Naperville Astronomical Association

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        Choosing A 'Scope



Magnitude is the measure of the brightness of an object. Most commonly for observers, it is how bright the object appears to us in the sky ("visual magnitude"). The brightest stars are around 1st magnitude; they give us about 2.5 times as much light as second magnitude stars, which are 2.5 times brighter than 3rd magnitude ones, and so on (The actual step is 2.512 or the 5th root of 100 between magnitudes). Brighter objects go into the negative numbers (the full Moon shining at about -12.7 mag.); in other words, the higher the number, the fainter the object is.
All the light from an object is totalled together in its regular magnitude measurement; an 8th magnitude star and an 8th magnitude galaxy both put the same amount of light into your telescope, but while that of the star is focused into a (relatively) bright point, that of the galaxy is smeared over an area, which causes it to look much fainter. The Surface Brightness measurement takes the spreading into account.
The magnitude scale is a logarithmic one; a star of mag. 1.0 is delivering 100 times more light than a star of mag. 6.0. This works out well for us, since the response of our eyes is also fairly logarithmic; a star of mag. 2.0 appears "twice as bright" as one of mag. 4.0.


Choosing A Telescope

If there was one "best" telescope then everyone would own it and life would be boring. Fortunately, there are many options and each will be a compromise between price, complexity, size and quality.

Most stargazers eventually own more than one telescope because each have their strengths. For example: A small non computerized telescope is great for a quick trip to the backyard to look at the moon, planets and other bright targets. Some are light enough to pick up and move to your backyard all in one piece. A scope that is easy to move is one that will be used often.

A larger computer controlled telescope is heavy and is made to break down into smaller sections that takes a few trips to get everything outside. Each of us have our limits on what we are willing to carry. Sometimes the older you get the lower the limit.

One is great for quick looks in the backyard and the other is more suited to traveling to a dark farm site to seek out the faintest of deep sky wonders.


450 X magnification or more?

Let's be very clear. Never buy a telescope if the seller, or the box it comes in, advertises its magnification. That is the mark of a cheap, horrible, unusable telescope. Never buy a telescope from the mall, a department store, warehouse club or from TV. Those will be an exercise in frustration and disappointment and ultimately a waste of money. Use extreme caution with Craigslist. Although some can be good scopes, the vast majority are department store junk that the buyers could never get to work. Where to buy comes later, for now lets talk options.


First Option: Refractor, Reflector or Catadioptric.

To see how each type of telescope focuses light go to the "How Telescopes Work" page.


Refractor: This is what most people think of when they hear the word "telescope". Modern glass has vastly increased this designs desirability but only at the higher end of the price scale. A quality reftactor is usually on every astronomers wish list.

Advantages: Potential for the best images, no obstruction in light path.

Disadvantages: The most expensive of the three designs per inch of aperture (often by a large margin). Some secondary color "(chromatic aberration)" is visible in all but the best units. Large aperture instruments are massive.


Reflector: Invented by Sir Isaac Newton to fix the refractors issues with chromatic aberration. Uses a parabolic mirror instead of a lens to focus light.

Advantages: Most economical of the three designs per inch of aperture (especially those on Dobsonian mounts).

Disadvantages: Secondary obstruction results in some loss of contrast. Can require occasional collimation (alignment) of optics.


Catadioptric: Schmidt-Cassegrain, Maksutov-Cassegrain, Schmidt-Newtonian are all Catadioptric designs that use both mirrors and lenses to fold the optical path back onto itself resulting in a compact design. The Schmidt-Cassegrain telescope is the most common variant and is referred to as an "SCT."

Advantages: Most compact per inch of aperture of the three designs.

Disadvantages: Images are typically the worst of the three designs.


Second Option: Size (Aperture).

The larger the aperture the more light you can gather and the dimmer the object you can see. If you plan on looking at the faintest objects like dim nebula and distant galaxies far from the typical suburban light pollution, then you want to consider looking for the largest aperture telescope you can afford. However, if you are concerned about the size and weight, then huge isn't for you.


Third Option: Focal Ratio (f/number).

When comparing scopes of equal aperture, the smaller the f/number, the wider the field of view. Focal ratios of f/4 to f/6 are best for low power wide field observing. Focal ratios of f/10 to f/15 are better suited to higher power lunar, planetary, and binary star observing. Medium f/7 to f/9 focal ratios are a good compromise to allow both fairly well.


 Comparing Object Sizes 

Sun/Moon Size


Solar System Objects
Deep-Sky Objects

The above graphic compares the apparent sizes of different objects as they appear in a typical telescope. Hover your mouse over the Solar System box to see planets and over the Deep Sky box for objects far beyond our Solar System. The planets look pretty small while some Deep Sky objects are much larger. Indeed, some deep sky objects are much bigger yet; the Orion Nebula is about 1° across, the Beehive Cluster about 1.5°, the Pleiades cluster 2°, and the Andromeda Galaxy a whopping 3° across! (The inner planets are shown in two sizes, largest when we're closest to them, smallest when we're farthest; the outer planets just at their largest, because they don't vary much).

