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Ott Planetarium - Buying a Telescope

Part 1: Don't Buy a Telescope
Part 2: Buy Binoculars
Part 3: But I Really Want a Telescope
Part 4: A Guide For The Serious Buyer

A guide for the serious buyer.

If you did not get to this page by way of the introductory page, I'd suggest that you read that page first!!!

"I already have binoculars, what telescope should I buy?"

For first time telescope buyers I still HIGHLY RECOMMEND that you go to several star parties before buying a telescope. I'll assume that you have either done that or have no access to star parties in your area.

Most people are well served with a Dobsonian ("dob") type telescope. This design of telescope gets you the most optics for the least money and has a good sturdy mount. Dobs are commonly available in 4 inch, 6 inch, 8 inch, 10 inch, 12 inch and larger sizes. The size mentioned here is the diameter of the main ("objective" or "primary") mirror.

The 4 inch diameter is terribly small for most observing and I would recommend against it. The cost of a 6 inch is not much more. I'd suggest that you consider either a 6, 8, or 10 inch telescope. The 8 inch is probably the most popular due to it's fairly manageable size and reasonable light gathering ability. 

"Where do I buy such a thing?"

Disclaimer: I will list various vendors/manufacturers for reference only, this is not an endorsement for any one supplier nor is it a complete list by any means. There are many small companies that have excellent products, prices, and service.

I'd suggest that you check out the ads in Astronomy magazine or in Sky & Telescope Magazine. You can also check out the want ads in Astromart on line which list lots of used telescopes for sale.

Celestron, Meade, and Orion all make dobs that are a good choice and good value. The questions you need to ask are:

1. "How much do I want to spend?" (Translation: "How big a scope can I afford?")

2. "What will fit into my car?" (Translation: "How big a scope can I move around?")

3. "What accessories do I need?" (Translation: "What do I need to be able to enjoy my scope?")

I strongly suggest that you not buy too many accessories. You can always add accessories next year or ten years from now. However, once you have bought the scope you are stuck with it. A bigger scope can be expanded upon more easily with better success than a smaller scope can. Thus, buy the biggest scope that you can afford and still be able to move.

The accessories that you will want are two or three eyepieces, a good finder scope, and a set of star maps.


I'd recommend three eyepieces: low, medium, and high power. If you can only afford two eyepieces then get the low and medium power ones. (Note that the high power eyepiece is not very useful and is the first choice to eliminate.) The actual choice of eyepieces depends on which scope you buy. You generally do not want to exceed about 25 power per inch of aperture. (As your skills improve you will learn that you can sometimes use over 50 power per inch, but not often.) The magnification is determined by dividing the focal length of the telescope's objective by the focal length of the eyepiece. An example follows:

Let's say you buy an 8 inch dob with a focal length of 1200 mm (a fairly typical value). First of all, the maximum magnification that you want to push this instrument to is 25 times 8 = 200 power. Since magnification = (focal length of objective) / (focal length of eyepiece) then solving for the eyepiece we find focal length of the eyepiece = (focal length of objective) / magnification thus focal length of the eyepiece = 1200 / 200 = 6 mm.

Hence, 6 mm would be your high power eyepiece. (And will likely be the eyepiece you will use the least.)

For a low power eyepiece you should select something in the range of 3 to 8 power per inch of aperture. Thus for our example scope this would be about 40 power. An eyepiece that would give a magnification of 5 power per inch of aperture would have a focal length of 1200/40 = 30 mm.

For the medium power eyepiece you should select about 10 to 15 power per inch of aperture. Again, for our example scope this would be 120 power. The eyepiece that would do this would be 1200/80 = 15 mm.

In summary, for an 8 inch reflector with a objective focal length of approximately 1200 mm (this would be called an f6 telescope) you should select eyepieces as:

low power = 40X = 30 mm eyepiece. (A 32 mm would be just as good.)

medium power = 80X = 15 mm eyepiece. (A 17 mm would be fine.)

high power = 200X = 6 mm eyepiece. (A 6.4 mm would be close enough.) Note that you could only use this eyepiece on nights with exceptionally clear viewing.

