Freeform aspherics in telescope design
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The document discusses the development of a fast speed wide angle telescope using freeform aspheric mirrors, focusing on design goals such as a 15-degree field of view and f/2.0 aperture. It details the evolution of the design from an initial 1980 concept to improved configurations allowing unobscured images with significant reduction in aberrations. Key advantages of the freeform design include enhanced image quality, uniform scaling, and superior performance when compared to traditional three-mirror systems.
Freeform aspherics in telescope design
- 1. The Evolution of a Fast Speed Wide Angle Telescope Design using Freeform Aspherics David Shafer David Shafer Optical Design
- 2. Design goals 1) 15 degree diameter field of view 2) f/2.0 3) A flat image 4) No obscuration or vignetting 5) Convenient image position 6) Diffraction limited over field at .50u for a 100 mm aperture diameter
- 3. Starting point In 1980 I discovered an unusual system – two spherical mirrors with equal but opposite radii and with 4 reflections between them. For one particular mirror spacing and a particular object/image conjugate pair the system is corrected for 3rd order spherical aberration, coma, astigmatism and Petzval. The magnification has to be 3.325X for this solution.
- 4. In 1983 I realized that this system can be unobscured if it is used for field points that are far enough off- axis. Then the four reflections do not overlap and there can be four separate mirror radii that can be unequal. Then it can be made to work well with collimated input and still be corrected for spherical aberration, coma, astigmatism and Petzval. Here is a picture from back in 1983.This was a 1983 design that was f/4 with a 30 degree by 1.0 degree strip field going in and out of the plane of the page. Spot size was 10 arc seconds over the whole field.
- 5. 4 spheres with common optical axis Aperture stop Flat imageSpheres have too much higher-order aberration to do well at f/2 and 15 degree field. Spot size is 1.0 millirad over the field
- 6. Making all four mirror be aspheric (conventional rotationally symmetric) with all the same optical axis improves the spot size by 10X compared to just spherical mirrors. Then you get about .10 milliard spot over the field, or 20 arc seconds. Because of being far off-axis and distortion there is scale change over the Y and X image area.
- 7. Aperture stop August 5, 2014 Freeform aspheric mirrors. 15 degree diameter flat field at f/2.0 with no obscuration or vignetting. Diffraction- limited over field at .50u for a 100 mm aperture diameter.
- 8. The design works about as well at f/1.5 over a 10 degree field, shown here, as the f/2.0 and 15 degree design
- 9. 1) The freeform aspherics give greatly improved image quality 2) They also allow the correction of the X and Y scale to be the same 3) The freeform asymmetries are not too great since the starting point is a good image quality design with conventional aspherics 4) There are other good designs with only 3 mirrors but this four mirror design here has better performance, a better image location and smaller volume. 5) There are still good new designs to explore. One 5 mirror design has amazing performance and there are probably several more yet to be discovered