What this shows is that you'll use relatively high magnifications (150-300x) to observe the moon and planets and relatively low powers (20-150x) to view "deep sky". The field of view a telescope delivers can be changed by puting in different eyepieces. Not all scopes do wide fields and some are better at high magnification. (see the Telescope Calculator page for more discussion of this).


Fourth Option: Telescope Mounts

The telescope's mount is critical for how you use and enjoy your telescope. There are two basic types: Alt-azimuth and Equatorial.

As the Earth rotates, the stars and planets appear to move in the sky. An Alt-azimuth requires constant repositioning of the telescope to compensate for the motion of the earth and to keep your target in the eyepiece.

The Dobsonian is an good example of an alt-azimuth mount combined with a reflector telescope Basic models have no motors, computers or batteries. Dobsonian telescopes are a popular option for first time buyers as well as seasoned observers and have a well earned reputation for the biggest bang for the buck.

The Equatorial mount was designed to make tracking easier by aligning one axis with the axis of the earth's rotation. Equatorial mounts were the first to have "clock" drives to keep the target in the eyepiece. Go-to computer systems became a natural follow on to the simple clock drive.


Fifth Option: Go-To or not to Go-To.

Even some lower cost telescope mounts are being sold with "Go-To" which uses a computer and electric motors to drive the telescope to the selected target. This type setup may sound ideal but any go-to telescope will be complicated. For the initial alignment process to be successful you need to know the location and names of some of the brighter stars. Star charts and especially astronomy apps can be a great help. Don't expect just because it is computer driven it will be easy. It will take some real effort to get it all to work properly.

Some newer mounts have become extremely advanced and have go-to computer systems with optional GPS receivers and cameras to support self-aligning. These systems are expensive and again, expect to spend some time learning how to make it all work properly.

Some stargazers find that having the computer do the work for them is less satisfying than doing it themselves. Before the go-to age, astronomers used "star hopping" to find deep sky objects. The sky now becomes your hunting grounds and there is real satisfaction in locating your targets. It is much different experience than just taking a computer guided tour.

Digital Setting Circles are an option for mounts without go-to computers. These are commonly added on to large Dobsonian telescope mounts to assist in locating deep sky objects and are sometimes referred to as "push-to" scopes.


Sixth Option: Eyepieces.

The field of view of a telescope and its magnification are changed by selecting different eyepieces. Good eyepieces are important to scope performance. A telescope can possess a fine lens or mirror but the image will suffer unless an eyepiece of equal quality is used. You don't need more than 3 or 4 carefully chosen eyepieces.

Eyepieces have an extreme price range ($25 to $800) because of a wide variety of complicated designs and is again where you have to balance price, performance and quality. Expect to pay higher prices for eyepieces with wider field of view for the equivalent magnification; greater eye relief and higher quality glass lenses. You should not buy a $700 eyepiece to put in a $300 telescope when a $80 eyepiece would work just as well. If you wear glasses you'll want to look for ones with good eye relief.

Don't concentrate on high magnifications. Sky conditions rarely allow their use. You'll also notice the higher magnification you use, the dimmer the planet, nebulae and galaxy look. Magnifying spreads out their light across the field of view and hence the dimmer image. Most of the time you will be using your lower power, wider field eyepieces.

Some telescope sellers offer an eyepiece kit. With a little research and not much more money you can do better.

The NAA's Telescope Calculator page will compute the values of a number of parameters for any telescope & eyepiece combinations you enter. It is a very useful tool.

Just a few eyepiece manufacturers to research. There are more.


Explore Scientific







Add-on Option: Astrophotography

When I get asked about getting into astronomy because they want to do astrophotography I equate it to someone who hasn't learned how to drive yet asking about how to enter the Indy 500. You can eventually get there but there is a lot to learn and be ready to have a bankroll to back up your aspirations.

The urge to photograph what you see is very common. So common that there are devices to hook up smart phones to the telescope eyepiece. This is one way to capture bright images like the moon but little else.

Real astrophotgraphy is way more involved and the costs skyrocket. Besides the telescope and a heavy-duty go-to mount, there is the camera, auto-guider camera, autoguider scope, computer, software to run the exposures and software to process the images. Don't forget hours of data gathering through multiple filters for each image. If you live in the typical light polluted urban area be advised you may be limited to narrow band astrophotography that can see through light pollution.

If you are still not discouraged and if astrophotography is one of your goals, you need to do considerably more research before choosing a telescope and mount. If you live under dark skies, building an observatory to house all the heavy equipment you will be buying should be a possible consideration as well. There is only so many times you are going to haul all this equipment out, spend hours gathering data and then haul it all back in again.


Add-on Option: Video Astronomy

Even though video astronomy is nowhere near as complicated as astrophotography, it is still not for a beginner just getting into the hobby. However, it is a new and powerful tool and something you might eventually want to persue.

Light pollution is increasing and it restricts viewing deep sky objects from the typical suburban backyard. Video assisted astronomy uses a specially designed camera and places it where the eyepiece would go. It allows you to view objects not accessable with just an eyepiece and telescope alone. The camera connects to a tv monitor or computer screen and that is how the image is viewed. The video camera has some impressive advantages over our eyes; sensitivity and exposure control being among them.

