Crossed Dragone

Lore & Background

Corrado Dragone first described the design in his 1978 paper. Several follow-up papers were published in the 1980s, but it was not until recently that astronomers were able to build focal planes large enough to warrant the extra construction costs associated with the large secondary mirror. Older millimeter or submillimeter telescopes have typically been of Gregorian or Cassegrain designs.

The main disadvantage is that the secondary mirror is of similar size to the primary mirror, making it expensive to make and heavy, requiring large supports. However, for professional applications where low systematic effects are critical—for example in cosmic microwave background experiments—the benefits of low systematics across a large field of view can far outweigh these disadvantages.

Variations exist: the field of view can be increased further by adding canceling aspheric terms in the primary and secondary mirror shapes, as used by the Simons Observatory Large telescope and the CCAT-prime telescope. This breaks the rotational symmetry of the mirrors, and when the mirror is made of many segments, each panel becomes different, adding to manufacturing costs.

Reader's Guide

The Crossed Dragone design is significant primarily for its use in cosmic microwave background experiments, where low systematic effects are critical. Its off-axis configuration eliminates secondary mirror blockage, reducing diffraction-related systematics at millimeter and submillimeter wavelengths. The design preserves polarization, a key requirement for such experiments. Although the large secondary mirror increases cost and weight, the benefits of a large field of view with low systematics have justified its adoption in professional telescopes. Examples include the QUIET experiment, the Atacama B-mode Search, EPIC-IM, the Fred Young Submillimeter Telescope, Simons Observatory, the CMB-S4 Experiment, and BINGO. The legacy of the design is tied to Corrado Dragone's 1978 paper and subsequent work in the 1980s, with recent advances in focal plane technology enabling its practical use.

Did You Know?

• The design was first described by Corrado Dragone in 1978.
• The focal plane is at right angles to the incoming light.
• The Simons Observatory Large telescope and CCAT-prime telescope use aspheric terms to increase the field of view.

Optical Architecture & Polarization Fidelity

The Crossed Dragone is an off-axis telescope configuration built around two mirrors: a parabolic primary and a large concave secondary. Their geometric arrangement directs the focal plane to sit at a right angle relative to the incoming beam of light. This particular orientation carries a significant physical consequence—polarization of the light is preserved as it passes through the optical train, which is critical for experiments that rely on measuring polarization signals. Because the secondary mirror is positioned off-axis rather than centered in the beam path, it does not obstruct any of the incoming radiation. This eliminates the central shadow that plagues on-axis designs like Cassegrain or Gregorian telescopes, which have been the standard for older millimeter and submillimeter instruments. The result is a cleaner optical path with reduced diffraction artifacts, particularly important at the short wavelengths where systematic errors can dominate measurements.

Engineering Trade-offs & Systematic Performance

The design delivers a generous field of view while remaining compact in overall volume, a combination that is difficult to achieve with conventional mirror arrangements. The absence of a central obstruction means that at millimeter and submillimeter wavelengths, systematic effects arising from diffraction are substantially reduced. For professional astronomy—especially cosmic microwave background experiments where even tiny systematic errors can mimic or obscure genuine cosmological signals—this cleanliness across a wide field is invaluable. The principal drawback, however, is mechanical: the secondary mirror must be nearly as large as the primary, making it costly to fabricate and heavy enough to demand substantial support structures. For decades this expense was prohibitive, but as focal planes grew large enough to justify the added construction cost, the benefits of low systematics over a broad field of view came to outweigh the financial and structural penalties.

From Theory to Practice: A Design's Long Wait

Corrado Dragone, an Italian optical designer, first laid out the principles of this configuration in a 1978 paper. A series of follow-up publications throughout the 1980s further developed the theory, yet the design remained largely academic for years. The practical barrier was clear: the large secondary mirror imposed construction costs that only made sense if the focal plane was sufficiently large to exploit the wide field of view. For much of the history of millimeter and submillimeter astronomy, instruments were built using Gregorian or Cassegrain architectures, which, while simpler and cheaper, suffer from central obstruction and associated diffraction systematics. It was only in recent decades that detector arrays and focal-plane technology advanced to the point where the extra engineering investment in a Crossed Dragone system became justifiable, finally moving the design from theoretical elegance to operational reality.

Modern Instruments & Aspheric Extensions

A growing roster of current and planned instruments now employs the Crossed Dragone architecture, including the QUIET experiment, the Atacama B-mode Search, EPIC-IM, the Fred Young Submillimeter Telescope, the Simons Observatory, the CMB-S4 Experiment, and BINGO. This breadth of adoption underscores the design's centrality to next-generation cosmology. Beyond the standard configuration, engineers have found a way to push the field of view even wider by introducing canceling aspheric terms into the surface profiles of both mirrors. The Simons Observatory's Large telescope and the CCAT-prime telescope, under construction in Chile as of 2021, both use this enhanced approach. The trade-off is a loss of rotational symmetry in the mirror shapes, meaning each segment or panel in a segmented mirror becomes unique, increasing manufacturing complexity and cost. Modern machining can produce these surfaces, but the segment-by-segment variation remains a significant engineering challenge.