This site introduces you to the design and characteristics of the refractor.
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. Hopefully after you read this, you'll have some idea if a
refractor telescope is the right type for your amateur astronomy interests.
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
rear.
Mousing over the 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 into an eyepiece
for more convenient viewing. Refractors with such a diagonal (called
a star diagonal) see an image that is rightside up, but reversed
left to right.
The original refractor telescopes used a single piece of glass for
the objective that caused horrible chromatic aberration (color
dispersion). 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 causes the converging
rays to come together at a much shallower angle, making the critical
focus region wider. This reduces the chromatic aberration caused by the
different colors coming to focus at different focal points.
Most modern telescopes, like the one shown here, 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, say f/15 or better.
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.
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 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. 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.
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
just starting out, the long time standard 60mm refractor is a great choice.
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 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
Use this astro-customized search to browse some excellent telescope
vendors.
Custom Search
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
available.
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
tripod).
