I get rid of the donut bokeh of mirror lenses by fitting an off-centre circular aperture on the front of the lens. The circular aperture has got to be small enough to avoid the central obstruction. Off-centre apertures can be made in smaller and smaller sizes thus stopping down a mirror lens; a procedure popularly thought to be impossible. Downside? The viewfinder image gets dim. Upside? Better bokeh and more depth of field.