Notch Filter Masks: Practical Image Masks for Planet-Finding   Coronagraphs
  An ideal coronagraph with a band-limited image mask can efficiently image off-axis sources while removing identically all of the light from an on-axis source. However, strict mask construction tolerances limit the utility of this technique for directly imaging extrasolar terrestrial planets. We present a variation on the basic band-limited mask design--a family of ``notch filter'' masks--that mitigates this problem. These robust and trivially achromatic masks can be easily manufactured by cutting holes in opaque material. 
Marc J. Kuchner and David N. Spergel