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How Do Telescope Covex Mirrors Work?

Reflecting telescopes overcome some of the problems associated with large lenses. They do not suffer from the same size restrictions as refracting telescopes, which must use a lens thin enough that light can still pass through. Mirrors do not suffer from sag to the extent that heavy, amorphous glass lenses do. Lenses also prevent ultraviolet light from passing through, which can be a problem in some applications.

Reflecting Telescopes
- Reflections from the primary mirror must be redirected by a secondary mirror.
  
  Telescope designs use lenses, mirrors or a combination of both to magnify and direct images from distant objects. The primary mirror in a reflecting telescope is typically concave. Its curved surface reflects light from far away and redirects it to a focal point much closer to the mirror. The methods for directing the reflected image from that focal point to a viewer's eye vary, but they often involve additional mirrors. Another mirror often rests at the primary mirror's focal point and directs the image elsewhere, and this method may employ either a flat mirror or a convex mirror.
Convex Mirrors
- Convex mirrors reflect wide angles of view into narrower viewing spaces.
  
  Convex mirrors are curved in the opposite direction. A parallel beam of light diverges when reflected from a convex mirror, and this creates a more compressed field of view that is accurate at the center and distorted at the edges. The field of view from a convex mirror is also wider than that from a flat or concave mirror, allowing more visual data to be transmitted to a narrower focal point.
Cassegrain Configuration
- The use of smaller convex mirrors in conjunction with large concave mirrors is common in reflecting telescopes. A larger concave mirror reflects a magnifying image toward its respective focal point. A smaller convex mirror, placed at this focal point, reflects the image back toward a new focal point. The convex mirror's focal point is behind the concave mirror, so the primary mirror has a hole at its center to allow redirected light through. A viewing lens or an additional angled mirror is placed at the second focal point. Several variants on the Cassegrain configuration are used in reflecting telescopes, such as the Maksutov-Cassegrain and Schmidt-Cassegrain telescopes, which add a refractive element, as well as the Ritchey-Chretien model, which uses two convex mirrors. These variants were designed to correct for some of the disadvantages of Cassegrain telescopes.
Disadvantages
- The primary mirror in a reflecting telescope does not capture a complete image. If the convex mirror is positioned to create a focal point behind the primary mirror, then it must have some of its reflective surface removed. Even angling the convex mirror to reflect an image to the side blocks some light, as the smaller mirror itself stands between the primary mirror and the distant object. Convex mirrors also produce distortions, such as spherical aberration and coma. Spherical aberration diminishes the sharpness of a reflected image near its edges. Coma is the visual appearance of a glow, haze or imperfection near the image's edge.

Astronomy