The Crossed Dragone Telescope is an off-axis telescope design consisting of a parabolic

primary mirror A primary mirror (or primary) is the principal light-gathering surface (the objective) of a reflecting telescope. Description The primary mirror of a reflecting telescope is a spherical, parabolic, or hyperbolic shaped disks of polished ...

and a large concave

secondary mirror A secondary mirror (or secondary) is the second deflecting or focusing mirror element in a reflecting telescope. Light gathered by the primary mirror is directed towards a focal point typically past the location of the secondary. Secondary mirro ...

arranged so that the focal plane is at right angles to the incoming light. In this configuration the polarization of light is preserved through the optics. A Crossed Dragone Telescope

Other advantages of this design are a large

field of view The field of view (FOV) is the angle, angular extent of the observable world that is visual perception, seen at any given moment. In the case of optical instruments or sensors, it is a solid angle through which a detector is sensitive to elec ...

in a compact volume. Due to its off-axis nature the secondary mirror does not block any of the incoming light. At millimeter and submillimeter wavelengths this greatly decreases systematic effects due to

diffraction Diffraction is the deviation of waves from straight-line propagation without any change in their energy due to an obstacle or through an aperture. The diffracting object or aperture effectively becomes a secondary source of the Wave propagation ...

. The main disadvantage is that the size of 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 The cosmic microwave background (CMB, CMBR), or relic radiation, is microwave radiation that fills all space in the observable universe. With a standard optical telescope, the background space between stars and galaxies is almost completely dar ...

experiments), the benefits of low systematics across a large field of view can far out-weight these disadvantages.

History

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

Examples

Some examples of existing and planned telescopes that use this design include: * The QUIET experiment * The Atacama B-mode Search (ABS) * EPIC-IM * Fred Young Submillimeter TelescopeNiemack, M. (2016)
Designs for a large-aperture telescope to map the CMB 10X faster
Applied Optics 55:7, 1688. * Simons Observatory * CMB-S4 Experiment *

BINGO (telescope) BINGO (Baryon Acoustic Oscillations from Integrated Neutral Gas Observations) is a transit telescope, transit radio telescope currently under construction that will observe redshifted hydrogen line emission (between z = 0.13 and 0.45) by intensi ...

Variations

Although the field of view of Crossed Dragone telescope is large, it can be increased further by the addition of canceling aspheric terms in the primary and secondary mirror shapes. This approach has been used by the Simons Observatory Large telescope and the CCAT-prime telescope currently (2021) under construction in Chile. This comes at the cost of breaking the symmetry of the mirrors – they are no longer rotationally symmetric around any axis. Modern machining techniques can cut such surfaces but on a large telescope the mirror will be made of many segments/panels. When aspheric terms are added, each panel becomes different adding to manufacturing costs.

References

{{Reflist Telescope types