A freeform aspheric version of the classic Dyson design gives much improved aberration correction and makes for designs that are fast speed and have a large field size, especially large rectangular strip fields

1. Dave Shafer
David Shafer Optical Design
Fairfield, CT 06824
203-259-1431
shaferlens@sbcglobal.net
Freeform Dyson Design

2. Classic Offner 1.0X monocentric design
Freeform Offer design with very
large rectangular strip field
Using freeform aspheric surfaces allows the classic Offner design with
spherical surfaces to have greatly increased field size with good correction
https://www.slideshare.net/operacrazy/freeform-aspheric-version-of-the-10-x-offner-relay-june-08-2019

3. Classic monocentric 1.0X Dyson design Classic monocentric 1.0X Wynne-Dyson design
Different glass types
The classic Dyson design is, like the Offner design, corrected for all the 3rd order aberrations.
The Wynne-Dyson design adds 5th-order correction due to two glasses with different index and
dispersion values. Chromatic variation of astigmatism and Petzval is also corrected. Only
spherical surfaces are used in these designs.

4. In order to avoid vignetting due to the small
convex mirror the Offner design has to be used
pretty far off-axis. That makes it impractical for
fast f# situations and also limits performance
due to higher-order field aberrations
The Dyson design has no vignetting problems
and so can work well at much faster speeds than
the Offner design, due to working closer to the
optical axis. But the long glass path and lack of
any working distance limits the practical size of
the system and its suitability when a sizable
working distance is required

5. By making the mirror surface and the convex
lens surface both be freeform aspherics the
design can be well-corrected over a very large
strip field that goes in and out of the plane of
the page. Or a well-corrected smaller square
field that is much larger in the plane of the
page than the classic Dyson design with similar
image quality.
If the freeform design is given a good working
distance, as shown here, then making the
surface closest to the object/image also a
freeform surface gives even better aberration
correction. But then the design has color that
needs to be corrected.

6. Color can be well
corrected by adding a weak
power lens near the mirror,
of the same glass type as
the main lens.