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 parabolic or spherical primary mirror and a flat diagonal secondary mirror. He first constructed it in 1669

Invention

The idea of a reflecting telescope had been around for some time before Newton’s invention. Niccolò Zucchi, an Italian Jesuit astronomer and physicist, is credited with producing the first one in 1616. James Gregory in his book Optica Promota (1663), pointed out that the surfaces of the lenses or mirrors are portions of spheres and that a reflecting telescope with a mirror that was shaped like the part of a conic section such as a Parabola would correct spherical aberration as well as the chromatic aberration caused by the lenses used in a refracting telescope. Gregory had no practical skill and he could find no optician capable producing a working version of his ideas and gave up trying to build one.

Durring the mid 1660’s Isaac Newton came to the same conclusion as James Gregory about refracting telescopes 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 that there was little you could do to correct aberration short of making lenses that were f/50 or more.. In February of 1669 Isaac Newton built his Newtonian reflector as a proof for his theory that white light is composed of a spectrum of colors. He chose an alloy (speculum metal) of tin and copper as the most suitable material for his objective mirror. He later devised means for grinding and polishing them, but chose a spherical shape for his mirror a instead of a parabola to simplify construction: he had satisfied himself that the chromatic—and not the spherical aberration—formed the chief faults of previous refracting telescopes. He added to his reflector what is the hallmark of the design, 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 allowed the image to be viewed with minimal obstruction of the objective mirror. He also made all the tube, mount, and fittings. Newton’s first compact reflecting telescope had a mirror diameter of 1.3 inches and a focal ratio of f/5. With it he found that he could see the satellites of Jupiter and the crescent phase of the planet Venus. Newton’s friend Isaac Barrow showed Newton’s telescope to small group from the Royal Society of London at the end of 1671. They were so impressed with it they demonstrated it for Charles II in January of 1672 and Newton and was admitted as a fellow of the society in the same year.

Newton’s choice of a spherical mirror instead of a parabola along with the problems with fast tarnishing speculum metal and the difficulty of grind mirrors of regular curvature meant it was more than a century before reflecting telescope became popular.

Advantages of the Newtonian design

• Newtonian telescopes are usually less expensive for any given aperture than comparable quality telescopes of other types.
• Since light does not pass through the objective (it only bounces off a mirrored surface) exotic glasses are not needed, the material only needs to be able to hold an accurate figure.
• Since the objective only has one surface that needs to be figured, overall fabrication is far simpler than other telescope designs (refractor objectives, for example, have four surfaces that have to be figured).
• A short focal ratio can be more easily obtained, leading to wider field of view.
• There are no lenses or corrector plates to cause chromatic aberration as in a refractor.
• The eyepiece at the top end of the telescope combined with short f-ratios allow a shorter and more compact mounting system, reducing cost and adding to portability.

A replica of Newton's first 6 inch focal length reflector

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 higher are considered to have insignificant coma for visual or photographic use. 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. 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.

• 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.

• 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.

Examples

• The Dobsonian telescope is a much used type of the Newton telescope.

References

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