Maybe someone with newton telescope experienced the same problem as I did. After cleaning primary mirror of my newt, BTW here is a nice tutorial how to do it, I noticed that the brighter stars have triangular shape. After some googling I found the indications that I have pinched primary mirror. Fortunately, it was just elastically deformed by the rubber clamps. When I placed the mirror back I overtightened the screws, therefore the mirror was not reflecting brighter stars properly.