Patent ID: 7408706

Claim:
A neutral-density optical-filter assembly for a solar telescope assembly comprising: a first optical structure with a front face adapted to receive an incoming beam and with a first reflective face reflecting internally a fraction of the incoming beam, said fraction being directed as an output beam toward an image plane; a second optical structure with a refracting facet in optical contact with said first reflective face of the first optical structure and with a second facet adapted to receive a first refracted fraction of said incoming beam propagating from said first reflective face; a third optical structure with a third face in optical contact with said second facet of the second optical structure and adapted to receive a second refracted fraction of said incoming beam propagating from said second facet and to retro-reflect said second refracted fraction along an optical path of said incoming beam; and an anti-reflection coating between said first reflective face of the first optical structure and said refracting facet of the second optical structure; wherein the first and second optical structures are made of different materials having substantially equal, but not identical, indices of refraction; the second and third optical structures are made of the same material; and the neutral-density optical-filter assembly is incorporated into a solar telescope and adapted to return said refracted fraction along said optical path of the incoming beam.