A new family of freeform mirror telescopes
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A variety of types of new freeform mirror designs are shown where there is an extra reflection between the mirrors. Three mirrors with four reflections can give very good image quality in attractive package configurations.
![In 1973 at the OSA annual meeting and later in 1977 in Applied Optics I showed [1] a new
family of two mirror telescopes, corrected for spherical aberration, coma, and astigmatism,
with three reflections between the two conventional conic mirrors. There are actually 4
distinct families, corresponding to the roots of a quartic equation. By tilting the mirrors and
making both of the mirrors have freeform aspheric shapes it is possible to get unobscured
designs with good anastigmatic correction. The primary mirror is seen twice. Here are two
design examples. The one on the left has two concave mirrors so there is a lot of image
curvature but the one on the right has a flat image due to Petzval correction. There is no
obscuration due to the mirrors but the image is buried in the light beam.](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
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A new family of freeform mirror telescopes
- 1. A New Family of Freeform Telescope Types David Shafer David Shafer Optical Design Fairfield, CT. 06824 shaferlens@sbcglobal.net 203-259-1431
- 2. In 1973 at the OSA annual meeting and later in 1977 in Applied Optics I showed [1] a new family of two mirror telescopes, corrected for spherical aberration, coma, and astigmatism, with three reflections between the two conventional conic mirrors. There are actually 4 distinct families, corresponding to the roots of a quartic equation. By tilting the mirrors and making both of the mirrors have freeform aspheric shapes it is possible to get unobscured designs with good anastigmatic correction. The primary mirror is seen twice. Here are two design examples. The one on the left has two concave mirrors so there is a lot of image curvature but the one on the right has a flat image due to Petzval correction. There is no obscuration due to the mirrors but the image is buried in the light beam.
- 3. Much more interesting is what happens with three mirror designs, with four reflections. On the left we see a f/4.0 conventional three mirror freeform design with no extra reflections. The middle and right hand designs show f/4.0 freeform designs with three mirrors and four reflections, with the primary mirror seen twice by the rays. Both of those designs have flat anastigmatic images. In some situations the ease of baffling, image tilt, and package size can be superior to what a conventional three mirror freeform design can provide.
- 4. A different type of design results when the second mirror is seen twice by the light, instead of the primary mirror. The right and left side examples here have a 200 mm aperture diameter and are f/4. They are diffraction-limited at .50u over a 2 X 4 degree flat field. All three mirrors have weak powers so the alignment tolerances should be relatively loose. The middle design shows how a slow speed long focal length design can fit into a very compact package [1] Anastigmatic two-mirror telescopes: some new types. Applied Optics / vol.16, no. 5 / May 1977