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This site introduces you to the design and characteristics of the
refractor. Hopefully after you read this, you'll have some idea if it's
the right telescope for you.
If you want to learn more about refractors, read on.
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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.
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Shown is 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.
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Shown: 50mm Jaegers Refractor on Pipe Fitting Mount
The original models 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.
Most modern telescopes, like the one shown here, use an objective
made of two tailor made pieces of glass. Such telescopes are called
achromats.
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Because the different components of the objective have different refractive
indices, it is possible to choose the right curves on each piece of glass and
greatly reduce the color problems. 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 that does allow
a bit more color distortion.
By either creating the objective out of three elements or using two
elements made from modern, more exotic glass, 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.
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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. Refractors also give the highest
contrast images, making them very good for planetary observing. They also give
star images uncluttered by the spikes prevalent in many types of reflector
telescopes. They tend to give steadier images.
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So why doesn't everyone use them? For the amateur astronomer, the biggest reason is
cost. Sizes of 4" 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.
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.
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Astro T-Shirt Designs
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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. The table is full of links that take
you to more complete descriptions of the indicated telescopes.
Telescope/Observing Preference Table
One online shopping mall that has telescopes in all of these categories
is
SHOP.COM
 . They carry very good prices on instruments and telescope accessories
from Celestron, Meade, Tele Vue, Vixen, and Rigel Systems, to name a few.
Just enter astronomical telescopes, or the specific type of telescope or accessory in their search engine.
Some Personal Notes
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A few years ago I was going through a transition in telescopes, having
finally given away an 8" Dob I'd had for 20 years, and not yet having decided
on a replacement. I spent a season with only this little 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 30 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.
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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 was actually quite good. It gave fine
star images. As with most inexpensive telescopes, it used the small 0.975"
eyepieces, which are usually a form of Kellner. They aren't bad, the main
objection is that there's a much smaller selection of the smaller size
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, replace a flimsy mount with a
sturdy pipe fitting mount as shown in the 50mm image (see
how to build a sturdy tripod).
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