masks work as a coded aperture camera and that they boost the angular
resolution of the focusing optics. The major advantage of this concept is that
a much better angular resolution an order of 2--3 or more than in the
conventional optics is achievable, while a high throughput (large effective
area) is maintained, which is crucial in photon-limited high-energy astronomy,
because any type of mirrors, including light-weight reflective mirrors, can be
employed in our concept. If the signal-to-noise ratio is sufficiently high, we
estimate that angular resolutions at the diffraction limit of 4" and 0"4 at
about 7 keV can be achieved with a pair of masks at separations of 1~m and
100~m, respectively, at the diffraction limit.
