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A star diagonal, erecting lens or diagonal mirror is an angled mirror or prism used in telescopes that allows viewing from a direction that is perpendicular to the usual eyepiece axis. It allows more convenient and comfortable viewing when the telescope is pointed at, or near the zenith (i.e. directly overhead). Also, the resulting image is right side up, but is reversed from left to right.
Star diagonals are available in 0.965", 1.25", and 2" diameters. The 2" diagonals allow longer-focal length, low-power 2" barrel eyepieces for a wider field of view. Star diagonals come in all price ranges, from as low as a few dollars up to hundreds of dollars.
These diagonals (often called star diagonals) use a mirror set at a 45° angle inside the diagonal to turn the telescope's image at a 90° angle to the rear cell. Mirror diagonals produce an image in the eyepiece that is correctly oriented vertically, but is reversed left-to-right horizontally. This causes image reversal, the view in the eyepiece is flipped left-right. The major advantage to mirror diagonals is that they cost less to produce to a high degree of optical accuracy compared to a prism and that they do not introduce any color errors to the image. The major disadvantage of mirror diagonals is that unless the reflective coating is properly applied they can scatter light rendering lower image contrast compared to a 90° prism. Also they deteriorate with age as the reflective surface oxidizes. The newer Dielectric mirrors have largely solved the deterioration problem, and if properly made the Dielectric mirrors scatter less light compared to conventional mirrors. With short-focal length instruments, a mirror diagonal is preferred over a prism.
A prism diagonal uses a simple 90°-angle prism, pentaprism, or an Amici roof prism rather than a mirror to bend the optical path.
On telescopes with a longer focal ratios, a well-made 90° prism diagonal is the optimum choice to deliver the highest image contrast short of using the telescope without a diagonal entirely. However, prisms seem to be falling out of favor probably due to marketing forces that have been favoring short-focal length instruments, which tend to function better with a mirror diagonal. [1] In some special cases however, the color dispersion effects of a prism diagonal can be used to advantage to improve the performance of undercorrected refractor objectives (regardless of focal length) by shifting the spherical and color correction of the objective closer to the design optimum. The natural color dispersion properties (overcorrection) of the prism works to lessen or nullify the undercorrection of the objective lens. [2]
On the other hand, a well-made conventional 90° prism star diagonal can transmit as much or more light as a mirror, and do so with higher image contrast since there is no possibility of light scattering from a reflective metallic surface as in a mirror diagonal. Also a prism will never degrade over time as a mirror will since there is no reflective metal coating to degrade from oxidation. However, prism diagonals may introduce chromatic aberration when used with short focal-length scopes although this is not a problem with the popular Schmidt-Cassegrain and Maksutov-Cassegrain telescopes, which have long focal lengths.
A pentaprism provides the same inverted image orientation as viewing without a diagonal would. A simple 90°-angle prism provides the same "flipped" or mirror reversed image as a mirror diagonal. Pentaprism diagonals are extremely difficult to find.
An Amici prism is a type of roof prism which splits the image in two parts and thus allows an upright image without left-right mirroring. This means that what is seen in the eyepiece is the same as what is seen when looking at the sky, or a star chart or lunar map.
The disadvantage of typical "correct image" Amici roof prism diagonals is that because the light path bounces around through a piece of glass, the total amount of light transmitted is less and the multiple reflections required can introduce optical aberrations. At higher magnifications (>100×), brighter objects have a bright line through the object viewed. Therefore, most Amici roof prisms are more appropriate for low-power viewing or in spotting scopes for terrestrial rather than astronomical use. But with low-power usage with a rich field, the field can easily be compared with star charts, as it is no mirror image.
They are available in two types: with a 90º angle (like an ordinary star diagonal) and with a 45º angle. Such prisms are often used in spotting scopes for terrestrial viewing, mostly with a 45º angle. Such telescopes rarely use magnifications over 60×. [3]
Even an expensive star diagonal will deliver poor performance if it is not in alignment with the optical axis of the telescope. A telescope in perfect collimation will be thrown out of collimation by a misaligned star diagonal and often this misalignment will determine the image quality of the telescope to a larger extent than the surface accuracy of the prism or mirror. Since the mirror or prism of the star diagonal is located nearly at the focal plane of the instrument, surface accuracy of greater that 1/4 wave is more in the line of advertising than any increase in optical performance. A 1/10 wave mirror or prism star diagonal that throws off the collimation of the telescope will perform worse than a 1/2 wave star diagonal that is in proper alignment.
Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviolet, and infrared light. Because light is an electromagnetic wave, other forms of electromagnetic radiation such as X-rays, microwaves, and radio waves exhibit similar properties.
Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes when viewing distant objects. Most binoculars are sized to be held using both hands, although sizes vary widely from opera glasses to large pedestal-mounted military models.
The history of the telescope can be traced to before the invention of the earliest known telescope, which appeared in 1608 in the Netherlands, when a patent was submitted by Hans Lippershey, an eyeglass maker. Although Lippershey did not receive his patent, news of the invention soon spread across Europe. The design of these early refracting telescopes consisted of a convex objective lens and a concave eyepiece. Galileo improved on this design the following year and applied it to astronomy. In 1611, Johannes Kepler described how a far more useful telescope could be made with a convex objective lens and a convex eyepiece lens. By 1655, astronomers such as Christiaan Huygens were building powerful but unwieldy Keplerian telescopes with compound eyepieces.
