Embodiments of the invention relate to a dome assembly. The dome assembly includes an upper dome comprising a central window, and an upper peripheral flange engaging the central window at a circumference of the central window, wherein a tangent line on an inside surface of the central window that passes through an intersection of the central window and the upper peripheral flange is at an angle of about 8° to about 16° with respect to a planar upper surface of the peripheral flange, a lower dome comprising a lower peripheral flange and a bottom connecting the lower peripheral flange with a central opening, wherein a tangent line on an outside surface of the bottom that passes through an intersection of the bottom and the lower peripheral flange is at an angle of about 8° to about 16° with respect to a planar bottom surface of the lower peripheral flange.