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Stargazing Aug 31st ✦ 7pm-10pm
Stargazing Aug 31st ✦ 7pm-10pm
Imaging with a Newtonian

Imaging with a Newtonian

The modern theory of optics is due in great part to Isaac Newton. Much has been written about the fantastic achievements of Newton, but it was a misconception of his that led to his greatest astronomical invention. While studying refracting telescope lenses, Newton came to the conclusion that it would be impossible to design a lens entirely free from chromatic aberration (see section on refractors). In a sense, he was correct: it has only been very recently that we have had the necessary technology to manufacture the exotic glasses required in apochromatic objectives. Fortunately for astronomers, Newton gave up on the idea of making better refracting telescopes and proceeded to invent the reflecting telescope which bears his name: the Newtonian.

A Newtonian telescope is one of the simplest designs, and one of the most common today. It is by far the easiest to manufacture, and is thus the least expensive design and is often built by first-time amateur telescope-makers. The Newtonian consists of a paraboloidal primary mirror and a flat secondary mirror tilted at 45° to reflect the light outside the telescope tube to the eyepiece.

Above: The optical layout of a Newtonian telescope.

Note: The term paraboloid is often replaced with "parabolic". In fact, parabolic is almost always used. Strictly speaking, paraboloidal is correct, as the mirror is a 3-dimensional object, and parabolic describes a 2-dimensional shape. But, we're not that picky, so use whatever you like!

Newtonians, especially the Dobsonian form of the telescope, are very popular for amateur astronomers. For deep-sky observing, no telescope gathers more light for the money. However, there are a few drawbacks to this design which make it less common for CCD imaging than the popular Schmidt-Cassegrain telescope (SCT).

Newtonians are generally long telescopes. An 8" aperture telescope is the most popular size, and taking this as a standard, one easy sees the advantage of a folded optical design such as the SCT. An 8" f/6 Newtonian (one of the most common sizes) is about 48" long. An 8" f/9 refractoris approximately 6 feet long, and will either suffer from tremendous amounts of chromatic aberration, or will cost more than a nicely equipped sports car. On the other hand, an 8" SCT is less than 18" long. Not only does this make the telescope more portable, but the lighter weight and reduced tube size require a smaller mount, which will be lighter, easier to set up, and less expensive.

If portability is less of an issue, a fairly large Newtonian (say, 10"-16" aperture) can provide excellent deep-sky views as well as a fairly fast focal ratio (usually f/4-f/6) for short CCD exposures. Equatorially mounted Newtonians require very large mounts to support the weight and length of the optical tube. Dobsonian telescopes, which are becoming very popular and very large (apertures over 20" are not uncommon), will work for short exposure imaging if an equatorial platform is employed. Without the ability to track, a Dob will not work for CCD imaging.

Optically, Newtonians are simple designs and do not suffer from the chromatic aberrations present in refractors. However, they do suffer from coma, an off-axis aberration that causes stars at the edge of the field to appear elongated. The faster the focal ratio, the worse this effect becomes. For imaging with most Newtonians, a coma corrector lens is required.

Above: Appearance of a star at the edge of the field of view in a telescope suffering from coma. The bright area points toward the center of the field.

Above: Image of the Crescent Nebula taken with a 10" f/6 Newtonian. Image by Dean Salman.

A related design is the Schmidt-Newtonian. The Schmidt-Newtonian (SN) combines design elements from the Newtonian and the Schmidt-Cassegrain. Instead of a paraboloidal primary mirror, the SN uses a spherical mirror which is much easier to make, especially in fast focal ratios. At the front of the telescope is a Schmidt corrector lens like that used in an SCT. This corrects for the spherical aberration inherent in the spherical mirror. This design typically has a focal ratio of around f/4, making it well suited to CCD imaging. It suffers from less coma than a Newtonian of the same focal ratio, although it still has some.

Above: Image taken through a 10" f/4 Schmidt-Newtonian. Image by Kazuyuki Tanaka.

Another similar design is the Maksutov-Newtonian. This is identical to the Schmidt-Newtonian, except that the Schmidt corrector (a thin, aspheric, and hard-to-manufacture element) is replaced with the Maksutov corrector (spherical and easier to make). These scopes are usually a bit slower (f/5 to f/6), but often have excellent optical quality. They tend to be more expensive designs, and the extra thickness of the corrector element requires a longer cool-down time than a SN or standard Newtonian.

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