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Newtonian Basics

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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 dollar.

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 altazimuth mount.

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.

Equatorial Newtonian
Equatorial Newtonian

Newtonian Optial Diagram

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 Newtonian Reflector Configurations

equatorial mount

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 hemisphere).

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.

Dobsonian Telescope Diagram

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 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, 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. Collimation of short focus Newtonians must also be more precise.

Newtonian Collimation

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 the telescope).

Newtonian Spider

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 centered.

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 also.

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 Tutorial.

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 FieldGeneral PurposeNarrow Field
15" f/4.5 DOB10" f/10 DOB12" SCT
12" f/4.5 DOB10" f/6 DOB6" Refractor
6" f/5 Newt8" f/10 SCT6" Maksutov
6" f/5 DOB6" f/8 Newt6" f/10 Newt
3.5" f/8 Refractor4" f/11 Refractor4" f/15 Refractor
4.5" f/4.5 DOB5" f/10 SCT5" f/15 Maksutov
2.4" Refractor4.5" f/10 Newt3.5" f/15 Refractor
Binoculars3" f/10 Refractor3.5" Maksutov

Personal Notes

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.