Modified concentric spectrograph

Abstract

A modified concentric spectrograph for diffracting light with high stray light rejection without astigmatism is provided. The modified spectrograph includes a grating, a lens, and at least one entrance port and one exit port. The grating has a concave surface and a meridian plane with a first side and a second side. The lens has a substantially planar surface and a convex surface. Preferably, the convex and concave surfaces are substantially concentric. The ports are substantially located on different sides of the meridian plane near a focal plane of the spectrograph. The position of a focal plane may be modified using an optically transmissive triangular prism with a reflective surface, and an optically transmissive block. The position of a focal plane may further be modified with one or more optically transmissive plates. Methods for using the spectrograph are also provided.

1. A modified concentric spectrograph comprising:
 a grating having an optical axis, a meridian plane, and a concave surface, said meridian plane having a first side and a second side; 
 a lens having a substantially planar surface, a convex surface, and an optical axis, wherein said convex surface is facing said concave surface, said optical axes being substantially coaxial; 
 a primary entrance port being located substantially out of said meridian plane toward said first side; and 
 a primary exit port being located substantially out of said meridian plane toward said second side for receiving an order of light that maximizes throughput and minimizes astigmatism. 
 
     
     
       2. The spectrograph of  claim 1  wherein at least one of said primary ports is near said planar surface of said lens. 
     
     
       3. The spectrograph of  claim 1  wherein said primary entrance port has a cross-sectional area, and wherein a majority of said cross-sectional area is on said first side of said meridian plane. 
     
     
       4. The spectrograph of  claim 1  wherein said primary entrance port has a center, and wherein said center is on said first side of said meridian plane. 
     
     
       5. The spectrograph of  claim 3  wherein said primary exit port has a cross-sectional area, and wherein a majority of said cross-sectional area is on said second side of said meridian plane. 
     
     
       6. The spectrograph of  claim 1  wherein said primary exit port has a center, and wherein said center is on said second side of said meridian plane. 
     
     
       7. The spectrograph of  claim 1  wherein said primary entrance port and said primary exit port are located at substantially the same perpendicular distance from said meridian plane. 
     
     
       8. The spectrograph of  claim 1  wherein:
 said entrance port is for receiving polychromatic light from a source, said spectrograph further comprising: 
 a housing for preventing stray light from contaminating said polychromatic light in said housing. 
 
     
     
       9. The spectrograph of  claim 1  further comprising at least one optical filter positioned near one of said ports. 
     
     
       10. The spectrograph of  claim 1  further comprising at least one optical filter positioned between one of said ports and said planar surface of said lens. 
     
     
       11. The spectrograph of  claim 1  further comprising:
 a secondary entrance port; and 
 a reflective surface between said primary entrance port and said lens. 
 
     
     
       12. The spectrograph of  claim 11  wherein said reflective surface is planar and has an axis normal to said reflective surface, said axis forming an angle with said grating optical axis, said angle being about 45°. 
     
     
       13. The spectrograph of  claim 1  further comprising:
 a secondary exit port; and 
 a reflective surface between said primary exit port and said lens. 
 
     
     
       14. The spectrograph of  claim 13  wherein said reflective surface is planar and has an axis normal to said reflective surface, said axis forming an angle with said grating optical axis, said angle being about 45°. 
     
     
       15. A modified concentric spectrograph comprising:
 a grating having an optical axis, a meridian plane, and a concave surface; 
 a lens having a substantially planar surface, a convex surface, and an optical axis, wherein said convex surface is facing said concave surface, said lens optical axis is substantially coaxial with said grating optical axis, and a primary focal plane is formed perpendicular to said optical axis facing said planar surface of said lens; 
 a primary entrance port near said primary focal plane at an intersection between a first axis and a second axis, wherein said first axis is parallel to, and offset in a first direction from, said meridian plane and said second axis is perpendicular to said meridian plane and offset from said optical axis; and 
 a primary exit port near said primary focal plane located at a second perpendicular distance from said meridian plane, in a second direction opposite said first direction for receiving an order of light that maximizes throughput and minimizes astigmatism. 
 
