Thinking About Buying a Telescope?

Seth Jarvis

‘Tis the season that many people think that a telescope would make a great gift for a loved one or for the whole family.  And of course, you are smart to think that.  But what to buy?  Here is a little telescope shopping advice:

1. Ask Yourself, Do You Really Need a Telescope?

Consider binoculars for their low-cost, variety of applications, simplicity, and transportability. You can have a marvelous time under the stars with a good pair of 7×50 (seven power with 50mm objective lenses) or 10×50 (ten power with 50mm objective lenses) or larger binoculars: Explore the dust lanes of the Milky Way, view the Andromeda Galaxy, Orion Nebula and the Hercules Globular Cluster, find the “coathanger” star cluster, count the stars in the Pleiades, and note the nightly change in the position of the Galilean Moons of Jupiter.

 

Hunter_7x50

Never underestimate how much good astronomy you can do with binoculars!

 

2. Bigger Buckets Are Better

All optics for nighttime viewing depend on their ability to act like “buckets” to collect light. In daylight the size of the “bucket” is less important because there’s alot of light, but for nighttime viewing it’s all about surface area.

Whether you’re considering a “reflecting” telescope (it gathers light with a large mirror) or a “refracting” telescope (which gathers light with a large lens) you are placing a collecting surface in the path of the light you’re trying to observe. The bigger the “bucket” you put under the light, the more light you gather. And the more light your telescope has to work with, the better the image it will display at the eyepiece.

A quick review of the two main types of telescopes, reflector and refractor:

 

Newtonian

refractor

 

Small changes in the diameter of the objective lens or primary mirror translate into big changes in the surface area for gathering light. Why? Area = Pi x Radius x Radius.

For example:

Telescope with a 6″ primary mirror = 3.14 x 3 x 3 = 28 square inches of light gathering.

Telescope with an 8″ primary mirror = 3.14 x 4 x 4 = 50 square inches of light gathering; nearly twice as much.

When you gather more light you get a brighter, higher resolution image and can support greater magnification. Therefore, buy the largest aperture telescope that your budget and transportability constraints will accommodate. You’ll never regret it.

 

3. Reflectors Give More Bang-For-The-Buck Than Refractors

It’s all about grinding glass and the application of optical coatings. A reflector has only one large glass surface that needs to be precision ground and optically coated (mirrored), while a refractor requires at least four surfaces (both sides of two lenses) to be precision ground and coated, and these coatings are a lot more complicated than just mirroring. This means that a high quality refractor is going to be a lot more expensive than a comparably-sized high quality reflector, and an inexpensive refractor is going to have lower quality optics. Translation: a low-cost 70mm refractor you find in discount department stores should be used only for daytime viewing and large, bright nighttime objects such as the Moon, Planets, and maybe looking at bright double-stars.

NOTE: A top-quality 6″ refractor (costing $5,000 – $10,000) is going to seriously outperform a 6″ reflector telescope in terms of color clarity and resolution. You get what you pay for.

 

4. The Telescope Must Accept Interchangeable 1.25″ Eyepieces

One of the first clues as to whether or not a telescope is to be taken seriously is its ability to accept interchangeable 1.25″ (the diameter of the eyepiece where it attaches to the telescope’s focuser) eyepieces. Some telescopes support both 1.25″ eyepieces and have 2″ focusers and adapters so they can use both 1.25″ and 2″ eyepieces.  A 2″ eyepiece offers the viewer a much larger viewing area and are popular among high-quality telescopes.

 

5. Rule of Thumb: Don’t attempt more than 25 Power Per Inch of Aperture

Don’t try to over-power the optics. Maximum nighttime power is 25x per inch of aperture (or put another way, 1 power per mm of aperture). For example, the maximum power you should try to use with an 8″ telescope is about 8×25=200 power.

As magnification increases the image becomes darker and more grainy, so too much magnification can produce an unacceptably dark and low-resolution image.

In general, magnification has a lot more to do with the type of eyepiece you’re using than with the telescope itself.  You can change a telescopes magnification by simply changing the eyepiece you’re using.  The smaller the focal length of the eyepiece, the more magnification it will provide, but that comes at the cost of brightness and resolution. Conversely, a longer focal length eyepiece provides a lower power, but a wider field of view and a brighter image.

How do you know how much magnification you’re using?

Magnification = Focal Length of the telescope ÷ Focal Length of the Eyepiece.

Example, an 8″ Dobsonian telescope has a focal length of 1200mm and comes with two eyepieces, a 28mm and a 10mm. The magnification obtained with the 28mm eyepiece in this telescope is 1200 ÷ 28 = 43, and for the 10mm eyepiece the power is calculated as 1200 ÷ 10 = 120.

Neither of these eyepieces used with this telescope exceed the 25-power-per-inch limit. BUT if you attempted to use a 6mm eyepiece in our 6″ Classic Dobsonian (also 1200 mm focal length) you’d end up with 1200 ÷ 6 = 200, which is more than the 150 power upper limit for a 6″ telescope.

 

sat_low-med-hi

As magnification increases, the image gets bigger, but it also gets more grainy and darker. This is Saturn at low, medium, and high power through a small amateur telescope. Is the highest magnification image the best? No, it is not. To get good high power you have to gather more light, which requires a bigger primary mirror (for a reflecting telescope) or objective lens (for a refracting telescope).

 

6. What about “Schmidt-Cassegrain” and “Maksutov” telescopes?

These are examples of “Catadioptric” telescopes that incorporate both lenses and mirrors as their primary light-gathering optics. These telescopes are very popular among amateur astronomers, especially the Schmidt-Cassegrain telescopes. Catadioptric telescopes have long focal lengths in short optical tubes, which make them both powerful and portable, and their optics design minimizes color distortions that are inherent in all optical systems.

