Above: Optical layout of a typical refractor
When most people think "telescope" they think of a refractor. The very
first telescopes, including Galileo's, were refractors. In fact, all
telescopes were refractors until Isaac Newton invented the first
reflecting telescope in 1669, six decades after
Galileo first pointed his telescope to the heavens.
How Refractors 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. Refractors
gather light by using an objective lens
located at the front (skyward) end of the telescope. The objective lens
consists of two or more pieces of glass which focus the light to a point at the
back of the telescope where an eyepiece is placed for viewing (or a camera for
photographing). The larger the objective lens, the more light the
telescope can gather and thus the more detail can be seen by the observer.
Refractors provide a correct image, meaning everything appears right-side-up.
This makes refractors well-suited as multi-purpose telescopes for both
terrestrial observing and stargazing, unlike
Newtonian reflectors which produce
inverted images and are only suited for astronomical observing.
Types of Refractors
Refractors come in two basic types, achromatic
and apochromatic. The difference lies in the
quality of the objective lens in terms of correcting an effect called
chromatic aberration. Chromatic aberration
occurs because of the familiar prism effect of glass. When light passes
through the objective glass to be focused, each wavelength (or color) of light
is bent by a different amount. In a simple lens consisting of a single
lens, this results in red light focusing farther from the objective than blue
light. In other words, the focal length of the objective is longer in red
than in blue.
Above: A simple lens focuses red light farther from the lens
than blue light, yielding chromatic aberration
To correct for chromatic aberration, two lenses can be used in the objective.
A two-lens objective is called a doublet.
Each piece of glass has different optical properties, allowing chromatic
aberration to be minimized (but not eliminated). This type of objective is
termed achromatic. In an achromatic objective, the design of the optic is
chosen so that red light and blue light focus to the same point. This
leaves green light focused to a slightly different point. When dark
adapted, the human eye is most sensitive to green light and much less sensitive
to red and blue. The green light is considered the primary wavelength in a
refractor as this is what the observer's eye will focus on when observing.
This means the red and blue light will end up slightly out of focus because
these colors are focusing slightly farther from the objective than the green
Above: An achromatic objective focuses different wavelengths of light
to different points
The diagram above greatly exaggerates the focus difference between green and
red/blue light for clarity. In actuality the difference is typically
around 1/2000th the focal length of the telescope, or around 0.5mm on a typical
small refractor. Still, this amount or chromatic aberration, or
false color, can be seen as a purple halo around
bright objects such as planets and bright stars. In order to bring red,
green, and blue light to the same focus point, three lenses must be used in the
objective. This type of color correction is known as apochromatic, and a
three lens objective is called a triplet. An
apochromatic objective can minimize chromatic aberration to the point where
colored halos cannot be seen around bright objects.
Above: An apochromatic objective focuses different
wavelengths of light closer to the same point than does an achromat
Advantages and Disadvantages of Refractor Types
The obvious disadvantage of an achromatic refractor is that it suffers from
some chromatic aberration which can detract from the quality of the image,
especially on certain objects such as the planets. However, the drawback
to an apochromatic refractor is that it is more expensive, due to the need for
extra glass (and for another more important reason that will be seen below).
Achromatic refractors are typically inexpensive telescopes (although more
expensive than a Newtonian
reflector of the same size). They are well-suited to general observing,
including both stargazing and terrestrial observing since they provide a correct
image. They are not, however, very good for photography because cameras
are much more sensitive to chromatic aberration than the human eye, so any
aberration will become much more significant in a photograph than it is
visually. For the most critical observing, an apochromatic refractor can
provide some of the best possible views. Apochromatic refractors are also
very well-suited to astrophotography.
Achromatic Refractor Pros & Cons
Not very good for photography
Usually less compact than a similar diameter apochromatic refractor
Apochromatic Refractor Pros & Cons
Excellent for photography
Usually more compact than a similar diameter achromatic refractor
Types of Glass in Refractors
Part of the reason apochromatic refractors are more expensive than achromatic
refractors is simply because they have more glass in them. But another
reason is that they use different types of glass which are much more expensive
to manufacture. While three lenses can bring three
wavelengths of light
(red, green, and blue) to a single point of focus, there are other wavelengths
of light (violet, for example) that are still focusing to a different point.
This means there is some remnant chromatic aberration. This leftover false
color can be further minimized by using exotic types of glass.
Fancier glass types such as extra-low dispersion
(ED) glass and fluorite spread out the different
colors of light less than regular glass types. This means the remnant false
color is less than it would be in an apochromatic objective using standard
glasses. Such an apochromatic refractor is ideal for photography and
high-resolution observing. Unfortunately, these types of telescopes are
also among the most expensive.
One other application of these glass types is in higher quality achromatic
refractors. By using ED glass or fluorite in a doublet objective,
chromatic aberration can be significantly minimized without the need for three
lenses. While not as well-corrected as an apochromatic refractor, these
telescope offer a considerable improvement over a standard achromatic scope for
a price below that of a typical apochromat. These high-quality doublet
refractors are almost always called "apo" refractors, although by the strict
definition of an apochromatic objective (bringing three colors to the same
focus) they are technically only very good achromats. The quality varies
among these types of telescopes, but even the inexpensive ones are quite a bit
better than standard achromats, and the best are almost every bit as good as a
Refracting telescopes probably cover the widest price range of any telescope
type. A good quality refractor will start around $200-300. This will
get you a small (70-90mm aperture) achromatic refractor on a small equatorial
mount. A good goto achromatic refractor will
start around $300-400. High-quality doublet "apo" refractors are dropping
considerably in price of late. With an equatorial mount, such scopes in
the 80-100mm aperture range start at under $1000. Better "apo" doublets
can cost $2000 or more for a 100mm aperture. True apochromatic triplets in
the 80-100mm range can start under $1000 if they do not incorporate exotic
glasses. The best apochromatic triplets will cost almost $1000 per inch of
aperture for the telescope only (not including a mount). A high-quality
10" refractor with a suitable mount might cost as much as a new BMW M3 sports
car. Although, as cool as an M3 is, a scope like that will take you a lot
farther a lot faster! But it can be seen why refractors are only common in
smaller sizes and why less-expensive reflectors are much more popular in larger
Above: When the item on the left is the
obvious choice, you're truly an addicted astronomer. 0 to 60 million
light-years in 4.8 seconds.
Is a Refractor Best for Me?
For beginners who want a telescope capable of both terrestrial and
astronomical observing, a refractor can be a good choice. Above about 5"
in aperture, refractors become very large and fairly
expensive, so 6" and larger achromatic refractors are not very common. For
advanced observers who are looking for a high-quality portable telescope or a
great wide-field photographic instrument, an apochromatic refractor can often be
the best choice.
Beginning observers who are not looking to do any terrestrial observing will
usually be better off with a Newtonian reflector which will provide much more
light-gathering power for the money. Schmidt-Cassegrain telescopes provide the ability to do both terrestrial and
astronomical observing with a much more compact package, so intermediate
observers often choose these telescopes over a smaller-aperture,