     
     
       16. The spectrograph of  claim 15  wherein said entrance port is for receiving light and wherein said grating diffracts said light into at least one non-zero diffraction order, said exit port being positioned along said first axis to receive a portion of said non-zero diffraction order. 
     
     
       17. The spectrograph of  claim 16  wherein said non-zero order is an order which is imaged most closely to said optical axis. 
     
     
       18. The spectrograph of  claim 16  wherein said non-zero order is a negative first order. 
     
     
       19. The spectrograph of  claim 15  wherein said exit port is elongated along said first axis. 
     
     
       20. The spectrograph of  claim 15  further comprising at least one optical filter positioned between one of said ports and said lens. 
     
     
       21. The spectrograph of  claim 15  wherein said entrance port is for receiving light from a primary light source, said spectrograph further comprising at least one optical filter between said light source and said entrance port. 
     
     
       22. The spectrograph of  claim 15  wherein said entrance port is for receiving light from a primary light source, said spectrograph further comprising a housing for preventing light coming from a secondary light source external to said housing from contaminating said light from said primary source in said housing. 
     
     
       23. The spectrograph of  claim 22  wherein at least one of said ports is mounted to said housing. 
     
     
       24. The spectrograph of  claim 15  further comprising:
 a secondary entrance port; and 
 a reflective surface between said primary entrance port and said lens, wherein said reflective surface forms a modified focal plane in which said secondary entrance port is located. 
 
     
     
       25. The spectrograph of  claim 15  further comprising a first body comprising an optically transmissive material, said transmissive material having an index of refraction, said first body having at least three planar surfaces, wherein any pair of said first body planar surfaces intersect to form a respective line of intersection, each respective line of intersection being substantially parallel to the other lines, a first of said first body planar surfaces being at least partially located between said primary entrance port and said lens and having a reflective surface disposed on said first plane forming a modified focal plane, a second of said first body planar surfaces facing said planar surface of said lens, and a third of said first body planar surfaces facing said modified focal plane. 
     
     
       26. The spectrograph of  claim 25  further comprising a second body comprising said optically transmissive material, said second body having at least two substantially parallel planar surfaces, a first of said second body parallel planar surfaces facing said primary exit port, a second of said second body parallel planar surfaces facing said lens. 
     
     
       27. The spectrograph of  claim 25  wherein an angle between said optical axis of said grating and an axis normal to said first of said first body planar surfaces is about 45°. 
     
     
       28. The spectrograph of  claim 27  wherein said second surface of said first body is fixedly attached to said planar surface of said lens. 
     
     
       29. The spectrograph of  claim 25  further comprising at least one optical filter positioned between said lens and said first body. 
     
     
       30. The spectrograph of  claim 26  further comprising at least one optical filter positioned between said lens and said second body. 
     
     
       31. The spectrograph of  claim 26  wherein said second surface of said second body is fixedly attached to said planar surface of said lens. 
     
     
       32. The spectrograph of  claim 31  further comprising an adhesive having an index of refraction that is substantially the same as the index of refraction of said second body for fixedly attaching said second body to said lens. 
     
     
       33. The spectrograph of  claim 26  wherein said second surface of said second body and said lens are integral. 
     
     
       34. The spectrograph of  claim 25  further comprising a secondary exit port and a reflective surface between said primary exit port and said lens, wherein said reflective surface forms a modified focal plane. 
     
     
       35. The spectrograph of  claim 15  further comprising a first body comprising an optically transmissive material, said optically transmissive material having an index of refraction, said first body having at least three planar surfaces, wherein any pair of said first body planar surfaces intersect to form a respective line of intersection, each respective line of intersection being substantially parallel to the other lines, a first of said planes being located between said primary exit port and said lens and having a reflective surface disposed on said first plane, thereby forming a modified focal plane, a second of said first body planar surfaces facing said lens planar surface, and a third of said first body planar surfaces facing said modified focal plane. 
     
     
       36. The spectrograph of  claim 35  further comprising a second body comprising said optically transmissive material, said second body having at least two substantially parallel planar surfaces, a first of said second body parallel planar surfaces facing said primary entrance port, a second of said second body parallel planar surfaces facing said lens. 
     
     
       37. The spectrograph of  claim 35  wherein an angle between said optical axis of said grating and an axis normal to said first of said reflective surface is about 45°. 
     
