  A method for the determination of the projection geometry of highly
magnifying cone beam micro computed tomography systems based on few rotating
fiducial markers of unknown position within the field of view is derived. By
employing the projection matrix formalism commonly used in computer graphics, a
very clear presentation of the resulting self consistent calibration problem
can be given relating the sought-for matrix to observable parameters of the
markers' projections. Both an easy to implement solution procedure for both the
unknown projection matrix and the marker assembly as well as the mapping from
projection matrices to real space positions and orientations of source and
detector relative to the rotational axis are provided.
  The separate treatment of the calibration problem in terms of projection
matrices on the one hand and the independent transformation to a more intuitive
geometry representation on the other hand proves to be very helpful with
respect to the discussion of the ambiguities occurring in reference-free
calibration. In particular, a link between methods based on knowledge on the
sample and those based on knowledge solely on the detector geometry can be
drawn. This further provides another intuitive view on the often reported
difficulty in the estimation of the detector tilt towards the rotational axis.
  A simulation study considering $10^{6}$ randomly generated cone beam imaging
configurations and fiducial marker distributions within a range of typical
scenarios is performed in order to assess the stability of the proposed
technique.