SchmidtCass

Schmidt-Cassegrain telescopes, like this little 5″ (127mm) Celestron, provide good optics in a compact design.

You’ll pay two to three times more for a Schmidt-Cassegrain or Maksutov telescope than for a comparable reflector of similar aperture, but you’ll be getting a high-quality, compact telescope with excellent optical performance in exchange for the higher price.

 

7. Pay Attention to the Mount!

A telescope’s mount needs to be strong enough to keep the instrument from moving while you are looking through it. A cheap mount will make even the best optics impossible to enjoy.

Department-store telescopes are notorious for coming with flimsy mounts attached to equally flimsy tripods that make the telescope wiggle crazily with every gust of wind.  They also tend to have lousy optics.

 

WhatNotToBuy

Cheap department store telescopes do more to discourage an interest in astronomy than anything I can think of. They give the viewer dark, fuzzy, wiggly images. These telescopes are the opposite of educational.

Dobsonian-mounted reflecting telescopes are popular among amateur astronomers. The “Dob” mount is a simple, robust design that cradles the telescope on an altitude-azimuth swivel base. Teflon pads in the mount allow the user to aim the telescope by simply grabbing it by the tube and aiming in the desired direction. The Dob’s rigid construction and low center of gravity provide excellent stability and place the eyepiece at a comfortable height for most astronomical observations.

 

8Dob

An 8″ Dobsonian-type telescope offers a large primary mirror for gathering lots of light, plus a rugged, simple mount. These telescopes can last a lifetime and make excellent low-cost (~$350) telescopes for beginners.

 

“Equatorial” mounts on telescopes mean that the telescope will move through the sky in a way that simplifies tracking objects as the Earth turns. Sometimes they are motorized, which makes it even nicer for tracking objects through the evening. When they’re not motorized, a simple turn of only one knob is all that’s required to keep an equatorially mounted telescope tracking. They’re a bit more complicated to set up, but once they have been set up they’re easy to use.

 

8. What about “Go-To” telescopes?

An increasingly popular type of amateur telescope incorporates a computer and an electric motor drive system to aim the telescope automatically for you. Typically these telescopes use a catadioptric system (to keep the optical tube short and manageable) paired with a computer controlled motor-driven mount.

With a Go-To telescope the user performs an initial alignment of the telescope by pointing it at two or sometimes three bright stars and telling the telescope’s computer its approximate location and what the date and time is. Once the telescope’s computer has used this information to establish its frame of reference everything else is a straightforward math problem for the telescope’s built-in computer to solve.

Each object in the heavens has a known set of celestial coordinates and the locations of planets and the Moon are calculated by knowing what the date and time it is. Electric motors controlled by the computer then aim the telescope for the user. These telescopes can even make recommendations of what you might like to see that’s currently in the sky, and then give you a little lesson on the computer controller’s text display about the object you’re seeing through the eyepiece!

 

Celestron 8SE

Schmidt-Cassegrain telescopes such as this Celestron Nexstar 8SE (8″ mirror) are often paired with computer-controlled and motor-driven “Go-To” systems. You use the keypad to tell the telescope what you want to see, and it points itelf for you! Expect to pay around $1,200 for a telescope like this.

Some telescopes, such as Orion’s “Intelliscope” design, use computers to tell you where the telescope is pointed, but you still have to manually move the telescope. To use these telescopes you tell the computer what you want to see and it then guides you with a digital readout as you move the telescope (up/down, left/right) to get it pointing at the desired target.

 

9. Do Some Reading

Buy and read at least two issues of the magazines read by amateur astronomers. This is important because it gives the prospective telescope buyer a chance to see what is available in the way of good quality optics from reputable dealers.

The two preeminent monthly magazines read by amateur astronomers are Astronomy (www.astronomy.com) and Sky & Telescope (www.skypub.com). In addition to many excellent articles on the subject of astronomy, these magazines contain many advertisements for high-quality telescopes and binoculars so you can gain some familiarity with what’s available.

I strongly recommend that everyone buying their first telescope also buy with it a copy of the excellent book, “Nightwatch: A Practical Guide to Viewing the Universe” by Terence Dickinson, available from our store. This book is an invaluable blend of star charts, instructions on how to use telescopes and binoculars, descriptions of telescopes, and general astronomy information.

Another excellent beginners guide to the stars is “The Stars” by H.A. Rey (yes, he’s also the author of the Curious George books!) Great explanations and lots of sketches and diagrams to help the beginning astronomer find their way around the nighttime sky.

 

10. Attend a Star Party

People learn best when they are sharing information with other people. After all, we are social animals, and “Star Parties” hosted by local astronomy clubs are great places to meet and talk with other telescope owners and astronomy enthusiasts.

Information about the relative merits and different types of telescopes, eyepieces, mounts and accessories flows freely among participants, and novices with questions about buying telescopes are enthusiastically welcomed.

In the Salt Lake area, try connecting with the people of the Salt Lake Astronomical Society. SLAS regularly hosts public star parties within Salt Lake County and operates a fantastic observatory complex in Stansbury Park. Information about SLAS can be found on their web site.

In Davis and Weber counties (north of Salt Lake City) try connecting with the folks in the Ogden Astronomical Society.

In Utah County (south of Salt Lake City), try connecting with the folks associated with the Utah Valley Astronomy Association.

And if you’re not in our area, just start Googling for “astronomy clubs” in your area.

Happy viewing!

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