     
       38. The spectrograph of  claim 35  wherein said second surface of said first body is fixedly attached to said planar surface of said lens. 
     
     
       39. The spectrograph of  claim 35  further comprising at least one optical filter positioned between said lens and said first body. 
     
     
       40. The spectrograph of  claim 36  further comprising at least one optical filter positioned between said lens and said second body. 
     
     
       41. The spectrograph of  claim 36  wherein said second face of said second body is fixedly attached to said planar surface of said lens. 
     
     
       42. The spectrograph of  claim 41  further comprising an adhesive having an index of refraction that is substantially the same as the index of refraction of said second body for fixedly attaching said second body to said lens. 
     
     
       43. The spectrograph of  claim 35  wherein said second body and said lens are integral. 
     
     
       44. The spectrograph of  claim 15  wherein at least one of said ports is substantially in one of said focal planes. 
     
     
       45. The spectrograph of  claim 15  wherein said convex surface has a center of curvature and said concave surface has a center of curvature, and wherein said centers of curvature are substantially concentric. 
     
     
       46. A modified concentric spectrograph with reduced stray light, said spectrograph comprising:
 a concave grating for dispersing light having an optical axis, a meridian plane, and a concave surface; 
 a lens having an optical axis, a planar surface, and a convex surface, wherein said optical axes are substantially coaxial, said convex surface is facing said concave surface, and wherein a primary focal plane is formed facing said planar surface of said lens; 
 an entrance port near said focal plane for permitting light to enter said spectrograph along an optical path, said entrance port being substantially located at a distance from said meridian plane in a first direction, said lens directing said light from said entrance port toward said meridian plane and said grating surface for diffraction; and 
 an exit port located near said focal plane for permitting a portion of said light to exit said spectrograph after said light is diffracted by said grating, said lens imaging said portion of said light at said exit port for receiving an order of light that maximizes throughput and minimizes astigmatism. 
 
     
     
       47. The spectrograph of  claim 46  wherein said entrance port and said exit port are optically connected by an optical path. 
     
     
       48. The spectrograph of  claim 46  wherein at least some of said portion of said light is a non-zero order of diffracted light. 
     
     
       49. The spectrograph of  claim 48  wherein said non-zero order of diffracted light is an order that is imaged most closely to said grating optical axis at said focal plane. 
     
     
       50. The spectrograph of  claim 48  wherein said non-zero order is a negative first order. 
     
     
       51. The spectrograph of  claim 46  wherein said exit port is at said perpendicular distance from said meridian plane in a direction opposite said first direction. 
     
     
       52. The spectrograph of  claim 46  wherein said lens has an optical axis that is parallel to and offset from said grating optical axis to improve image quality and reduce stray light at said exit port. 
     
     
       53. The spectrograph of  claim 46  further comprising a housing in which said lens and said grating are placed, said housing for preventing light coming from any secondary light source outside said housing from contaminating said light in said housing. 
     
     
       54. The spectrograph of  claim 46  further comprising:
 a reflective surface between said entrance port and said lens, thereby forming a modified focal plane; and 
 a secondary entrance port near said modified focal plane for receiving light from a light source. 
 
     
     
       55. The spectrograph of  claim 46  further comprising:
 a reflective surface between said exit port and said lens, thereby forming a modified focal plane; and 
 a secondary exit port near said modified focal plane for permitting dispersed light to exit said spectrograph. 
 
     
     
       56. The spectrograph of  claim 46  wherein said primary entrance port and said primary exit port are located near a primary focal plane near said planar surface of said lens. 
     
     
       57. The spectrograph of  claim 46  wherein said convex surface has a center of curvature and said concave surface has a center of curvature that is substantially concentric with said convex surface center. 
     
