Telescope Design Twist with Freeform Optics

Recommended reading in Optical Engineering

01 October 2015
Michael T. Eismann

Freeform optics is an emerging optical manufacturing technology that gives optical designers expanded degrees of freedom. In theory, this should allow them to optimize image quality and other design performance metrics with fewer optical elements.

Since advanced freeform manufacturing capabilities are not restricted to the symmetries of traditional manufacturing methods, asymmetric aberrations such as coma and astigmatism can potentially be reduced. The challenge for the optical designer, however, is to effectively exploit these new degrees of freedom.

In the February issue of Optical Engineering, the use of freeform optics provides a new twist to the design of a classical two-mirror telescope in “Optical design of off-axis Cassegrain telescope using freeform surface at the secondary mirror.”

The traditional Cassegrain design is often coupled with a refractive relay lens group to compensate off-axis aberrations, and it exhibits a central obscuration that reduces image quality. Authors Suryakant Gautam, Amit Gupta, and Ganga Sharan Singh of the Academy of Scientific and Innovative Research, Central Scientific Instruments Organisation (India) use a freeform secondary mirror in an off-axis design to eliminate both of these issues.

As freeform optics present a quite unbounded design space, much of the article is devoted to the design optimization methodology. Ultimately, the authors arrive at an f/4.4 reflective design using a prolate spheroid off-axis primary mirror and freeform secondary mirror to obtain near diffraction-limited performance at visible wavelengths over a 2-degree field of view.

Employing only two reflective elements without obscuration, the design provides achromatic performance with potentially high transmission over a broad spectral range. To simplify alignment during assembly, the designers used decentered as opposed to tilted optics.

From the standpoint of history, it may be a misnomer to call an off-axis design such as this a Cassegrain because the fundamental concept attributed to the 17th- century Catholic priest was to achieve an on-axis design, specifically, using a suspended convex mirror to provide an imaging path through a hole in the primary mirror.

Practical implementation of the Cassegrain concept did not occur for a century after the original publication in 1672. While it should not take as long to produce this new variant, the next step is to demonstrate the practical implementation of the design.


SPIE Fellow Michael T. Eismann is editor-in-chief of Optical Engineering.

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