The Newtonian Telescope
Named after Sir Isaac Newton, who built and presented the first one of its type to the Royal Society in the 17th century, the Newtonian has become arguably the most popular serious telescope for amateur astronomy. Compared to many more complicated optical designs, the Newtonian has a relatively simple prescription of a concave primary mirror and a small flat surface secondary mirror, suspended centrally above the primary, and its reflective surface at 45o to the axis of the primary. Light travels down the tube and reflects off the primary. A converging light cone moves back up the tube, a second reflection off the secondary mirror, and the cone comes to focus outside the side of the tube at the eyepiece position. Because of its relatively simple construction, the Newtonian during the 20th century, became the most popular design for those amateurs that preferred to build their own telescope. The main tube need not be circular, and many successful instruments have been built using square section tubing, octagonal section, and even those with no tube at all. All that is required for a workable Newtonian is a support cell for the primary, a support cell holding the secondary in place, and the focuser positioned to one side and at the same height as the secondary. Even some commercial designs today opt for collapsible accordion type tubes, or even frame types and truss tube instruments particularly for the very large aperture Newtonians.
The most successful Newtonian telescopes have a parabolic curvature to the surface of the primary. The easiest surface for the optician to generate is spherical, however spherical mirrors, particularly with focal ratios found in modern commercial Newtonians, suffer from lower order spherical aberration, which is destructive to on-axis image resolution.
An accurate parabolic mirror focused at infinity has no spherical aberration, but if the optician does not generate an accurate parabolic surface, mild spherical aberration can be present, and the mirror is described as either under-correct or over-correct. If the focal ratio of the primary is at least f/8 and the aperture is not greater than 150mm, it is still possible to use a spherical primary Newtonian, and indeed some low cost small aperture commercial Newtonians with either long focal ratios, or shorter but with a small correcting lens at the bottom of the focuser, can still be found offered by some of the mass producers. The time and skill required to produce a smooth parabolic mirror does not lend itself to the economics of the lowest cost small commercial Newtonians.
With apertures of 150mm to 300mm (considered medium aperture Newtonians), in order for practicality in use, and physical manageability, focal ratios commonly found are usually f/6 down to f/4, with a few at the smaller end of the size range made at f/8. Above 300mm it becomes increasingly necessary for the manufacturer to keep the focal ratio to a maximum of f/5, as the instruments simply become too long to be manageable. The difficulty then for the manufacturer is to produce accurate and smooth parabolic mirrors at short focal ratios. The difficulty in making a smooth parabolic mirror increases with decreasing focal ratio and with increasing aperture. It is therefore fortunate for us amateurs, that the major Chinese and Taiwanese manufacturers are now producing good quality medium and large aperture Newtonians at more budget prices than ever before.
The images from parabolic mirrors suffer from off-axis coma, and increasingly so with decreasing focal ratio. At f/6 images are not noticeably affected in normal use for deep-sky, and as the planets are always observed at or near the centre of the field, they are considered on-axis. At f/5 and below, coma becomes increasingly noticeable off-axis as the diameter of the diffraction limited field reduces with focal ratio. Fortunately, there are optical accessories that can correct for off-axis coma. A coma corrector is added to the focuser before the eyepiece or camera, and the visual and photographic images can be sharp to the field edge.
The Newtonian is a first class instrument for wide-field photography. A 200mm Newtonian with a focal length of 1000mm becomes a 1000mm f/5 photographic lens, simply by replacing the eyepiece with a camera and coma corrector.
The flat secondary mirror has an elliptical shape. This is so that when tilted to 45o to the primary axis and to the axis of the eyepiece, it appears circular from the eyepiece position. It therefore also presents a circular profile to the incoming light waves, so that the obstruction in the light path and at the centre of the diffraction image is also circular. As a result of geometry, the required set position of the secondary shifts slightly with different focal ratios. This is known as secondary off-set. At focal ratios of around f/8, the off-set requirement is so small, the secondary can sit at the centre of the primary diameter without an issue. Below f/6 there is an increasing need for the secondary to be shifted off the dead centre of the primary, away from the eyepiece position and toward the primary, in order to catch all of the light cone from the primary. The amount of off-set is usually in the order of a few millimetres. The need for this off-set occurs because in order to keep the secondary obstruction to a minimum size, the secondary mirror needs to be just larger than the diameter of the incident light cone from the primary. The secondary is elliptical, and if we mark the centre of the ellipse and place it centrally over the primary, tilting it to 45o, the centre of the cone from the primary does not pass through the our centre mark of the elliptical secondary. We need to move it slightly until it does. If we don't, with shorter focal ratio Newtonians we run the risk of introducing asymmetrical vignetting.
Collimation (optical alignment) of a Newtonian is achieved via tilting of the secondary and tilting of the primary. Incorrect secondary off-set does not affect collimation. Precise collimation becomes more necessary as focal ratio decreases.
Amongst all types of astronomical telescopes, the Newtonian is the most cost effective for its aperture. A 200mm Newtonian telescope complete with equatorial mount can be purchased for less than £600. A 200mm Schmidt-Cassegrain tube assembly only for less than £1000. By comparison, an extremely heavy 200mm achromatic refractor tube assembly would be a few thousands of pounds, and a 200mm apochromatic refractor tube assembly, > £15,000. The value for money for a medium or large aperture Newtonian cannot be overstated. As high resolving power and the detection of faint deep-sky objects rely on aperture, the Newtonian becomes even more desirable.
Mounting a Newtonian.
As Newtonians are relatively inexpensive, many amateurs opt for as large an aperture as they can afford. At apertures up to around 250mm, a sturdy German equatorial mount is ideal for supporting the tube assembly for observing and astrophotography. Over 250mm and the mount requirement becomes increasingly heavy, non-portable and expensive. It is very common for the larger aperture Newtonians to be mounted in a low centre of gravity alt-azimuth mount known as a Dobsonian mount (after John Dobson, the founder of the San Francisco Sidewalk Astronomers and the man who popularised the mount). The Dobsonian mount is a remarkably effective and stable mount. It works effectively because the telescope tube is supported at two opposite positions, similar to a fork mount. Holding a wide tube between two hands is more stable than holding the same tube with one hand. As the heaviest part of the tube is the primary mirror and its cell, this weight is always lower than the altitude pivot point. A typical Dobsonian Newtonian has two circular bearing rings placed at opposite sides of the tube, and at the centre of gravity of the tube. The bearing rings sit in a v-block, sometimes bearing against two Teflon pads per ring. The v-block is part of the azimuth rocker box. The rocker box is larger than the tube diameter and itself sits down on further bearing pads or cone roller bearings so it can swivel in azimuth on a large ground board. With the rocker box swivelling in azimuth and the tube assembly swivelling in altitude, the Dobsonian mount is an excellent choice for mounting a larger aperture Newtonian.