     
       58. A modified concentric spectrograph comprising:
 a grating having an optical axis, a meridian plane, and a concave surface; 
 a lens having a substantially planar surface, a convex surface, and an optical axis, wherein said convex surface is facing said concave surface, said optical axes are substantially collinear and said surfaces are substantially concentric, and a primary focal plane is formed perpendicular to said optical axis facing said planar surface of said lens; 
 a primary entrance port near said primary focal plane at an intersection between a first primary axis and a second primary axis, wherein said first primary axis is parallel to and offset from said meridian plane and said second primary axis is perpendicular to said meridian plane and offset from said grating optical axis; 
 a primary exit port near said primary focal plane located at a first perpendicular distance from said meridian plane, said first perpendicular distance being in a second direction opposite said first direction for receiving an order of light that maximizes throughput and minimizes astigmatism; 
 a secondary entrance port near said primary focal plane at an intersection between a first secondary axis and a second secondary axis, wherein said first secondary axis is parallel to and offset from said meridian plane and said second secondary axis is perpendicular to said meridian plane and offset from said grating optical axis; and 
 a secondary exit port near said primary focal plane located at a second perpendicular distance from said meridian plane in said second direction. 
 
     
     
       59. The spectrograph of  claim 58  wherein said primary entrance port is positioned to receive polychromatic light and wherein said grating diffracts said light into at least one non-zero diffraction order, said primary exit port being positioned to receive a portion of said non-zero diffraction order. 
     
     
       60. The spectrograph of  claim 59  wherein said non-zero order is an order which is imaged most closely to said optical axis of said grating. 
     
     
       61. The spectrograph of  claim 60  wherein said non-zero order is a negative first order. 
     
     
       62. The spectrograph of  claim 58  wherein said primary entrance port is for receiving light from a primary light source, said spectrograph further comprising a housing around in which said grating and said lens is placed. 
     
     
       63. The spectrograph of  claim 58  wherein at least one of said ports is in said primary focal plane. 
     
     
       64. A modified concentric spectrograph comprising:
 a grating having an optical axis, a meridian plane, and a concave surface; 
 a lens having a substantially planar surface, a convex surface, and an optical axis, wherein said convex surface is facing said concave surface, said optical axes are substantially collinear, and wherein a primary focal plane is formed perpendicular to said optical axis facing said planar surface; 
 a first body comprising an optically transmissive material, said first body having at least three planar surfaces, wherein any pair of said first body planar surfaces intersect to form a respective line of intersection, each respective line of intersection being substantially parallel to the other lines, a first of said first body planar surfaces being located between said primary focal plane and said planar surface of said lens and having a reflective surface disposed thereon, a second of said first body planar surfaces facing said planar surface of said lens, and a third of said first body planar surfaces facing a modified focal plane, said reflective surface forming said modified focal plane at an optical length from said planar lens surface; 
 a primary entrance port near said modified focal plane at an intersection between a first axis and a second axis, wherein said first axis is parallel to and offset in a first direction from said meridian plane and said second axis is perpendicular to said meridian plane; and 
 a primary exit port near said primary focal plane located at a first perpendicular distance from said meridian plane, said first perpendicular distance being in a second direction opposite said first direction for receiving an order of light that maximizes throughput and minimizes astigmatism. 
 
     
     
       65. The spectrograph of  claim 64  further comprising a second body comprising said optically transmissive material, said second body having at least two substantially parallel planar surfaces, a first of said second body parallel planar surfaces facing said primary exit port, a second of said second body parallel planar surfaces facing said lens. 
     
     
       66. A method for dispersing light comprising:
 passing polychromatic light through an entrance port located substantially on a first side of and at a perpendicular distance from a meridian plane of a concave diffraction grating; 
 directing said polychromatic light with a lens toward said grating so that said light is incident on said grating at least at said meridian plane; 
 diffracting said light with said diffraction grating, thereby dispersing said light; and 
 imaging said dispersed light with said lens at an exit port located substantially on a second side of said meridian plane for receiving an order of light that maximizes throughput and minimizes astigmatism. 
 
