If one starts now Analysis>Physical optics>Physical Optics Propagation with the following settings, than will get an odd result (note the small point in the center of the POp output drawing):
    

The reason is in the high losses seen by each pass: if the magnification is about 100x then only 1/(100^2) of the overall number of rays are kept in the resonator for the next pass. This problem may be overcome by following actions:

The most crucial place is reflection by the output mirror of the beam after each pass. Here we have only one such reflection. The radiation to this surface must come with the largest sampling that the PC can provide (I use 1024x1024 at 1Gb RAM PC). To propagate further after this reflection the radiation must be resampled to make effective use of the data. Let’s study propagation from the entrance pupil:

•  The initial beam is defined a top hat with sampling 512x512 and diameter of 0.4 mm because after first reflection at secondary mirror will be kept only
  
  d_in = d_max/b = 14/101 ~ 0.14 mm
  
  and the rest (0.4-0.14=0.26mm) diameter is used to provide a good FFT working guard band to prevent aliassing
• At the primary mirror we have only 180x180 (because (0.14/.4)*512 ~ 180) effective sampling. We keep in mind that next reflection will be with huge spatial losses, so here we may resample the beam again. Apertures between primary and secondary mirrors will not influence the final field distribution, so the sampling to width ratio must be as large as possible considering computer RAM size and stability of output beam profile: if the sampling per mm is not enough, then non-symmetrical modes will appear as shown below (sampling 1024 and width 20 used):
  
  
  
• Physical optics settings at surface 5 will be as follows for satisfactory results (if sampling of 1024 is not acceptable due to insufficient RAM, a smaller width can be used):
  
  

After 2 apertures, radiation is incident on the secondary mirror and only 7x7 effective beam sampling is kept ((0.14/10.2)*1024 ~ 7!). We need to resample to get back to the original sampling:



After next reflection from primary mirror at 2-passes system the final beam will be produced, so the width of the beam array must be larger than the beam width for good FFT functionality (a good guard band).



The next 2 apertures are accounted without any additional settings.
Then light hits surface 17, which has a variable transmission filter surface representing the output coupler. Because of this surface transmission function, the pilot beam parameters must be recalculated for correct propagation after this surface:

With these changes, the final field distribution will be as follows (at the surface 19 with settings as for first analysis):

This is in excellent agreement with experimental measurements.