Afocal beam relay

1. An afocal beam relay comprising:

first and second primary concave reflective surfaces and first and second secondary convex toroidal reflective surfaces,

wherein the centers of curvature of each of the first and second primary reflective surfaces and first and second secondary reflective surfaces lie on an axis;

wherein the first and second secondary convex reflective surfaces face toward the first and second primary concave reflective surfaces and are disposed to relay a decentered entrance pupil to a decentered exit pupil; and

an aspheric corrector element that is disposed in the path of an input beam of light that is directed by the primary and secondary surfaces to the decentered entrance pupil,

wherein the directed beam of light between the first and second secondary convex mirrors is collimated in one direction and focused in mid air in an orthogonal direction.

2. The afocal beam relay of claim 1 wherein the aspheric corrector element is a lens.

3. The afocal beam relay of claim 1 wherein the aspheric corrector element comprises one or more reflective surfaces.

4. The afocal beam relay of claim 1 wherein the first and second primary concave reflective surfaces are non-spherical.

5. The afocal beam relay of claim 1 wherein the first and second secondary convex reflective surfaces are non-spherical.

6. The afocal beam relay of claim 1 wherein the entrance and exit pupils are substantially circular in shape.

7. The afocal beam relay of claim 1 wherein the entrance and exit pupils are non-circular in shape.

8. The afocal beam relay of claim 1 further comprising:

a first scanning element actuable to scan the input light at the entrance pupil in a first direction;

a second scanning element that is actuable to scan the relayed input light in a second direction that is substantially orthogonal to the first direction; and

a scan lens disposed to direct the scanned light from the exit pupil toward a surface.

9. The afocal beam relay of claim 8 wherein at least one of the first and second scanning elements is a micro-electromechanical systems device.

10. A pre-objective scanning apparatus comprising:

an afocal beam relay comprising:

first and second primary concave reflective surfaces and first and second secondary convex toroidal reflective surfaces,

wherein the centers of curvature of each of the first and second primary surfaces and first and second secondary surfaces lie on an axis;

wherein the first and second secondary convex reflective surfaces face toward the first and second primary concave reflective surfaces, disposed to relay a decentered entrance pupil to a decentered exit pupil; and

an aspheric corrector element that is disposed in the path of an input beam of light that is directed by the primary and secondary surfaces to the decentered entrance pupil,

wherein the directed beam of light between the first and second secondary convex mirrors is collimated in one direction and focused in mid air between the first and second secondary surfaces in an orthogonal direction;

a first scanning element that is actuable to scan the input light at the entrance pupil in a first direction;

a second scanning element that is disposed near the exit pupil and that is actuable to scan the relayed input light in a second direction orthogonal to the first direction;

and

a scan lens that is disposed to direct light from the exit pupil toward a surface.

11. The pre-objective scanning apparatus of claim 10 wherein the entrance and exit pupils are optically coplanar.

12. The pre-objective scanning apparatus of claim 10 wherein at least one of the first and second scanning elements is a micro-electromechanical systems device.

13. A method for scanning an image surface comprising:

relaying an entrance pupil to an exit pupil by:

providing an afocal beam relay that has first and second primary concave reflective surfaces and first and second secondary convex toroidal reflective surfaces, wherein the centers of curvature of each of the first and second primary reflective surfaces and first and second secondary reflective surfaces lie on an axis; wherein the first and second secondary convex reflective surfaces face toward the first and second primary concave reflective surfaces and are disposed to relay a decentered entrance pupil to a decentered exit pupil; wherein the afocal beam relay further has an aspheric corrector element that is disposed in the path of an input beam of light that is directed by the primary and secondary surfaces to the decentered entrance pupil, wherein the directed beam of light between the first and second secondary convex mirrors is collimated in one direction and focused in mid air in an orthogonal direction;

directing a collimated input beam to the entrance pupil;

scanning, at the entrance pupil, a first scanning element in a first direction;

and

scanning, at the exit pupil, a second scanning element that directs light in a second direction that is orthogonal to the first direction.

14. The method of claim 13 wherein the phase wavefront variation over the beam at the exit pupil is less than a quarter wave for the collimated input beam.

15. A pre-objective scanning apparatus comprising:

an afocal beam relay comprising:

(i) a concave reflective surface having a first center of curvature;

(ii) a convex reflective surface having a second center of curvature that is substantially coincident with the first center of curvature;

wherein the convex reflective surface faces toward the concave reflective surface to relay an entrance pupil to an exit pupil;

(iii) an aspheric corrector element that is disposed in the path of input light at the entrance pupil and that has correction values substantially centered on the first center of curvature;

a first micro-electromechanical systems scanning element that is actuable to scan the input light at the entrance pupil in a first direction;

a second micro-electromechanical systems scanning element that is disposed at the exit pupil and that is actuable to scan the relayed input light in a second direction orthogonal to the first direction;

and

a scan lens that is disposed to direct light from the exit pupil toward a surface.