     
     
       67. A method for diffracting two beams of light using a modified concentric spectrograph, said spectrograph comprising:
 a grating having an optical axis, a meridian plane having a first side and a second side, and a concave surface, 
 a lens having a substantially planar surface, a convex surface, and an optical axis, wherein said convex surface faces said concave surface, said optical axes being substantially coaxial, 
 a primary entrance port being located substantially on said first side of said meridian plane, 
 a primary exit port being located substantially on said second side of said meridian plane, 
 a secondary entrance port being located substantially on said first side of said meridian plane, and 
 a secondary exit port being located substantially on said second side of said meridian plane; said method comprising: 
 diffracting a first light beam comprising:
 providing said first beam at said primary entrance port, 
 directing said first beam with said lens toward said grating so that at least a portion of said first beam is incident on said grating surface, 
 reflectively diffracting said first beam with said grating to form a first diffracted beam, and 
 imaging said first diffracted beam with said lens at said primary exit port; and 
 diffracting a second light beam comprising:
 providing said second beam at said secondary entrance port, 
 directing said second beam with said lens toward said grating so that at least a portion of said second beam is incident on said grating surface, 
 reflectively diffracting said second beam with said grating to form a second diffracted beam, and 
 imaging said second diffracted beam with said lens at said secondary exit port. 
 
 
 
     
     
       68. A method for diffracting two beams of light using a modified concentric spectrograph, said spectrograph comprising:
 a grating having an optical axis, a meridian plane having a first side and a second side, and a concave surface, 
 a lens having a substantially planar surface, a convex surface, and an optical axis, wherein said convex surface is facing said concave surface, said optical axes being substantially coaxial, 
 a primary entrance port being located substantially on said first side of said meridian plane, 
 a primary exit port being located substantially on said second side of said meridian plane for receiving an order of light that maximizes throughput and minimizes astigmatism, 
 a secondary entrance port being located substantially on said second side of said meridian plane, and 
 a secondary exit port being located substantially on said first side of said meridian plane; said method comprising:
 diffracting a first light beam comprising:
 providing said first beam at said primary entrance port, 
 directing said first beam with said lens toward said grating so that at least a portion of said first beam is incident on said grating surface, 
 reflectively diffracting said first beam with said grating to form a first diffracted beam, and 
 imaging said first diffracted beam with said lens at said primary exit port; and 
 
 diffracting a second light beam comprising:
 providing said second beam at said secondary entrance port, 
 directing said second beam with said lens toward said grating so that at least a portion of said second beam is incident on said grating surface, 
 reflectively diffracting said second beam with said grating to form a second diffracted beam, and 
 imaging said second diffracted beam with said lens at said secondary exit port. 
 
 
 
     
     
       69. A concentric spectrograph for spectrally dispersing polychromatic light comprising:
 a grating having a concave surface and an optical axis; 
 a lens having a substantially planar surface, a convex surface, and an optical axis, wherein said convex surface is facing said concave surface, said optical axes are substantially coaxial, said convex and concave surfaces are substantially concentric, and wherein said lens and said grating are positioned at a distance to form a primary focal plane; 
 a first port facing said planar surface of said lens; 
 a first body comprising an optically transmissive material, said transmissive material having an index of refraction, said first body having at least three planar surfaces, wherein any pair of said first body planar surfaces intersect to form a respective line of intersection, each respective line of intersection being substantially parallel to the other lines, a first of said first body planar surfaces being at least partially located between said first port and said lens and having a reflective surface disposed on said first plane forming a modified focal plane, a second of said first body planar surfaces facing said planar surface of said lens, and a third of said first body planar surfaces facing said modified focal plane; and 
 a second port facing said planar surface of said lens and being located near said primary focal plane for receiving an order of light that maximizes throughput and minimizes astigmatism. 
 
     
     
       70. The spectrograph of  claim 69  further comprising a second body comprising said optically transmissive material, said second body having at least two substantially parallel planar surfaces, a first of said second body parallel planar surfaces facing said second port, a second of said second body parallel planar surfaces facing said lens. 
     
     
       71. The spectrograph of  claim 69  wherein an angle between said optical axis of said grating and an axis normal to said first of said first body planar surfaces is about 45°. 
     
     
       72. The spectrograph of  claim 69  wherein said second surface of said first body is fixedly attached to said planar surface of said lens. 
     
     
       73. The spectrograph of  claim 72  further comprising an adhesive having an index of refraction that is substantially the same as the index of refraction of said optically transmissive material for fixedly attaching said first body to said lens. 
     
     
       74. The spectrograph of  claim 69  wherein said first body and said lens are integral at said second surface of said first body. 
     
