Refractor Telescope Basics
This site introduces you to the design and characteristics of the refractor.
Refractirs are very popular among amateur astronomers, in fact the survey
results shown above indicate that refractors are the most used type of
telescope among amateur astronomers.
From the simplistic telescope of Galileo to the giants of the astronomical
refractor period, refractors seem to have a place in the hearts of most
astronomers. At star parties, people will still line up to look through modest
sized refractors, even though much larger reflecting type telescopes are
available. The venerable refractor just seems to look like what most people
think a telescope should look like. Hopefully after you read this, you'll have
some idea if a refractor telescope is the right type for your amateur astronomy
If you want to learn more about refractors, read on.
The Refractor Telescope Design
The oldest and easiest to understand telescope type is the refractor. Used
since the early 1600's, the refractor consists of an objective (the imaging
forming element) at the front of the telescope, and an eyepiece at the
The bottom of the two images above telescope images shows a ray trace
diagram of a refractor. The objective lens (right side of picture) converges
incoming light into a focused image.
It's common for a modern refractor to also have a diagonal mirror (left side
of image) that reflects the converging rays at a 45 or 90 degree angle into an
eyepiece for more convenient viewing. Refractors with such a diagonal (called a
star diagonal) see an image that is right side up, but reversed left to
Schrodingers Cat Small Poster
Refractor Telescope Characteristics
The original refractor telescopes used a single piece of glass for
the objective. That caused horrible chromatic aberration (color
dispersion), making color fringes around bright objects, and seriously
softening detail. In an effort to reduce this distortion, some of the old
refractors were made exceedingly long, over 100 feet.
Increasing the focal length of the telescope caused the converging
rays to come together at a much shallower angle, making the critical
focus region wider. This reduced the chromatic aberration caused by the
different colors coming to focus at different focal points. But it left
the user with a very small field of view, and the exceedingly difficult
task of wielding a 100 foot long telescope.
Most modern telescopes, like the Orion
9024 AstroView 90mm Equatorial Refractor Telescope, use an objective made of two tailor made pieces of
glass. Such telescopes are called achromats.
The different components of the objective are chosen to have
different refractive indices. It is thus possible to choose the right
curves on each piece of glass to greatly reduce the color problems. The
multi-element objectives are designed to bring the red and blue ends of
the visible spectrum to the same focal point. These classic two-element
designs work quite well at long focal ratios.
The focal ratio size
necessary for acceptable color dispersion varies by telescope diameter.
Chromatic Aberration Chart illustrates this well. A 60mm telescope starts to be acceptable
as to color dispersion at only about f/7, or with a focal length of only 420mm.
But a 100mm or 4 inch telescope has to be f/12 or better to deliver
acceptable dispersion. That's a whopping 1200mm or 4 feet long. Not a bad
length for a low setting Dobsonian reflector, but pretty unwieldy for a
tall mounted, heavy refractor.
A number of imported refractors are made at about f/10 or f/11, which make
them much more portable in sizes over 3 inches in diameter, but allows a bit
more color distortion. Most observers find the amount of color distortion
in these popular telescope acceptable.
By either creating the objective out of three elements or using two
elements made from modern, more exotic glass that has lower color dispersion
characteristics, the apochromat can be constructed. The Orion
9895 ED80 80mm f/7.5 Apochromatic Refractor Telescope is an example of such a telescope. The apochromat
greatly reduces chromatic (color) distortion, and also allows quality
refractors to be made in focal ratios down to about f/6. For the first time,
quality rich-field views can be had with a refractor -- for a price.
The modern refractor telescope has a number of features making it
desirable. One is that the refractor requires minimal maintenance. A good
example is my 50mm refractor, shown on a homemade Pipe Fitting Tripod. It
stands ready to go at a moments notice. Keeping the lenses covered when not in
use, and very occasionally cleaning the objective surface is all that's
required. Unlike reflectors, refractors never need to be re-aligned.
Refractors also give the highest contrast images, making them very good for
planetary observing. This is because refractors have a clear light path from
objective to eyepiece. Most reflector designs necessarily have a secondary
mirror in the light path to deliver the image to the eyepiece. Because of the
clear light path available in refractors, they give star images uncluttered by
the spikes prevalent in many types of reflector telescopes.
Refractors tend to give steadier images. This is for a number of reasons,
including that fact that the telescope tube is closed at both ends,
preventing air moving in the light path. Also, there's an averaging effect
because the light travels through multiple elements.
So why doesn't everyone use refractors? For the amateur astronomer, the
biggest reason is cost. Sizes of 4 inches diameter or bigger begin to cost in
the thousands of dollars. Because the achromat works best at long focal
ratios, the larger refractors are also very heavy and unwieldy.
