Newtonian telescope

Newtonian Telescope

The Newtonian telescope is a type of reflecting telescope invented by the British scientist Sir Isaac Newton (1643-1727), 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.^[1] The Newtonian telescope's simple design makes them very popular with amateur telescope makers^[2].

History

Newton’s idea for a reflecting telescope was not a new one. Galileo Galilei and Giovanni Francesco Sagredo had discussed using a mirror as the image forming objective soon after the invention of the refracting telescope,^[3] and others, such Niccolò Zucchi, claimed to have experimented with the idea as far back as 1616.^[4] Newton may have even read James Gregory's 1663 book Optica Promota which described reflecting telescope designs using parabolic mirrors^[5] (a telescope Gregory had been trying unsuccessfully to build).^[6]

A replica of Newton's second reflecting telescope that he presented to the Royal Society in 1672.^[7]

During the mid 1660s Isaac Newton came to the conclusion that refracting telescopes could never be made free of chromatic aberration after his work on the theory of colour showed him that lenses behaved the same as prisms, breaking white light into a rainbow of colors around bright astronomical objects, and he thought that there was little you could do to correct aberration short of making lenses that were f/50 or more.^[8][9] In 1668 Isaac Newton built his first reflecting telescope as a proof for his theory that white light is composed of a spectrum of colors.^[10] 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^[11] to polish the optical surface. He chose a spherical shape for his mirror instead of a parabola to simplify construction; he had satisfied himself that the chromatic – and not the spherical – aberration was the chief fault of previous refracting telescopes. He added to his reflector what is the hallmark of the design of a "Newtonian telescope", namely a secondary "diagonal" mirror near the primary mirror's focus to reflect the image at 90° angle to an eyepiece mounted on the side of the telescope. This unique addition produced a compact telescope that also 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 mirror diameter of 1.3 inches and a focal ratio of f/5.^[12] With it he found that he could see the four Galilean moons of Jupiter and the crescent phase of the planet Venus. Newton's friend Isaac Barrow showed a second telescope to a small group from the Royal Society of London at the end of 1671. They were so impressed with it they demonstrated it to Charles II in January of 1672. Newton was admitted as a fellow of the society in the same year.

Like Gregory before him, Newton found it hard to construct an effective reflector. It was difficult to grind the speculum metal to a regular curvature. The surface also tarnished rapidly; the consequent low reflectivity of the mirror and also its small size meant that the view through the telescope was very dim compared to contemporary refractors. Because of these difficulties in construction, the Newtonian reflecting telescope was initially not widely adopted. It wasn't until 50 years later in 1721 that John Hadley showed a much-improved model to the Royal Society^[13]. Hadley had solved many of the problems of making a parabolic mirror. His Newtonian with a mirror diameter of 6 inches (~15 cm) compared favorably with the large aerial refracting telescopes of the day^[14]. The aperture of reflecting telescopes would subsequently grow rapidly, with designs doubling in aperture about every 50 years. ^[15]

Advantages of the Newtonian design

Newtonian optical assembly showing the tube (1), the primary mirror (2), and the secondary diagonal mirror support (also called a "spider support") (3).

• They are free of chromatic aberration found in refracting telescopes.
• Newtonian telescopes are usually less expensive for any given aperture 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 far simpler than other telescope designs (Gregorians, cassegrains, and early refractors had two surfaces that need "figuring". Later achromatic refractor objectives had four surfaces that have to be figured).
• A short focal ratio can be more easily obtained, leading to wider field of view.
• The eyepiece 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.

Disadvantages of the Newtonian design

• Newtonians suffer from coma, an off-axis aberration which causes imagery to flare inward and towards the optical axis. This flare is zero on-axis, and is linear with increasing field angle and inversely proportional to the square of the mirror focal ratio, equal to the mirror focal length divided by the mirror aperture. The formula for third order tangential coma is 3θ / 16F², where θ is the angle off axis to the image in radians and F is the focal ratio. Newtonians with a focal ratio of f/6 or lower (f/5 for example) are considered to have increasingly serious coma for visual or photographic use.^[16] Newtonians having a focal ratio of less than f/4 have considerable coma but are the most compact systems, and can still yield beautiful wide-field, low-power imagery.^{[citation needed]} Commercial lenses are also available for Newtonian telescopes that correct for coma from low focal ratio primary mirrors and restore image sharpness over the field.^[17]

A large Newtonian reflector from 1873 with structure to access the eyepiece.

• Newtonians have a central obstruction due to the secondary mirror in the light path. This obstruction and the diffraction spikes caused by the support structure (called the spider) of the secondary mirror reduces contrast. Visually, these effects can be reduced by using a two or three-legged curved spider. This reduces the diffraction sidelobe intensities by a factor of about four and helps to improve image contrast, with the potential penalty that circular spiders are more prone to wind-induced vibration. Although a four-legged spider causes less diffraction than a three-legged curved spider, the three-legged curved spider often gives a more aesthetically pleasing view.^{[citation needed]}

• For portable Newtonians collimation can be a problem. The primary and secondary can get out of alignment from the shocks associated with transportation and handling. This means the telescope may need to be re-aligned (collimated) every time it is set up. Other designs such as refractors and catadioptrics (specifically Maksutov cassegrains) have fixed collimation.

• Cheaper Newtonian telescopes use spherical primary mirrors as opposed to parabolic. This can result in poor optical quality due spherical aberration. However, this aberration is less important for longer focal ratios, and can eventually be considered negligible for systems above around f/10.^{[citation needed]}

• Since the eyepiece is at an asymmetrical point and at the top of the optical tube assembly, accessing it can be difficult. For visual observing on an equatorial mount, tube orientation can put the eyepiece in a very poor viewing position^[18], and larger telescopes require ladders or support structures to access it. For research telescopes, counter balancing very heavy instruments mounted at the focus is difficult.

Gallery

Newtonian Reflector	Very large trailer mounted Newtonian and its ladder	Newtonian (Truss-tube Dobsonian)	Altazimuth mounted Newtonian
Newtonian eyepiece mount	Amateur built 150mm Newtonian telescope	Astroscan, a commercial wide-field Newtonian reflector

Notes

References

• Smith, Warren J., Modern Optical Engineering, McGraw-Hill Inc., 1966, p. 400