     
       75. The spectrograph of  claim 70  wherein said second surface of said second body is fixedly attached to said planar surface of said lens. 
     
     
       76. The spectrograph of  claim 75  further comprising an adhesive having an index of refraction that is substantially the same as the index of refraction of said second body for fixedly attaching said second body to said lens. 
     
     
       77. The spectrograph of  claim 69  further comprising one or more optically transmissive plates placed so that said polychromatic light passes through said at least one of said plates for varying the position of one of said focal planes. 
     
     
       78. The spectrograph of  claim 77  wherein said at least one of said plates is placed between one of said ports and said lens. 
     
     
       79. The spectrograph of  claim 77  wherein said at least one of said plates is placed between one of said bodies and one of said ports. 
     
     
       80. The spectrograph of  claim 77  wherein said at least one of said plates is placed between one of said bodies and said lens. 
     
     
       81. The spectrograph of  claim 77  wherein said at least one of said plates has an index of refraction which is different from said optical bodies. 
     
     
       82. The spectrograph of  claim 81  wherein said at least one of said plates comprises sapphire. 
     
     
       83. The spectrograph of  claim 77  further comprising at least one optical filter positioned between said lens and one of said bodies. 
     
     
       84. A method for diffracting two beams of light employing a concentric spectrographic apparatus, the apparatus including a grating, a lens, a primary entrance port, a primary exit port, a secondary entrance port and a secondary exit port, the method comprising the elements of:
 (a) providing a first polychromatic light beam to the primary entrance port;   (b) refracting the first light beam with the lens to diverge the beam toward the grating;   (c) reflectively diffracting the first light beam at the grating to form a first diffracted beam;   (d) imaging the first diffracted light beam with the lens at the primary exit port;   (e) providing a second polychromatic light beam to the secondary entrance port;   (f) refracting the second light beam with the lens to diverge the beam toward the grating; reflectively diffracting the second light beam at the grating to form a second diffracted beam; and   (g) imaging the second diffracted light beam with the lens at the secondary exit.   
     
     
       85. A method according to claim 84 wherein elements (e)-(f) are performed during the performance of elements (a)-(d). 
     
     
       86. A method according to claim 84, wherein performance of elements (e)-(f) is alternated with performance of elements (a)-(d). 
     
     
       87. A method according to claim 84 wherein the first and second polychromatic light beams are each provided to the lens at respective lens locations displaced from the meridian plane. 
     
     
       88. A method according to claim 84 wherein the first and second diffracted light beams are each received from the lens at respective locations displaced from the meridian plane. 
     
     
       89. A spectrograph comprising:
 a grating having an optical axis, a meridian plane, and a concave surface, said meridian plane having a first side and a second side;   a lens having a substantially planar surface, a convex surface, and an optical axis, wherein said convex surface is facing said concave surface, said optical axes being substantially coaxial or parallel to each other;   a primary entrance port being located substantially out of said meridian plane toward said first side; and   a primary exit port being located substantially out of said meridian plane toward said second side for receiving an order of light that tends to maximize throughput and minimize astigmatism.   
     
     
       90. A modified concentric spectrograph comprising:
 a grating having an optical axis, a meridian plane, and a concave surface;   a lens having a substantially planar surface, a convex surface, and an optical axis, wherein said convex surface is facing said concave surface, said lens optical axis is substantially coaxial with said grating optical axis, and a primary focal plane is formed perpendicular to said optical axis facing said planar surface of said lens;   a primary entrance port near said primary focal plane at an intersection between a first axis and a second axis, wherein said first axis is parallel to, and   offset in a first direction from, said meridian plane and said second axis is perpendicular to said meridian plane and offset from said optical axis; and   a primary exit port near said primary focal plane located at a second perpendicular distance from said meridian plane, in a second direction opposite said first direction for receiving an order of light that maximizes throughput and minimizes astigmatism.   
     
     
       91. The spectrograph of claim 90 wherein the primary exit port is located for receiving a negative first order of diffracted light. 
     
     
       92. The spectrograph of claim 90 wherein said exit port is elongated along said first axis. 
     
     
       93. The spectrograph of claim 90 wherein said entrance port is for receiving light from a primary light source, said spectrograph further comprising a housing for preventing light coming from a secondary light source external to said housing from contaminating said light from said primary source in said housing. 
     