A collimated beam of light or other electromagnetic radiation has parallel rays, and therefore will spread minimally as it propagates. A perfectly collimated light beam, with no divergence, would not disperse with distance. However, diffraction prevents the creation of any such beam.
A pentaprism is a five-sided reflecting prism used to deviate a beam of light by a constant 90°, even if the entry beam is not at 90° to the prism. The beam reflects inside the prism twice, allowing the transmission of an image through a right angle without inverting it as an ordinary right-angle prism or mirror would.
An optical telescope is a telescope that gathers and focuses light mainly from the visible part of the electromagnetic spectrum, to create a magnified image for direct visual inspection, to make a photograph, or to collect data through electronic image sensors.
A reflecting telescope is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century by Isaac Newton as an alternative to the refracting telescope which, at that time, was a design that suffered from severe chromatic aberration. Although reflecting telescopes produce other types of optical aberrations, it is a design that allows for very large diameter objectives. Almost all of the major telescopes used in astronomy research are reflectors. Reflecting telescopes come in many design variations and may employ extra optical elements to improve image quality or place the image in a mechanically advantageous position. Since reflecting telescopes use mirrors, the design is sometimes referred to as a catoptric telescope.
An optical prism is a transparent optical element with flat, polished surfaces that refract light. At least one surface must be angled — elements with two parallel surfaces are not prisms. The traditional geometrical shape of an optical prism is that of a triangular prism with a triangular base and rectangular sides, and in colloquial use "prism" usually refers to this type. Some types of optical prism are not in fact in the shape of geometric prisms. Prisms can be made from any material that is transparent to the wavelengths for which they are designed. Typical materials include glass, acrylic and fluorite.
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light. Because light is an electromagnetic wave, other forms of electromagnetic radiation such as X-rays, microwaves, and radio waves exhibit similar properties.
The Newtonian telescope, also called the Newtonian reflector or just the Newtonian, is a type of reflecting telescope invented by the English scientist Sir Isaac Newton (1642–1727), using a concave primary mirror and a flat diagonal secondary mirror. Newton's first reflecting telescope was completed in 1668 and is the earliest known functional reflecting telescope. The Newtonian telescope's simple design has made it very popular with amateur telescope makers.
An eyepiece, or ocular lens, is a type of lens that is attached to a variety of optical devices such as telescopes and microscopes. It is so named because it is usually the lens that is closest to the eye when someone looks through the device. The objective lens or mirror collects light and brings it to focus creating an image. The eyepiece is placed near the focal point of the objective to magnify this image. The amount of magnification depends on the focal length of the eyepiece.
A catadioptric optical system is one where refraction and reflection are combined in an optical system, usually via lenses (dioptrics) and curved mirrors (catoptrics). Catadioptric combinations are used in focusing systems such as searchlights, headlamps, early lighthouse focusing systems, optical telescopes, microscopes, and telephoto lenses. Other optical systems that use lenses and mirrors are also referred to as "catadioptric", such as surveillance catadioptric sensors.
The Cassegrain reflector is a combination of a primary concave mirror and a secondary convex mirror, often used in optical telescopes and radio antennas, the main characteristic being that the optical path folds back onto itself, relative to the optical system's primary mirror entrance aperture. This design puts the focal point at a convenient location behind the primary mirror and the convex secondary adds a telephoto effect creating a much longer focal length in a mechanically short system.
The octant, also called reflecting quadrant, is a measuring instrument used primarily in navigation. It is a type of reflecting instrument.
An Amici roof prism, named for its inventor, the Italian astronomer Giovanni Battista Amici, is a type of reflecting prism used to deviate a beam of light by 90° while simultaneously inverting the image. It is commonly used in the eyepieces of telescopes as an image erecting system. It is sometimes called an Amici prism or right angle roof prism. The non-dispersive Amici roof prism should not be confused with the dispersive Amici prism.
The Gregorian telescope is a type of reflecting telescope designed by Scottish mathematician and astronomer James Gregory in the 17th century, and first built in 1673 by Robert Hooke. James Gregory was a contemporary of Isaac Newton. Both often worked simultaneously on similar projects. Gregory's design was published in 1663 and pre-dates the first practical reflecting telescope, the Newtonian telescope, built by Sir Isaac Newton in 1668. However, Gregory's design was only a theoretical description, and he never actually constructed the telescope. It was not successfully built until five years after Newton's first reflecting telescope.
The first reflecting telescope built by Sir Isaac Newton in 1668 is a landmark in the history of telescopes, being the first known successful reflecting telescope. It was the prototype for a design that later came to be called the Newtonian telescope. There were some early prototypes and also modern replicas of this design.
In optics, a relay lens is a lens or a group of lenses that receives the image from the objective lens and relays it to the eyepiece. Relay lenses are found in refracting telescopes, endoscopes, and periscopes to optically interfere with the light path, extend the length of the whole optical system, and usually serve the purpose of inverting the image. They may be made of one or more conventional lenses or achromatic doublets, or a long cylindrical gradient-index of refraction lens.
A Herschel wedge or Herschel prism is an optical prism used in solar observation to refract most of the light out of the optical path, allowing safe visual observation. It was first proposed and used by astronomer John Herschel in the 1830s.