Newtonian Telescope Basics
This site describes the features of the Newtonian telescope, one of the
simplest and cheapest telescope designs made for astronomy. And yes, it
is named after Isaac Newton, who invented the design.
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
My 6 inch f/5 Newtonian, similar to the
Orion 9827 AstroView 6 Equatorial Reflector Telescope,
, was purchased from Discovery telescopes some years
ago. My equatorial Newtonian, reviewed here is overall my
favorite telescope. It's a good general purpose workhorse. 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. My f/10 was designed for me by Stargazer Steve. It was a special design, not available on
his website, but his 6 inch f/8, also a very good planetary telescope, is
available. His telescopes come in a kit, so you have a minimal amount of
assembly to do before observing. Another way to go is to get the Orion 8944 SkyQuest XT6 Classic Dobsonian Telescope, which may get you up and running quicker.
If you want to know more about Newtonians, read on.
As to Newtonians, the graphs above, shown with many others on the
User Survey page,
show a puzzling finding relating to Newtonian ownership and use. As you
probably know, the Dobsonian telescope is a Newtonian on a simple but
amazingly capable altazimuth mount. So the term Newtonian to most amateur
astronomers means an equatorial mounted Newtonian telescope, and a
Dobsonian refers to a Newtonian telescope on a Dobsonian type
As the above graphs illustrate, Newtonian equatorial telescopes are
perhaps the second most owned telescopes. Yet the graph on the bottom shows
that Dobsonian telescopes get far more use than equatorial mounted
Newtonians. If I had to guess why, I'd suspect that Dobsonians are preferred
for use because of their simplicity and ease of setting up and operating.
The Newtonian Reflector Configurations
After the refractor, the next telescope design that came along was the
Newtonian. The 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, an easier to create spherical
mirror will suffice.
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.
The Modern Amateur Astronomy Newtonian
Newtonian reflecting telescopes tend to come these days in one of three
mount configurations, the classical Equatorial mount, the computerized
altazimuth mount, and the Dobsonian Mount. Above you see the equatorial mount.
The idea of the equatorial mount is to make it easier to keep your telescope on
its target. You might think that the equtorial mount is a bit complicated, in
that it must be properly adjusted to compensate for your latitude, and aligned
with the Earth's spin axis (pointed at Polaris if you live in the Northern
The classical equatorial mount has counter-weights on the declination
axis to offset the weight of the telescope. The knob that is visible just
beneath the telescope is a Declination adjustment knob. There is another
for the Right Ascension (RA) axis. Once on a target, one only needs to
turn the RA knob to stay on a target. And, if motorized to track the stars,
only one constant speed motor is required to turn the equatorial mount
into a tracking mount.
In this modern era of computerized telescopes, you can a 130mm (5 inch)
Newtonian, as shown, on a computerized mount. The Celestron NexStar 130 SLT Computerized Telescope is one such telescope. This is a pretty inexpensive way to get
into a computer driven telescope and yet take advantage of the simplicity
of the Newtonian design. With this kind of mount you are getting the
advantage of a computerized system, but are restricted to smaller versions
of the Newtonian design, 5 inch being about the maximum.
The Newtonian is often used on a much simpler mount called the Dobsonian
mount. A model of one is shown above, with the basic parts labeled. The
Dobsonian is a simple altazimuth mounting, no counterweights or complicated
angle adjustments. However, if you want to use setting circles on a Dobsonian,
it is helpful if the Dob base is level. This elegant mount design is what you
may want to consider if you are interested in a large Newtonian, say 8 inch
to 15 inch.
The Dob style of mount is described in more detail on the
Dobsonian web page.
It's main advantage is that it is inexpensive to make, and can hold a rather
big telescope. But it is generally for observing only, not photography.
Newtonian Telescope Characteristics
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
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
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, like the Orion
10014 SkyQuest XT4.5 Classic Dobsonian Telescope , need not be parabolic to give very good
performance. These long focal ratio Newtonian telescopes are often sold with
spherical mirrors, which perform quite adequately. 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. Examine, for example the popular Orion 10015 StarBlast 4.5 Astro Reflector Telescope (Teal). It's a short focus Newtonian on a simple mount usable on a table top (or sturdy tripod), and it's as functional as it is cute. This model illustrates the low cost available because of the simplicity of the Newtonian design.
If you happen to be an Astroscan fan, you are
probably bummed that the venerable Astroscan in its classic form is no longer
manufactured. If you know about the Astroscan, you probably know it was also a
Newtonian, but it had a window on the open end which acted to both hold the
secondary in place, and keep dust out of the tube. But don't dispair, there is
a more modern version now avaiable, the Astroscan
Millennium Dobsonian Reflector. It doesn't have quite the charm of the original,
but actually solves some of the problems the classic had, like the new one can
be collimated by the user.
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. The Orion
8946 SkyQuest XT10 Classic Dobsonian Telescope is a good example of a rather big aperture (10 inch)
telescope that's actually not that large to contend with, nor that expensive.
Shorter focus Newtonians do, however, suffer from coma. This aberration causes
objects near the edge of the field of view to reveal a comet shaped distortion.
short focus Newtonians must also be more precise.
To get good performance out of a Newtonian, you must check and possibly
adjust the collimation occasionally, every few months at least. The
collimation process is not difficult, but is more demanding the smaller
the f ratio of your telescope. There are adjustments for both the secondary
mirror (the diagonal), and the primary (the main mirror at the bottom of
Above is a representation of what you see when you look down the open
tube of the Newtonian. You'll see the main mirror at the bottom, and
what's called the
Spider or secondary support that holds the
secondary mirror in proper position.
There may be 3 or 4 vanes holding the secondary in position, but in
addition there are usually 3 screws on the hub where the secondary is
mounted. The secondary is mounted on the back side of the hub, be careful
not to touch the surface of the secondary.
Secondary adjustment for proper collimation is done with the 3 screws
on the spider hub.
Primary adjustment is done with 3 screws on the back of the primary mirror
cell. Some, like those shown on my telescoped shown in the Discovery Newtonian
Review, are in
push-pull pairs, with the outer screws being used to
hold the inner adjusting screws in position.
A very quick alignment guide is illustrated by the animated image at left.
One easy way to do alignment is with a Collimation
Eyepiece , a handy alignment tool. The illustration at left
is a depiction of what you see looking into a Cheshire collimation eyepiece
inserted into a Newtonian telescope focuser. The colored rings represent as
follows: Cheshire eyepiece and cross hair,
secondary mirror, primary mirror, and cross hair
reflection. The initial image is what you typically see, a primary
that doesn't appear to be centered, and a reflection of the Cheshire that's not
The procedures are to first make adjustments to the secondary adjustment
screws to make the primary appear to be centered. The process is aided
considerably if you paint a dot on the very center of your primary, or glue a
notebook re-inforcement ring to the center of the mirror. The secondary blocks
off the center of the primary anyway, so a dot or re-inforcement ring on the
center does not harm your primary mirror.
Then, adjust the primary alignment screws to move the
reflection of the Cheshire cross hair to also be centered on
the directly seen cross hair. Walla, you've achieved alignment. A more detailed
collimation instruction can be found at Newtonian
Collimation. These same instructions apply to Dobsonian telescopes
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. A good procedure is described at
Starizona's Optics Cleaning
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
for 50 years, 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.
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
(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
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 Making Your Own Telescope (Dover Books on Astronomy)
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.
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.
If you want an inspirational telescope making book, check out Making Your Own Telescope by Thompson.