     
       94. The spectrograph of claim 90 further comprising:
 a secondary entrance port; and   a reflective surface between said primary entrance port and said lens, wherein said reflective surface forms a modified focal plane in which said secondary entrance port is located.   
     
     
       95. A modified concentric spectrograph comprising:
 a grating having an optical axis, a meridian plane, and a concave surface;   a lens having a substantially planar surface, a convex surface, and an optical axis, wherein said convex surface is facing said concave surface, said optical axes are substantially collinear and said surfaces are substantially concentric, and a primary focal plane is formed perpendicular to said optical axis facing said planar surface of said lens;   a primary entrance port near said primary focal plane at an intersection between a first primary axis and a second primary axis, wherein said first primary axis is parallel to and offset from said meridian plane and said second primary axis is perpendicular to said meridian plane and offset from said grating optical axis;   a primary exit port near said primary focal plane located at a first perpendicular distance from said meridian plane, said first perpendicular distance being in a second direction opposite said first direction for receiving an order of light that maximizes throughput and minimizes astigmatism;   a secondary entrance port near said primary focal plane at an intersection between a first secondary axis and a second secondary axis, wherein said first secondary axis is parallel to and offset from said meridian plane and said second secondary axis is perpendicular to said meridian plane and offset from said grating optical axis; and   a secondary exit port near said primary focal plane located at a second perpendicular distance from said meridian plane in said second direction.   
     
     
       96. The spectrograph of claim 95 wherein said non-zero order is a negative first order. 
     
     
       97. The spectrograph of claim 95 wherein said primary entrance port is for receiving light from a primary light source, said spectrograph further comprising a housing in which said grating and said lens is placed. 
     
     
       98. The spectrograph of claim 95 wherein at least one of said ports is in said primary focal plane. 
     
     
       99. A concentric spectrograph comprising:
 a diffraction grating having an optical axis, a meridian plane, a grating concave surface and a set of grating grooves on said concave surface, said grating grooves generally extending in a groove direction, and said meridian plane containing the grating optical axis and extending transversely to the grating grooves;   a field lens having a lens convex surface, a relatively planar lens surface, said relatively planar lens surface being relatively planar relative to the shape of said, lens convex surface, and a lens optical axis, wherein said lens convex surface faces and is substantially concentric with said grating concave surface, said optical axes of said grating and said lens being substantially coincident and said relatively planar lens surface extending transversely to said lens optical axis;   an entrance port positioned to introduce incident polychromatic light to the relatively planar lens surface at a location on said relatively planar lens surface on one side of said meridian plane; and   an exit port located to receive diffracted light emerging from said lens planar surface at a location on the other side of the meridian plane from the incident polychromatic light, said entrance and exit ports being positioned relative to said lens convex surface and said grating concave surface to reflect light which is reflected by said lens convex surface towards said grating concave surface for reflection generally towards said lens convex surface to follow a path which avoids said exit port.   
     
     
       100. A spectrograph as in claim 89, wherein said optical axes are parallel to and offset from each other. 
     
     
       101. A spectrograph as in claim 89, wherein said lens is spherical. 
     
     
       102. A concentric spectrograph comprising:
 a diffraction grating having an optical axis, a meridian plane, a grating concave surface and a set of grating grooves on said concave surface, said grating grooves generally extending in a groove direction, and said meridian plane containing the grating optical axis and extending transversely to the grating grooves;   a field lens having a lens convex surface, a relatively planar lens surface, said relatively planar lens surface being relatively planar relative to the shape of said lens convex surface, and a lens optical axis, wherein said optical axes of said grating and said lens substantially coincide and said relatively planar lens surface extends transversely to said lens optical axis;   an entrance port positioned to introduce incident polychromatic light to the relatively planar lens surface at a location on said-relatively planar lens surface on one side of said meridian plane; and   an exit port located to receive a non-zero order of diffracted light emerging from said lens planar surface at a location on the other side of the meridian plane from the incident polychromatic light, and, overall, to tend to maximize throughput and tend to minimize astigmatism.