Newtonian telescope
The earliest known functional reflecting telescope, invented by Isaac Newton.
The Newtonian telescope, also known as the Newtonian reflector, is a reflecting telescope created by Sir Isaac Newton. It uses a concave primary mirror and a flat diagonal secondary mirror. Newton built his first working model in 1668, which is the earliest known functional reflecting telescope. Its straightforward design has made it a favorite among amateur telescope builders.
A Newtonian telescope has a primary mirror, usually parabolic, that gathers light from a specific area of the sky. A smaller, flat secondary mirror then redirects this light at a right angle out of the optical axis, allowing it to be viewed through an eyepiece.
One advantage of the Newtonian design is that it avoids chromatic aberration, a problem in refracting telescopes. For a given aperture, Newtonians are typically less expensive than comparable quality telescopes of other types. Because only one surface needs to be ground and polished into a complex shape, fabrication is simpler than in other designs like Gregorian, Cassegrain, or early refractors. A short focal ratio is easier to achieve, providing a wider field of view. The eyepiece sits at the top of the telescope, which, with short f-ratios, allows for a more compact and portable mounting system.
However, Newtonians suffer from coma, an off-axis aberration that makes stars near the edge of the field look comet-like. This flare is zero on-axis and increases linearly with field angle, inversely proportional to the square of the mirror's focal ratio. The formula for third-order tangential coma is 3θ / 16F², where θ is the off-axis angle in radians and F is the focal ratio. Newtonians with a focal ratio of f/6 or lower (like f/5) are considered to have serious coma for visual or photographic use. Low focal ratio mirrors can be paired with coma-correcting lenses. The secondary mirror creates a central obstruction, and its support structure (the "spider") causes diffraction spikes that reduce contrast. Using a two- or three-legged curved spider can reduce these diffraction sidelobes by about four times, though circular spiders are more prone to wind vibration. Collimation can be an issue for portable Newtonians, as shocks from transport can misalign the mirrors, requiring realignment each time the telescope is set up—unlike refractors or Maksutov-Cassegrains.
- Inventor
- Sir Isaac Newton
- Year first completed
- 1668
- Primary mirror diameter first version
- 1.3 inches (33 mm)
- Focal ratio first version
- f/5
- Primary mirror material first version
- speculum metal (alloy of tin and copper)
Lore & Background
Newton built his reflecting telescope because he suspected it could prove his theory that white light is composed of a spectrum of colours. Colour distortion (chromatic aberration) was the primary fault of refracting telescopes of Newton's day, and there were many theories as to what caused it. During the mid-1660s with his work on the theory of colour, Newton concluded this defect was caused by the lens of the refracting telescope behaving the same as prisms he was experimenting with, breaking white light into a rainbow of colours around bright astronomical objects. If this were true, then chromatic aberration could be eliminated by building a telescope which did not use a lens – a reflecting telescope.
In late 1668 Isaac Newton built his first reflecting telescope. He chose an alloy (speculum metal) of tin and copper as the most suitable material for his objective mirror. He later devised means for shaping and grinding the mirror and may have been the first to use a pitch lap to polish the optical surface. He chose a spherical shape for his mirror instead of a parabola to simplify construction; even though it would introduce spherical aberration, it would still correct chromatic aberration. He added to his reflector what is the hallmark of the design of a Newtonian telescope, a secondary diagonally mounted mirror near the primary mirror's focus to reflect the image at a 90° angle to an eyepiece mounted on the side of the telescope. This unique addition allowed the image to be viewed with minimal obstruction of the objective mirror. He also made the tube, mount, and fittings. Newton's first version had a primary mirror diameter of 1.3 inches (33 mm) and a focal ratio of f/5. He found that the telescope worked without colour distortion and that he could see the four Galilean moons of Jupiter and the crescent phase of the planet Venus with it.
Newton's idea for a reflecting telescope was not new. The astronomer Galileo Galilei and the mathematician Giovanni Francesco Sagredo had discussed using a mirror as the image forming objective soon after the invention of the refracting telescope, and others, such as Niccolò Zucchi, claimed to have experimented with the idea as far back as 1616. Newton may even have read James Gregory's 1663 book Optica Promota which described reflecting telescope designs using parabolic mirrors (a telescope Gregory had been trying unsuccessfully to build). However, Newton's telescope is considered the first reflecting telescope due to previous designs never being put into practice or ultimately ending in failure when attempted.
Reader's Guide
The Newtonian telescope's simple design has made it very popular with amateur telescope makers. It is free of chromatic aberration found in refracting telescopes, and is usually less expensive for any given objective diameter than comparable quality telescopes of other types. Since there is only one surface that needs to be ground and polished into a complex shape, overall fabrication is much simpler than other telescope designs. A short focal ratio can be more easily obtained, leading to a wider field of view, and the eyepiece location at the top end of the telescope combined with short f-ratios can allow for a much more compact mounting system, reducing cost and adding to portability. However, Newtonians suffer from coma, an off-axis aberration that causes imagery to flare inward and towards the optical axis, with stars towards the edge of the field of view taking on a comet-like shape. They also have a central obstruction due to the secondary mirror, and diffraction spikes caused by the support structure reduce contrast. For portable Newtonians, collimation can be a problem, as the primary and secondary can get out of alignment from transport and handling. The focal plane is at an asymmetrical point and at the top of the optical tube assembly, which can put the eyepiece in a poor viewing position on equatorial mounts. Variations such as the Schmidt–Newtonian and Maksutov–Newtonian add a corrector plate to reduce coma and diffraction effects, while the Jones–Bird design uses a spherical primary mirror with a sub-aperture corrector lens to reduce cost.
Did You Know?
- Newton's first reflecting telescope was completed in 1668 and is the earliest known functional reflecting telescope.
- Newton's friend Isaac Barrow showed a second telescope to a small group from the Royal Society of London at the end of 1671.
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