This recommendation is summarized in the following table for various scopes.

Table 1. Suggested eyepieces for a first scope.

Scope Size "f number" 

(typical value)

focal length of objective 

(size X f#)

Low power eyepiece 

(~ 5X per inch)

Medium power eyepiece 

(~ 10X per inch)

High power eyepiece 

(~ 25X per inch)

6 inch f/6 914 mm 32 mm (29X) 17 mm (54X) 6.4 mm (143X)
6 inch f/8 1220 mm 40 mm (30X) 20 mm (61X) 6.4 mm 


8 inch f/6 1220 mm 32 mm (38X) 17 mm (72X) 6.4 mm (190X)
10 inch f/4.5 1140 mm 25 mm (46X) 10 mm (114X) 4.7 mm (243X) or 

use a barlow and 9.7 mm (235X)

12 inch f/4.8 1460 mm 32 mm (46X) 17 mm (86X)


10 mm (146X)

4.7 mm (311X) 

or, use a good barlow and a 10 mm (292X)

Note 1: The difference between 11 inch, 12 inch, and 12.5 inch telescopes is fairly minimal.

Note 2: Varying the eyepiece focal length by plus or minus 10% will have little effect. If anything, I'd err towards the lower magnification (larger eyepiece focal length).

Note 3: Smaller focal length eyepieces are harder to look through. Using a good quality 2X barlow lens and an eyepiece with twice the focal length will yield the same magnification and will be easier to use. As you get more skilled (and recover financially) you can always add the short focal length eyepieces. Barlow lenses are never as good as a lone eyepiece, but the difference is minimal to all but the more experienced observers. 

Finder Scopes.

You will need to be able to point your telescope. Rather than use fancy computer systems, I'd recommend a Telrad or similar finder scope. These finder scopes have no magnification, they simply present a cross hair, spot, or bulls eye type target to position over the spot in the sky that you are interested in.

It is my strong opinion that you should not use electronics to point your scope when you are first learning the sky. I feel that these devices are wonderful accessories for more experienced amateur astronomers, but the beginning enthusiast should learn the sky first. You will be much more at home in the starry vault of the universe if you have taken the time to learn your way around the sky.

Star Maps.

A telescope is useless if you don't know where to point it. There are numerous star guides available. I'd recommend a simple introduction to astronomy level field guide. A slightly more advanced but very complete guide is the Norton's star atlas. I'd also recommend that you get a subscription to one of the two major astronomy magazines: Astronomy (slightly more introductory level) or Sky & Telescope (slightly higher level, but still very accessible to beginning amateur astronomers). Both of these have monthly star maps and detailed listings of things that you can see in the sky with both your binoculars and new telescope.

A final note.

Many cheaper telescopes save money by using low end focusers. Beware of the helical type focusers, they screw in and out to adjust the focus and are dreadful to use. The extra few dollars needed to get a rack and pinion focuser is well worth it.

Also, don't ignore the used telescope market. There are lots of people who want to upgrade and sell their other scope. With good care a telescope will last for many years, a used telescope will work as well as a new one as long as the mirror coatings are in good condition.


This document contains the opinions of John Sohl, the director of the Ott Planetarium and a professor in the Department of Physics at Weber State University. These are my opinions voiced in an effort to help people avoid wasting money on poor quality telescopes. These are not "official" university opinions, they are my own. These are not to be construed as a final recommendation. I strongly recommend that anyone seriously considering buying a telescope should attend meetings of the local astronomy club and local star parties. Every buyer is different and you should get as many opinions as possible especially those that are geared specifically for your particular needs. Good luck and happy star gazing.

(Note, I'll soon be adding a page on building your own telescope as another option.)

Don't Buy a Telescope | Buy Binoculars | But I Really Want a Telescope | A Guide For the Serious Buyer

Weber State University > Physics Department > Ott Planetarium > here about us | contact us