Above: Optical layout of a typical Newtonian
For sixty years after the invention of the astronomical telescope, all
telescopes were refractors.
However, refractors suffer from chromatic aberration
because the different colors of light do not focus to the same point. Isaac
Newton studied this problem (in his free time between inventing calculus,
developing modern physics, and being beaned on the noggin by
gravitationally-accelerated apples) and came to the conclusion that a refractor
free of chromatic aberration could not be built. (Eventually, modern glass
types have allowed excellent quality refractors to be built, but not until 300
years after Newton's time.) Newton's ingenious solution was to use a
mirror instead of a lens to gather light. A telescope with only mirrors does not suffer from chromatic aberration. Newtonian telescopes are
much less expensive to make than refractors of similar size, so they are a very
How Newtonians Work
The main purpose of all telescopes is to gather light. This is contrary
to the common belief among first-time stargazers that the most important
function of a telescope is to magnify objects. While telescopes do magnify
objects, the most important thing they do for astronomical observing is to
gather much more light than the observer's eye alone could. Reflectors
gather light by using a primary mirror
located at the back of the telescope. This mirror reflects light back up
the telescope tube to a small flat secondary mirror.
This mirror is tilted at a 45° angle to redirect
the light out the side of the telescope tube where an eyepiece is located for
viewing (or a camera for photographing). The secondary mirror is supported
on a spider consisting of (usually) four metal vanes. These spider vanes
cause diffraction spikes which appear as the familiar cross patterns seen around
bright stars in many astrophotos. While pretty in pictures, these
diffraction spikes can slightly (if usually imperceptibly) degrade the image
quality. Normally this is not a concern.
Newtonian telescopes produce an inverted image,
making them poorly suited for terrestrial observing. For stargazing,
having the image upside-down doesn't matter because the orientation of objects
in space is arbitrary. For a multi-purpose telescope for daytime and
nighttime observing, a refractor or Schmidt-Cassegrain is a better choice, but for strictly stargazing, a
Newtonian can be ideal.
The primary mirror in a reflecting telescope is curved to focus the incoming
light to a point. The simplest shape for a curved mirror is spherical.
This means the reflecting surface of the mirror conforms to the shape of a
sphere. The curve on a typical telescope mirror is very slight. The
total depth of the curve depends on the diameter and focal length of the mirror, but typically is around a couple millimeters.
The diagrams on this page greatly exaggerate the curves for clarity.
The problem with using a spherical mirror is that a spherical reflector does
not focus all of the incoming light to the same point. The outer parts of
the mirror will focus light to a point closer to the mirror than will the
central parts of the mirror. This effect is called
spherical aberration and is demonstrated in the
Above: A spherical mirror focuses light rays from different
off-axis distances to different points, causing spherical aberration
In order to focus all the light to the same point, the shape of the mirror
must be parabolic, as shown below.
Above: A parabolic mirror focuses off-axis light rays to a
Other reflectors such as satellite dishes use this same principal and are
parabolic in shape. Almost all Newtonian telescopes will have parabolic
primary mirrors. The only exceptions are in the case of small
aperture telescopes with slow
focal ratios. In such telescopes (such as the
very common 4.5" f/8 Newtonians made by just about every major telescope
manufacturer) a spherical mirror is adequate because at that scale the
difference between a spherical shape and parabolic shape is insignificant.
For more telescopes larger than 5" in aperture, a parabolic mirror is necessary.
Aberrations in Newtonians
While parabolic mirrors eliminate spherical aberration (which would blur the
entire image), they do suffer from a less problematic aberration called
Coma is an off-axis aberration, meaning it only affects the outer parts of the
image and not the center of the image. Coma results in the stars at the
edge of the field having a comet-like shape with the narrow end pointed toward
the center of the field.
Above: Shape of an off-axis star affected by coma. The
center of the field is down in this diagram.
This effect is not too problematic for visual observation, except over very
large fields of view. At high magnification,
such as for planetary viewing, coma is not noticeable. Coma becomes worse
for larger apertures and shorter focal ratios, so the large, fast Newtonians
that might seem ideal for photography suffer the most coma and therefore require
additional corrective optics to produce sharps stars across large photographic
Some eyepieces provide better correction
for coma in Newtonians. Standard Plössl eyepieces will show the full
effect of coma in a Newtonian, but certain wide-field designs such as the
TeleVue Panoptic eyepieces will provide some coma correction and show much
sharper stars at the end of the field. Naturally, these are more expensive
eyepieces. For most observers, coma will not be problematic. In
large, short-focal-ratio Newtonians where coma is more of a problem, there are
coma-corrector devices (such as TeleVue's Paracorr) which minimize the effects
Newtonians and Dobsonians
Dobsonians are a type of Newtonian reflector. The optical design is
the same as any Newtonian, but what makes a Dobsonian a Dobsonian is the type
of base that it uses. Dobs have a lazy-Susan type base that allows easy
movement in the up-down and left-right directions. This means Dobs are
very inexpensive and very easy to use, but they do not have automatic tracking
capabilities (usually) like an equatorially mounted Newtonian would.
Dobsonians are covered in more detail on the
Dobsonian Telescope page.
Non-Dobsonian Newtonians are normally mounted on
equatorial mounts which allow automatic tracking. These are the types
Newtonian telescopes provide the most bang for the buck. They are the
least expensive type of telescope for a given size. This makes them very
popular for deep-sky observing where having a lot of aperture is important.
A good-quality Newtonian reflector will start around $200-250. This will
be a non-computerized scope around 4-5" in aperture on an equatorial mount.
Such a telescope will provide automatic tracking, but will not automatically
locate objects. A good-quality computerized Newtonian will start around
$350 and it will track as well as find objects automatically. Probably the
most popular size for Newtonians is in the 6" to 8" aperture range. Such
scopes cost around $400-600 for equatorially mounted but non-computerized models
and around $800-1000 for computerized versions.
Because of their length (compared to similar-aperture Schmidt-Cassegrains,
for example), larger Newtonians require very large equatorial mounts, especially
if they are going to be used for photography. These scopes can be much
more expensive (mostly due to the cost of the mount) and are not very portable.
Large equatorial Newtonians are very uncommon now that Schmidt-Cassegrains have
become popular. Equatorial Newtonians over 10" in diameter are rare and
usually cost over $2000.
Is a Newtonian Best for Me?
For beginning observers who are not interested in terrestrial observing,
Newtonians provide the most scope for the money. For strictly visual
observing, a Dobsonian is often the best choice since it is simpler to use and
less expensive than an equatorial Newtonian. However, for observers who
want automatic tracking capabilities, an equatorial Newtonian can be a good
choice. However, keep in mind that 8" and larger aperture scopes get to be
quite large and cumbersome. Equatorial Newtonians can provide some
photographic capabilities, but most commercial models do not have adequate
mounts for such applications (although some do). For those interested in
serious deep-sky astrophotography, a Schmidt-Cassegrain is often a better
choice, while for visual observing only, Dobsonians are much more popular.