NGC 4151: There is a source 5arcmin NNW of the target which
has similar brightness to NGC 4151 at some energies.  The
spectra will not overlap with that ot NGC 4151 if the position
angle of the s/c is set to values of 200deg +/- 50 deg or to
20 deg +/- 50 deg (measured W from North).  Only the range from
300 to 360 deg (W from N) would not be allowed according to
the observation visualizer plots.