Aperture: A Telescope's Heart

All astronomical telescopes, large or small, are designed to do two things: to brighten and magnify your views of celestial objects. Refractors, reflectors, and compound (catadioptric) telescopes do this in different ways, each with its benefits and drawbacks.

Whatever the telescope, its most important spec is its aperture: the diameter of its main, light-gathering lens or mirror. (This lens or mirror is called the telescope's objective.) The bigger the aperture, the sharper and brighter the view will be.

Therefore, a bigger aperture allows you to use more magnification. You can actually make any telescope provide any magnification at all (just by changing eyepieces), but without large aperture, high magnification is worthless — it just shows a blurry, dim mess. A telescope that can only be pushed to 50x (50 times magnification) before the view goes blurry will reveal Jupiter's moons, Saturn's rings, and some detail in the brightest star clusters, nebulae, and galaxies. But to discern Martian surface features or to see both members of a tight double star, you really would like to have sharp views at 150x or more.

Depending on optical quality and observing conditions, you can expect to get anywhere from 20x to 50x of useful magnification per inch of aperture. In other words, a 4-inch scope tops out at 200x under ideal conditions, but a 6-inch scope can work well as high as 300x under ideal conditions.

But that's the maximum; most of the time, you'll find that the best views are actually had at the telescope's lowest power. If the advertising on the box hypes super-high power, the manufacturer is trying to fool you.

Perhaps more importantly, big aperture also lets you see fainter objects. This is different from providing magnification. In fact, the problem with most astronomical objects is not that they're too small and need more magnifying, it's that that they're too faint and need more light — in other words, more aperture.

For example, several dozen galaxies beyond our Milky Way can be discerned through my 4½-inch (105-mm) reflector. Some are more than 50 million light-years away. Not bad for a telescope I can tuck under my arm and carry on a plane! But with my 12½-inch Dobsonian, hundreds of galaxies are within reach. Even if I use the same magnification on both scopes!

Focal Length

If a telescope's aperture is its most important spec, its focal length comes next. Say you have two telescopes with the same aperture but different focal lengths. The one with the longer focus (that is, a higher-numbered f/ratio) will generally lend itself better to high-magnification viewing. (The f/ratio is just the focal length divided by the aperture.) One reason: you can stick with longer-focus eyepieces, which are easier to use, especially for eyeglass wearers. Another reason: "fast" objectives, those with low f/ratios, are harder to manufacture well, and thus they tend to make fuzzier images unless you've paid a premium for top-quality optics.

"So it's simple: I should go for the largest, longest telescope I can afford." Maybe; maybe not! A long focal length is preferable if your primary targets are high-power objects like the Moon, planets, or double stars. And a large objective is a necessity if you dream of viewing numerous galaxies. But if you want to take in large swaths of the Milky Way or sparkling showpieces like the Pleiades in a wide view, then a short, small, scope is called for — one that works nicely at low power.

"Why's that?" Because high power only let you see a small patch of sky at once. With standard eyepieces (those with 1¼-inch-wide barrels), a focal length of 20 inches (500 mm) can provide a 3° field of view — enough to take in all of Orion's Sword. A scope with a focal length of 80 inches (2000 mm), by contrast, barely lets you encompass M42, the Orion Nebula in the Sword's center.

"What if I want to do a bit of everything?" Don't worry, there are plenty of midway compromises. Many astronomers think of the 6-inch reflector as an ideal "do-it-all" instrument. But even with that aperture, you still face a tradeoff between a wide-field performance (f/5 or thereabouts) and high-power performance (optimal at f/8 and up). And remember that the long-focus unit will be bigger and heavier and so will require a beefier mount — making it harder to carry, set up, and store. Everything's a tradeoff.