The Naperville Astronomical Association's two observatories are well within the light pollution dome of the greater Chicago area. To complement our 16" visual telescope and to contine to show the public deep sky wonders, we built an observatory dedicated to video astronomy. The DuPage Valley Observatory's video astrograph can display images in color or black and white depending on choice of camera. The observatories are routinely open for public observing sessions.

The Night Skies Network website hosts amateur astronomers that broadcast their live viewing sessions using Mallincam cameras. You can get a better idea of what is possible by watching. You can even ask questions and have them answered in real time. Check in during the daytime and you might catch someone from South Africa or Australia showing southern hemisphere deep sky targets.

Video astronomy is still expensive. It has some of the same requirements of astrophotography like a specialized camera, a sturdy go-to mount and a short focal length telescope. Some cameras also require a computer for contol. Because the camera can be adjusted for exposure, aperture is a lesser consideration vs. a short focal length.

Some very low priced cameras are able to image the moon but that is about all. To image dim deep sky objects in full color you will need to increase your budget.

Here are some links if you are interested in more reading.

Getting Started with Video Astronomy :Curtis V. Macchioni and Nico V. Macchioni.

Observing with Astro-Video Cameras :Sky and Telescope.

Observing with Astro-Video Cameras :Rod Mollise.

Night Skies Network.

Viewing the Universe in Near Real-Time on Night Skies Network :Chris Appleton.


How large of a telescope do you really need?

The view through a smaller telescope which you popped out into the backyard for a quick observing session wins by default over that monster scope which you didn't bother to haul out at all.

Some stargazers would argue that you can never have a big enough telescope. While the views of a faint object under a prestine dark sky will always be spectacular through a huge telescope, that doesn't always make the larger one superior. Large scopes are not able to do very wide fields of view. If your target is one of the many large objects amatures observe, then the view in a smaller 'scope, which takes in the whole object and frames it nicely would be better. It is another example why most observers eventually have more than one scope.

The diameter of the telescope objective also relates directly to the instrument's resolving power; that is how small details can be seen through it. This is not an absolute because it assumes perfect optics and perfect seeing conditions. Oddly enough, sometimes in bad seeing, a small 'scope will resolve more than a large one because the large one catches more turbulence!

The Telescope Calculator page will give you the magnitude of the faintest star you should be able to see with any given telescope).


Making Decisions

Budget: Setting your budget is usually a good starting point. For an entry level telescope plan on spending around $400. This 8" Dobsonian has a reasonably large aperture and comes with one eyepiece and a quality red dot finder all on a sturdy dobsonian mount. This type of scope eventually ends up in in the collection of every stargazer because it is the most affordable per inch of aperture, is easy to set up and start observing, and for a mass produced scope has consistantly good optics. At f/5.9 it has the ability to do both wide field and fairly high power equally well.

Our observatories have two 6" version of this telescope to demonstrate to our visitors. This is one place you can actually test drive a telescope. If you live in the area you are more than encourage to come out and try it out.

At the top, well, there is no limit but this 14" Celestron telescope gives you an idea of what $10,000 will get you. You can go much, much higher.

Portability: How heavy are the parts? How long will it take to haul out, set up, take down and put away? Where will you store it? Telescopes which are too big, too heavy or too much trouble to set up do not get used much.


More research

Most towns don't have a telescope supermarket where you can go see every model and take them for test drive. In place of that, many areas do have amateur astronomy clubs. The best advice is to join one. A typical club has both public and private observing sessions and will feature all sorts of 'scopes and mounts which you can see and compare views.

There is no substitute for spending time with real equipment out in the field. You may discover that you much prefer the views through a good refractor or you like the portability of Schmidt-Cassegrains, or that the 14" Dobsonian you saw in the catalog is much more of a handful than you imagined. There's no substitute for experience. Clubs are a tremendous resource for the person considering a telescope purchase.


Where to buy: New equipment

Since few of us that live near a telescope dealer most of us purchase over the internet or through a catalog. Here are just a few options.

Orion Telescopes and Binocular :Largest mail order supply house. Great catalog.

Astronomics :Norman, OK

Anacortes Telescope and Wild Bird :Anacortes, Washington.

Analytical Scientific :San Antonio,Texas.

APM Telescopes :Optics from Germany.

Skies Unlimited :Potterstown PA

High Point Scientific :NJ

OPT :Oceanside, CA


Where to buy: Used equipment

One way to save 25% - 50% is buy buying used. Equipment of all kinds is bought and sold on the used market. You can find some great deals if you are willing to check often. These two sites are heavily used and the best deals go quickly. Cloudy Nights is a free service and Astromart charges a yearly fee.

Cloudy Nights Classified


Craigslist is also an option but the vast majority of scopes there are cheap department store junk that the buyers could never get to work so use extreme caution. Remember if the box is advertizing its magnification, stay away.

Local astonomy club sometimes have a used used equipment section on their website. Prices on all sites are sometimes negotiable.