Freeform aspheric talk

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Freeform Aspheric Telescope with an External Pupil Dave Shafer David Shafer Optical Design Fairfield, Connecticut [email protected]

Transcript of Freeform aspheric talk

1. Freeform Aspheric Telescope with an External Pupil Dave Shafer David Shafer Optical Design Fairfield, Connecticut [email protected] 2. A 30 meter diameter telescope is being built for a new observatory in Hawaii 3. Its mosaic mirror with 492 segments will be 6 times larger than the single mirror Mt. Palomar 5 meter Hale telescope in California. The tertiary mirror directs light to very large size instruments, such as a spectrograph. 4. After the star light has been dispersed by some very large prisms the dispersed rays are collected and focused by a camera. There are two cameras, one for the blue light and one for the red light. Each needs to be 300 mm aperture f/2.0 with a 22 degree full field. The pupil must be 600 mm in front of the first element. The spectral range is very broad, from the near UV to the near IR. The dispersed light seems to be coming from a pupil in front of the focusing camera. 5. Because of the large size of the camera a refractive design is almost impossible to make, and would require some calcium fluoride lenses about 500 mm in diameter for secondary color correction. The distant front external pupil makes a lot of problems, and so does the color correction in such large size lenses. 6. The project wants a camera design with no obscuration, because of diffraction effects. A catadioptric design would still require some very large lenses. The best design would be an unobscured all-reflective design, with a front pupil. Freeform aspherics help a lot in getting to a 22 degree f/2.0 unobscured design. Here is a 15 degree f/2.0 unobscured freeform aspheric design with very good performance but with an internal pupil. 7. Front pupil Image plane A catadioptric design was considered but the lens sizes would still be impractical and this freeform aspheric design shown here would still need color correction and even more large size lenses. External pupil 8. This 4 mirror freeform aspheric design has an external pupil but is too slow speed and narrow angle. Two convex field mirrors correct for Petzval The big appeal of a reflective design is not having to deal with the enormous spectral range of the project. 9. There is an interesting 5 mirror unobscured design with an external pupil in my 1978 patent (US 4,226,501). All the mirrors have spherical surfaces. 10. The basis of this design is the well-known two concentric spheres, with spherical aberration correction and no coma or astigmatism A concentric design stays well corrected if it is used backwards a very useful fact to know. Here it then gives a virtual image. 11. Both versions of the concentric spheres can be made unobscured by decentering the pupil. We are interested in the virtual image version. 12. A combination of decentered pupil and off-axis field makes for the best unobscured configuration 13. Convex grating surface Spectrum By making the convex mirror in the Offner 1X relay become a grating, you get a high performance spectrograph. 14. This design, as a telescope/spectrograph, was made and sent on the Cassini spacecraft to Saturn. It is unobscured, 5 spherical mirrors, and has loose tolerances. 15. Another one of this telescope/spectrograph designs was sent on the Dawn spacecraft to the asteroid Vesta and is now orbiting the dwarf planet Ceres. In case you have not been following this, there are several bright lights coming from the surface of Ceres!! 16. NASAs theory that these bright spots are reflections of the sun from ice is clearly false since the brightness does not change as the reflection angle changes a lot. The spots are also too bright to be defuse reflections, like from snow. 17. Fortunately the internet has the answer to this mystery. The bright lights are clearly city lights of the aliens who live on Ceres. The answers to many of lifes mysteries can be found on the internet. Citizen of Ceres 18. A third example of my design was sent on the Rosetta spacecraft to orbit a comet. 19. The success of this design on several space missions makes it an interesting candidate to consider for the start of a freeform aspheric design for astronomy that is unobscured, f/2.0, 22 degree field and with an external pupil. But there is a very similar design that is a better starting point. 20. The design already discussed starts out as two independently corrected systems, with just Petzval at the 3rd-order level. Then the combined design is optimized and both Petzval and distortion can be corrected, with just spheres. But if the eventual design will have aspheres then there is better starting point, with one asphere. It looks very similar but has some important differences. 21. First design front part = concentric mirrors, corrected for spherical aberration, coma, and astigmatism but not Petzval. Exit pupil located at entrance pupil. Second design front = convex mirror M2 at focus with same radius as M1 mirror. Stop at center of curvature of M1 mirror. Corrected for coma, astigmatism and Petzval but not spherical aberration. Telecentric exit pupil Center of curvature of M1 Center of curvature of M1 M2 at focus of M1 22. New design, with one aspheric Decentered pupil is imaged onto 4th mirror, which is made aspheric Telecentric rays Telecentric rays This 2nd type of front end gives better correction results than the 1st type . 23. M3, M4, and M5, the Offner 1X relay, can be made any size. M2 always makes the rays leaving M2 be telecentric and M4 is then always an aperture stop. The center of curvature of M1 will always be imaged onto M4, which is then made aspheric. The two subsystems (M1-M2) and (M3,M4, and M5) are independently corrected for coma, astigmatism and Petzval but not for spherical aberration. Aperture stop aspheric Aspheric at a stop only affects spherical aberration Best starting point for a freeform aspheric design 24. 300 mm diameter front pupil, >600 mm in front of M2 Flat image, f/2.0 unobscured 22 degree field All are freeform aspheric mirrors. f/2.0 and 600 mm focal length 90% energy spot diameter