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This site describes the features of the Newtonian, one of the simplest and
cheapest instruments made for astronomy.
I admit I'm partial to this inexpensive and capable design. Given its
capabilities versus its price, it provides the best overall seeing for the
dollar.
My 6 inch f/5, reviewed here is overall my favorite telescope. It's a good general purpose
work horse. Portable, great for wide star views, and capable of delivering
decent planetary images.
I also have a 6 inch f/10 Newtonian designed especially for lunar and
planetary observing, and it is reviewed here.
If you want to know more about Newtonians, read on.
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If you want to investigate the market for Newtonians, use this
astro-customized search engine:
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The Newtonian Reflector Design
After the refractor, the next telescope design that came along was the
Newtonian, named after, you guessed it, Isaac Newton. This design uses a
parabolic curved mirror (left side of picture) for the objective instead of a
lens. The curved surface of the mirror reflects the light to create the image
for the eyepiece. Newtonian telescopes of long focal ratio (f/10 for a 4.25
inch or 6 inch for example) need not have parabolic mirrors to produce good
images.
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Because light entering the open end of the telescope tube (right side of
picture) is reflected back through the tube, a flat diagonal mirror is needed
to deliver the image out the side of the tube to an eyepiece.
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Shown: A Discovery 6 inch Equatorial
Since all wavelengths of light reflect off of the surface of a mirror in
the same way, the reflecting telescope is not plagued with the color problems
of the refractor. The only trouble with the reflectors of old was that the
mirrors of the day were made of speculum metal. It provided a smooth
reflecting surface, but one whose reflectivity deteriorated rapidly. So a given
diameter reflector would provide much dimmer images than the same sized
refractor. |
That's not nearly as true today. Most mirrors of today are made of glass
with a thin aluminum coating. Aluminum is very reflective, above 90%. So
today's reflectors are good performers. The parabolic curve on the mirrors
surface is necessary, especially in focal ratios less than f/10, in order to
cause light striking the entire mirror surface to be focused to the same
point.
As it turns out, in longer f ratios the difference between a sphere and a
parabola is insignificant. So longer focus Newtonian reflectors, common in the
4.25 inch and 4.5 inch diameters, need not be parabolic to give very good
performance, and are often sold with spherical mirrors. Just don't buy
a short focus reflector with a spherical mirror and expect good performance.
The Newtonian has it's own advantages that appeal to observers. First and
foremost, they give the most seeing per dollar. That is, for a given diameter
of instrument, they are the cheapest.
Does that mean they are inferior in any way? Not at all. They are just a
simple design that is inexpensive to make.
Since the Newtonian has no chromatic distortion, it can be made to
a wider range of focal ratios without problem. Very large instruments
(12 inch diameter and larger) can be made at focal ratios of f/4 or f/5, making
them still usable by the amateur astronomer at only about 4 to 5 feet long.
Shorter focus Newtonians do, however, suffer from coma. This aberration
causes objects near the edge of the field of view a comet shaped distortion.
Alignment of short focus Newtonians must also be more precise.
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The features that compromise the design are less to do with observing and
more to do with maintenance. Newtonians expose all optical components to the
elements when they are in use. So delicate first-surface mirrors occasionally
must be cleaned, and this takes care. To be cleaned, they must be removed from
the telescope. This means a complete realignment of the optics will have to be
performed when all elements are reassembled. In fact, alignment tweaking needs
to be done routinely for best performance. Once learned, this is not a
difficult procedure, but a number of observers would rather avoid this
issue.
If you don't mind learning how to carefully clean your optics and
realign them, a Newtonian is a excellent choice. I've been using Newtonians
since I was about 16, so it's not a big deal. I've owned a number of them,
and find them my personal favorite for a general use telescope.
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If you think a Newtonian might be what you're interested in, I suggest
you give a moment to the following table. It shows which types of
telescopes are most often used for different types of observing, including
provision for larger, moderate, or small portable telescopes.
Telescope/Observing Preference Table
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Use this astro-customized search to browse some excellent telescope
vendors.
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Personal Notes
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I've owned a number of Newtonian reflecting telescopes over the years, some
in the Dobsonian configuration. My first was a Gilbert 2.5 inch Newtonian --
anyone else ever have one of those? Then I obtained a 6 inch f/12 Newtonian in
sort of a kit, I had to assemble it. It was great, but so long I had to use a
small step ladder to look through it much of the time.
Then I decided to make one of my own -- from scratch. Allyn J. Thompson's
book Making Your Own Telescope gave me the inspiration, and the guidance
of what to build. I was first thinking an 8 inch, but Allyn stated that the
perfect match to a 6 inch f/12 was a 6 inch Richest Field Telescope (RFT).
That was it, I went for a 6 inch f/4.5. It turns out that short focus
telescopes aren't the easiest to construct, but I managed it. So for a time
there, I had a great combination. Then I got aperture fever, going to an 8
inch, then a 10 inch behemoth. In the fray I sold my 6 inchers.
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Now after all these years I'm almost back where Allyn said I should be, I
have a 6 inch f/10 telescope for planetary viewing, and a 6 inch f/5 telescope
for stellar work. It is still a great combination, as he wrote in his book.
Allyn's books is still available at used book stores, if you want an
inspirational telescope making book.
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