Houghton Cassegrain
Introduction
The Houghton-Casegrain offers an important advantage: all surfaces can be left spherical, because the Houghton corrector can correct both spherical aberration and coma for every position of the corrector. This is not the case for Schmidt and Maksutov corre´ctors, which eliminate spherical aberration, but correct coma for one position only, if both mirrors are spherical. The Houghton corrector can correct both because it consists of two elements. This also implies that the color aberration can be corrected. If the elements are placed close together and have equal power of opposite signs, they can be made of the same type of glass. Furthermore, if we make radii of curvature in pairs (i.e. R3=-R1 and R4=-R2), then the lenses can be tested against each other by means of interference.
Design and Optimization
For the Houghton-Cassegrain the mirrors are always spherical. The design of a Houghton-Cassegrain is similar to that for the Schmidt- and Maksutov-Cassegrain. Spherical aberration and coma can be corrected for every given position of the corrector. Astigmatism can only be corrected when the corrector is near the center of curvature of the primary.
Optimizing a systems such as the Houghton is more difficult than optimizing a Schmidt or Maksutov system because there are more degrees of freedom. JODAS start with automatically fit of the optimal bending factor, and the correspond radii of curvature of both lenses in pairs at the same rate.
The distance between the lenses is another free parameter to test for its effect, but such changes should be made systematically (only when color aberration cannot be suppressed sufficiently with lenses made of the same glass, should the designer resort latter in surface editor, two different glasses with the same design index but different dispersion). Spherical aberration must be eliminated. Unless the axial spot diagram is smaller than the Airy disk, the system will never have value as an astronomical telescope.
The Houghton-Casegrain offers an important advantage: all surfaces can be left spherical, because the Houghton corrector can correct both spherical aberration and coma for every position of the corrector. This is not the case for Schmidt and Maksutov corre´ctors, which eliminate spherical aberration, but correct coma for one position only, if both mirrors are spherical. The Houghton corrector can correct both because it consists of two elements. This also implies that the color aberration can be corrected. If the elements are placed close together and have equal power of opposite signs, they can be made of the same type of glass. Furthermore, if we make radii of curvature in pairs (i.e. R3=-R1 and R4=-R2), then the lenses can be tested against each other by means of interference.
Design and Optimization
For the Houghton-Cassegrain the mirrors are always spherical. The design of a Houghton-Cassegrain is similar to that for the Schmidt- and Maksutov-Cassegrain. Spherical aberration and coma can be corrected for every given position of the corrector. Astigmatism can only be corrected when the corrector is near the center of curvature of the primary.
Optimizing a systems such as the Houghton is more difficult than optimizing a Schmidt or Maksutov system because there are more degrees of freedom. JODAS start with automatically fit of the optimal bending factor, and the correspond radii of curvature of both lenses in pairs at the same rate.
The distance between the lenses is another free parameter to test for its effect, but such changes should be made systematically (only when color aberration cannot be suppressed sufficiently with lenses made of the same glass, should the designer resort latter in surface editor, two different glasses with the same design index but different dispersion). Spherical aberration must be eliminated. Unless the axial spot diagram is smaller than the Airy disk, the system will never have value as an astronomical telescope.
Applet Tag:
- param name = "Title" value = "Two Mirrror Catadioptric: Houghton Cassegrain"
- param name = "Corrector Diameter" value = "150.0"
- param name = "Edge Thickness Lens A" value = "10.0"
- param name = "Center Thickness Lens B" value = "10.0"
- param name = "Primary Focal Length" value = "500.0"
- param name = "System Focal Length" value = "2000.0"
- param name = "Back Working Distance" value = "200.0"
- param name = "Lens Glass" value = "BK7"
- param name = "Half Field Angle" value = "0.45"
- param name = "Central Obstruction" value = "0.00"
- param name = "Plot Scale" value = "0.05"
- param name = "TF Start Angle" value = "0.00"
- param name = "TF End Angle" value = "0.45"
- param name = "TF Defocus" value = "0.05"
- param name = "Number of Arms" value = "36"
- param name = "Number of Rings" value = "10"
- param name = "Ray Density" value = "10"
- param name = "Scale Type Index" value = "3"
- param name = "Ray Pattern Index" value = "0"
- param name = "Merrit Function Index" value = "2"
- param name = "Design Type Index" value = "0"
- param name = "Lens Catalog Index" value = "2"
- param name = "Monochr. Color Index" value = "10"
- param name = "WL1 Color Index" value = "1"
- param name = "WL2 Color Index" value = "4"
- param name = "WL3 Color Index" value = "9"
- param name = "Text Color Index" value = "10"