I'm often asked to recommend a telescope, especially around the holidays, when gift-giving is on our minds. Today I'll share a few suggestions, but before we talk specifics, let's look at the different types of telescopes out there. There are three basic designs — refractor, reflector and Schmidt-Cassegrain (abbreviated to SCT).
Refractors have a spyglass design with a glass lens called an objective at the end of a tube. The objective is like a giant eyeball that gathers light from distant celestial objects and funnels it into an eyepiece at the other end of the tube. The eyepiece is a fancy magnifying glass and sits inside a focuser with a knob to bring the image into sharp focus. The design is simple and elegant. The word refractor comes from refraction, which is the bending of light when it passes from one medium (air) through another, in this case a curved piece of glass. The objective bends incoming light rays into a narrow cone so they can be brought to a focus.
A reflecting telescope similarly gather light and directs it down a tube to an eyepiece but instead of using lenses it accomplishes this with mirrors. Unlike a bathroom mirror a telescope mirror has a very precise curve ground into it. When light from an object strikes the curved mirror, it reflects and narrows into a cone toward the open end of the tube. But before it can escape, a much smaller, flat mirror called the secondary intercepts the light and directs the beam through a hole in the side of the tube to the eyepiece.
Two reflections of light — one from the main mirror and the other from the secondary — flip the final image upside down and backwards. While it takes a little getting used to it's not really a big concern because you're looking at celestial objects and not, say, ships at sea.
Schmidt-Cassegrains are also called compound telescopes because they use a combination of mirrors and lenses to fold the light path in half. This makes for a much shorter tube, which means you more more easily transport a large telescope. In a typical SCT light first passes through a glass lens at the tube's front end, strikes the main mirror and then bounces forward to small, curved mirror attached to the inside of the lens. That mirror funnels the light through a hole in the center of the main mirror and into the eyepiece.
Advantages and disadvantages
While advertisements often tout magnification, the most important part of a telescope is the size or aperture of the objective (refractors) or the main mirror (reflectors and SCTs). That's what gathers the light. The more light you can collect, the brighter things look. Outside of the moon, planets and the occasional comet, most objects in the sky — galaxies, nebulas, star clusters — are faint. Plain and simple, a bigger telescope will show them in their true glory compared to a smaller instrument. For planets and the moon, smaller telescopes work well because they're bright and relatively nearby, especially the moon.
As far as magnification, you can buy eyepieces to make any telescope magnify 1000x if you like. But you'd be wasting your money. High power makes objects appear dimmer, and it also magnifies ever-present atmospheric turbulence, resulting in dim, blurry images. Most nights I use magnifications between 50x and 250x with the sweet spot at 75x-150x.
Now that we know a little bit about the different types of telescopes, let's explore the advantages and disadvantages of each.
Refractor — Objective lenses are made of 2-3 individual pieces of high-quality, carefully ground and polished glass called elements. That makes them more expensive to make, the reason even a relatively small aperture refractor costs good money. Cheap ones with plastic lenses or single-element glass lenses should be avoided. These are the kind you'll see in big box stores costing around $30-50. Not only are they poorly made, the scopes are mounted on flimsy tripods that shake at the slightest touch. Since you're magnifying images, any wobble or shake is magnified, too.
These mounts also move in fits and starts. As soon as you center an object of interest, it's next to impossible to follow it across the sky as the Earth rotates. That's right. When you point your telescope at a sky object, our spinning planet hurries it out of the field of view. And the more you magnify, the faster it departs! If the mount controls aren't smooth, it's VERY difficult to re-center or follow a star.
Less expensive refractors also suffer from color-fringing known as chromatic aberration. Remember, the objective lens bends light similar to a prism, and spreads it out into a bit of a rainbow. That creates unwanted pink and green fringes of color around bright objects like the planets and moon. Good refractors have modest fringing, and excellent ones get rid of it completely by using objectives made of three separate glass elements, but you'll pay for the privilege. Inch per inch of aperture, refractors are much more expensive than reflectors.
That said, a small, well-made refractor, despite what it may lack in light-gathering ability, provides a wide field of view and very sharp images. Refractors also have closed tubes and rarely need maintenance and are ideal for astrophotography.
In a reflecting telescope only one surface of the main mirror is ground and polished, making big mirrors much cheaper to make than big refractors. Mirrors are also easier to mount because they sit at the bottom of the tube. Importantly, they don't suffer from color fringes because mirrors reflect light, not refract it.
