Cassegrain reflector
Folded optical path with concave primary and convex secondary mirrors.
A Cassegrain reflector is a combination of a primary concave mirror and a secondary convex mirror, often used in optical telescopes and radio antennas. Its main characteristic is that the optical path folds back onto itself relative to the optical system's primary mirror entrance aperture, putting the focal point at a convenient location behind the primary mirror and creating a much longer focal length in a mechanically short system.
- First published
- April 25, 1672
- Publication
- Journal des sçavans
- Attributed to
- Laurent Cassegrain
- Primary mirror shape classic
- parabolic
- Secondary mirror shape classic
- hyperbolic
- Primary mirror conic constant classic
- -1
Lore & Background
The Cassegrain reflector is named after a published reflecting telescope design that appeared in the April 25, 1672 Journal des sçavans, attributed to Laurent Cassegrain. Similar designs using convex secondary mirrors have been found in Bonaventura Cavalieri's 1632 writings describing burning mirrors and Marin Mersenne's 1636 writings describing telescope designs. James Gregory's 1662 attempts to create a reflecting telescope included a Cassegrain configuration, judging by a convex secondary mirror found among his experiments.
In a symmetrical Cassegrain, both mirrors are aligned about the optical axis, and the primary mirror usually contains a hole in the center, permitting light to reach an eyepiece, camera, or image sensor. Alternatively, as in many radio telescopes, the final focus may be in front of the primary. In an asymmetrical Cassegrain, the mirror(s) may be tilted to avoid obscuration of the primary or to avoid the need for a hole in the primary mirror.
The classic Cassegrain configuration uses a parabolic reflector as the primary while the secondary mirror is hyperbolic. Modern variants may have a hyperbolic primary for increased performance (e.g., the Ritchey–Chrétien design); either or both mirrors may be spherical or elliptical for ease of manufacturing. The Cassegrain design is also used in catadioptric systems.
Reader's Guide
The Cassegrain design is significant for folding the optics, making it compact while achieving a long focal length. In the classic configuration, a concave parabolic primary reflects all incoming light rays parallel to its axis to a single focus, and a convex hyperbolic secondary has two foci, reflecting light directed at one focus toward the other. The mirrors share one focus, and the second focus of the hyperbolic mirror is at the observation point, usually just outside the eyepiece. On smaller telescopes and camera lenses, the secondary is often mounted on an optically flat, clear glass plate that closes the tube, eliminating star-shaped diffraction effects from a straight-vaned support spider and keeping the tube clean, at the cost of some light loss. In most Cassegrain systems, the secondary blocks a central portion of the aperture, creating a ring-shaped entrance aperture that significantly reduces a portion of the modulation transfer function over low spatial frequencies, lowering image contrast when imaging broad features. The support spider may also introduce diffraction spikes. The design's legacy includes its use in both optical telescopes and radio antennas, and its adaptation in modern variants like the Ritchey–Chrétien.
Did You Know?
- Similar convex secondary mirror designs appear in Bonaventura Cavalieri's 1632 writings on burning mirrors and Marin Mersenne's 1636 writings on telescope designs.
- The classic Cassegrain uses a parabolic primary mirror and a hyperbolic secondary mirror.
Frequently Asked Questions
Who is Cassegrain reflector?
The design is attributed to Laurent Cassegrain, a Scottish astronomer who first described the layout in April 1672 in the Journal des sçavans. His key idea was pairing a concave primary mirror with a small convex secondary to fold the light path back through the primary.
What are Cassegrain reflector's powers/role?
Its signature trick is bouncing light off a convex secondary and back through a central hole in the primary, so the focal point lands behind the primary mirror in a tube far shorter than the effective focal length. That makes it the go-to choice when you need a very long focal ratio without a correspondingly long structure.
Why is Cassegrain reflector important?
By delivering the focus at a convenient spot behind the primary mirror, it lets observers mount heavy cameras and spectrographs at the back of the tube instead of at the top of a tall open framework. That mechanical practicality is why the design dominates both professional optical observatories and large radio dishes.
What are Cassegrain reflector's classic mirror specs?
In the original configuration the primary is a parabolic mirror (conic constant −1) paired with a hyperbolic convex secondary. Together the two conic surfaces cancel the spherical aberration the paraboloid would otherwise introduce and redirect the beam to a clean focus behind the primary.
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