  We present a new technique - photon folding - for imaging in non-focusing
telescopes. Motivated by the epoch-folding method in timing analysis, the
photon folding technique directly provides the statistical significance of
signals in images, using projection matrices. The technique is very robust
against common imaging problems such as aspect errors and non-uniform
background. The technique provides a deterministic recursive algorithm for
improving images, which can be implemented on-line. Higher-order photon folding
technique allows a systematic correction for coding noises, which is suitable
for studying weak sources in the presence of highly variable strong sources.
The technique can be applied to various types of non-focusing telescopes such
as coded-aperture optics, rotational collimators or Fourier grid telescopes.