Click On Image For Full Size
Image Taken with 50mm Refractor
However, if the low maintenance and steady images, especially for planetary
viewing, is what you crave, a refractor is an excellent telescope. If you are
on a budget, you can start small. Lest you think there's nothing you can see
with a small refractor, check out the images of the solar eclipse of 1994.
Click on the image block above for a full-sized version. Those images were
taken with my little 50mm refractor.
Lunar Straight Wall Region w/60mm Telescope
If you are just starting out, the long time standard 60mm refractor is a
great choice. And again, if you think such a size is too small, just check out
the above image of the Straight Wall region of the Moon. It was taken with my
Refractor. You'll be able to see more with a bigger scope of another
variety, but the simplicity of using a refractor will increase the likelihood
that your first telescope won't end up quickly in a garage sale.
If simplicity is what you are after, and on a budget, I know a fellow that
has a Celestron
60LCM Computerized Telescope. Though the computerized 60mm is advertised for the
beginner, the fellow I know isn't a beginner, he's an old guy like me who's
long grown tired of lugging big telescopes out. So he often uses the
computerized 60mm for the ultimate in low maintenance and ease of use. No
aligning, no mirrors to clean, cools down quick, and has a computer find the
targets for you. Interested in a refractor yet?
Vixen A70lf on Pipe Tripod
Another good choice if you are on a budget is the nice sized 70mm
Vixen Optics Mini Porta Mount and A70LF Telescope. I recently bought the Vixen A70lf, and have found it to have very good optics. It has an f/12.9 focal ratio, so
it delivers images that have no detectable color distortion. Yet, at a 36
inch focal length it still has a repectably large field of view. If you have
a tripod already, or are interested in making the pipe-fitting tripod, you
can save even more by buying the Vixen 2602 A70Lf Telescope without
tripod. I've written a review at Vixen Telescope Review. You'll notice on the image above that the
Vixen uses the same pipe tripod shown with the 50mm telescope.
If you're pretty sure a refractor is right for you, I suggest you
examine the following table to see what kind of telescopes work best
for different kinds of observing..
Telescope/Observing Preference Table
(Small Instruments At Table Bottom)
|Wide Field||General Purpose||Narrow Field
|15" f/4.5 DOB||10" f/10 DOB||12" SCT
|12" f/4.5 DOB||10" f/6 DOB||6" Refractor
|6" f/5 Newt||8" f/10 SCT||6" Maksutov
|6" f/5 DOB||6" f/8 Newt||6" f/10 Newt
|3.5" f/8 Refractor||4" f/11 Refractor||4" f/15 Refractor
|4.5" f/4.5 DOB||5" f/10 SCT||5" f/15 Maksutov
|2.4" Refractor||4.5" f/10 Newt||3.5" f/15 Refractor
|Binoculars||3" f/10 Refractor||3.5" Maksutov
Some Personal Notes
A few years ago I was going through a transition in telescopes, having
finally given away an 8 inch Dob I'd had for 20 years. While shopping for a
replacement, I spent a season with only a 50mm refractor to use for my
astronomy hobby. It was really a fun season. I had a small equatorial mount
from another scope, and it had a clock drive to which I'd added a fast/slow
control. The 50mm and tripod could be easily carried around, so I could leave
the unit assembled.
In ten minutes I could carry it out, set it in place, and be observing.
Within 15 minutes, the optics would cool to the point of providing good
images. I was able to observe some Mars features on a 17 arc-second sized
planet -- I was amazed. I also got some good Saturn observing that year.
The rings were easily visible, but I could not make out the Cassini division
with this instrument. That kind of convenience is what's available with
a modest sized refractor.
My own experience is that images are at their best through a quality
refractor. I once bought a Bushnell 60mm refractor that was on sale. Bushnell
telescopes have many critics. I found, as one writer had commented, that the
optics on the inexpensive instrument were actually quite good. It gave fine
star images. As with most inexpensive telescopes, it used the small 0.975 inch
eyepieces, which are usually a form of Kellner. They aren't bad, the main
objection is that there's a much smaller selection of these eyepieces
The issue I had with the bargain telescope was the inadequate mount. At
higher magnifications, the slightest touch or breeze would start vibrations
that seemed to take forever to die out. That's something to keep in mind if
you are bargain hunting -- don't get a telescope with too flimsy a mount. Or,
if you're handy with tools, replace a flimsy mount with a sturdy pipe fitting
mount as shown in the 50mm image (see how to build a sturdy