A 6-inch telescope —one built around a 6-inch mirror — equipped with a basic mount costs about $300 while the same size refractor runs about $900. And that's just for the telescope. Add another $400-500 for a mount. You can buy a reflector on a tripod-style base called an equatorial mount. "Equatorials," as they're called, are designed to make tracking celestial objects easy. Once you align an equatorially-mounted telescope with the North Star, you can compensate for the Earth's rotation with a simple push of a finger. Or you can spend the extra cash and get a clock drive, a motor you attach to the mount to automatically track the object.
Equatorial mounts and motors quickly escalate the price of a reflector or refractor package. There is however a wonderful alternative — the Dobsonian mount (above). Named for its inventor, John Dobson, this simple mount is remarkably inexpensive and stable. No wobbling or vibration here.
To point to a star you just grab the top of the tube and move it up and down in altitude and from side to side in azimuth until it's centered in the field of view. When the star leaves the view it's a simple matter of shifting the tube a smidge up and smidge over to bring it back. Dobsonians set up in seconds. You place the mount on the ground, set the telescope tube in it, and you're good to go.
Reflectors aren't perfect. They're not as suitable for astrophotography as other types, and their open tubes invite dust. You'll also occasionally need to collimate them by using screws on the back of the main mirror to nudge it this way and that so it squares up with the secondary mirror. It may sound tricky, but it's easy in practice.
One of the big advantages of a Schmidt-Cassegrain scope is the short tube. An 8-inch SCT is less than half as long as an 8-inch reflector and packs up in a jiffy. By the way, some reflectors get around this issue by using trusses that you assemble on the spot instead of a tube.
The SCT design is also ideally suited for astrophotography because it has lots of backfocus, the distance between the back of the telescope and the point where light comes to focus. You can stick a camera and other accessories at the back end of an SCT and still get everything in focus. Reflectors generally have limited backfocus, the reason they're not as good as refractors and SCTs for astrophotography.
SCTs have their drawbacks, too. Because so many surfaces have to be ground and polished — front lens, main mirror and secondary — and the fact that most are equatorially-mounted, SCTs are expensive. An 8-inch on a mount will set you back around $1,200, while a Dobsonian reflector the same size costs about $450.
Because of their design, Schmidt-Cassegrains don't have especially large fields of view. In other words you can only see a small slice of sky unless you outfit the scope with high-end eyepieces costing extra $$$.
My goal was to find telescopes that combined quality and affordability for $300 or less. These are free-will choices — I'm not associated with the companies that make or sell these instruments. You'll find full details and photos about each of the telescopes at the links below.
* Orion 4.5-inch Starblast Tabletop reflecting telescope — A small, but high-quality instrument on a portable Dobsonian-style mount that you can set on a picnic bench or sturdy table. The telescope will show thousands of moon craters, the rings of Saturn, Jupiter and its moons, the phases of Venus along with the brighter galaxies, star clusters, nebulae and comets. Includes very basic eyepieces, a laser-dot finder scope to help you point at objects, a moon map and planisphere.
Cost: $219 (shipping included)
* Orion SkyQuest XT6 Classic Dobsonian reflector — Excellent quality, easy to use and lightweight. I bought a 6-inch reflector with paper route money when I was 12 — and still own it. You can go deeper with this telescope than the 4.5-inch and see more detail in Jupiter's cloud belts, up to three rings around Saturn and brighter and richer views of nebulae, galaxies, clusters and comets. The moon will amaze. Includes one decent quality eyepiece, a laser-dot finder scope and astronomy software.
Cost: $300 (shipping included)
* Sky-watcher 6-inch f/8 Traditional Dobsonian reflector — Similar to the above telescope. Excellent quality, easy to use and lightweight. I own the 10-inch version and can vouch for the quality of the optics and ease of use. Includes a traditional finder scope, two basic eyepieces (low and medium power) and a 2-inch focuser you can use to upgrade to 2-inch eyepieces (a larger variety with wider fields of view) in the future.
Cost: $315 (shipping included)
* Orion VersaGo E-series 90mm (3.5-inch) Altazimuth refractor — A smaller aperture, yes, but still a nice first telescope that will give fantastic views of the moon. You'll also be able to see Saturn's rings, Jupiter's four brightest moons (and its most prominent cloud belts) and get decent views of the brightest galaxies, star clusters and nebulae from dark skies. Includes a laser-dot finder scope and two modest eyepieces for low and high power and a moon map.
Cost: $180 (shipping included)
If I were to pick just one of these, I'd go with either of the 6-inch models. Keep in mind you may also want to buy a few accessories down the line such as better quality eyepieces, a star atlas and a red flashlight to preserve your night vision while observing. For an awesome little atlas I highly recommend the Pocket Sky Atlas ($24.95).
I hope you found this brief guide helpful. In a future installment, we'll take a look at binoculars.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.