  ASPIICS is a novel externally occulted coronagraph that will be launched
onboard the PROBA-3 mission of ESA. The external occulter (EO) will be placed
on one satellite ~150 m ahead of the second satellite with an optical
instrument. During part of each orbit, the satellites will fly in a precise
formation, constituting a giant externally occulted coronagraph. Large distance
between the EO and the primary objective will allow observations of the
white-light solar corona starting already from ~1.1RSun. We analyze influence
of shifts of the satellites and misalignments of optical elements on diffracted
light. Based on the quantitative influence of misalignments on diffracted
light, we will provide a "recipe" for choosing the size of the internal
occulter (IO) to achieve a trade-off between the minimal height of observations
and sustainability to possible misalignments. We implement a numerical model of
the diffracted light and its propagation through the optical system, and
compute intensities of diffracted light throughout the instrument. Our
numerical model extends axi-symmetrical model of Rougeot et al. 2017 to
non-symmetrical cases. The computations fully confirm main properties of the
diffracted light obtained from semi-analytical consideration. Results: relative
influences of various misalignments are significantly different. We show that:
the IO with R=1.1RSun is large enough to compensate possible misalignments in
ASPIICS, apodizing the edge of the IO leads to additional suppression of the
diffracted light. Conclusions: the most important misalignment is the tilt of
the telescope WRT the line connecting the center of the EO and the entrance
aperture. Special care should be taken to co-align the EO and the coronagraph,
i.e. co-aligning the diffraction fringe from the EO and the IO. We suggest that
the best orientation strategy is to point the coronagraph to the center of the
EO.
