Every telescope, whether refractor, reflector or the hybrid Schmidt-Cassegrain, has two fundamental specifications which determine its optical charateristics and its subsequent performance. These properties are not related to the quality of manufacture. They are the results of the wave nature of light. A high quality optical system will perform better than one of low quality–but only up to a point. Physics sets us natural limits that cannot be exceeded no matter how much we spend.

The first is aperture, or the diameter of the objective (main) lens or mirror. Size matters, the bigger the aperture, the more light the telescope can collect, and hence, the fainter objects it can see. Light collecting ability rises as the square of the aperture, hence, an 8″ (200mm) objective can collect four times as much light as a 4″ (100mm), so the human eye can see objects that are four times fainter. Aperture also determines the resolving power of the telescope, that is, how much detail can be seen in the image. For example, a double star which is easily “split” in an 8″ scope may appear as a single point of light in the 4″. The bigger the aperture, the better the resolution, but the relationship is linear. If a telescope can just barely resolve a double star of a given separation, an objective of twice that aperture will just barely be able to split a double of half that separation. In general, and all else being equal, a larger aperture will be able to detect fainter objects and it will show more minute detail in those objects. Incidentally, cost of an objective goes up as the cube of the aperture, so even though an astronomer will try and get as big an objective as he can afford, economics quickly forces him to limit his expectations.

The second fundamental characteristic of an objective is its focal length, or the distance at which that objective forms a focussed image. This is where the eyepiece (a compact collection of lenses designed to magnify the focussed image), or a photographic film or detector, is placed to capture the image. Focal length is expressed in the same units as the aperture, so for example, my 4″ telescope has a focal length of 24″. It is convenient for focal lengths to be expressed as the ratio of focal length to aperture, the way photographers talk about their lenses. So my scope is an f/6 (pronounced “eff six”). Again, using photographic nomenclature, this is what they would call a “fast’ lens, since it would form a latent image on film quickly. If my scope had the same aperture but a focal length of 48″, it would be an f/12, a “slower” lens. A fast lens forms a smaller image than a slow one, so the light collects on the detector quickly. A slow lens makes a bigger image, so the light is more spread out and film takes longer to expose. This makes no difference when viewing stars, because they are point sources of light, they don’t spread out. But it makes all the difference in the world when looking at extended sources, such as faint nebulae or galaxies. The fast lens has a brighter, but smaller, image. For a given aperture, a fast lens is better for faint, extended deep sky objects and a slow one for bright, borderline detail planetary images and double star work.

Incidentally, by “size” of the image I mean its absolute field of view, or how much sky is displayed. The apparent field of view is the size that “hole” in the eyepiece appears to your eye, and is solely a result of eyepiece design. So for example, if I have an eyepiece with an apparent field of 50 degrees, and I am looking at two degrees true field in the sky, I have a 25X eyepiece, because it blows up 2 degrees of sky (about 4 full moons) to 50 degrees, about 1/7 of a full circle.

Image size is important when using the scope as a camera. But amateurs are interested in actually looking at things with their own eyes, and magnification is really not the key performance parameter in a telescope. Remember, the primary purpose of an astronomical telescope is not to make small objects look bigger, but to make faint objects look brighter. For the faintest nebulae and galaxies you want low magnification (power), in fact, you may want as low a power as your optical system can deliver. Every celestial object will have some optimum magnification at which it is best observed, depending on its brightness, its size, the amount of detail you would like to see, the resolution you need, atmospheric conditions, and what your objective can provide. So it is important to be able to vary the magnification of your optical system. Tracing the subtle dust lanes in the outer reaches of the Andromeda galaxy requires different magnification than resolving minute details on bright planetary surfaces. It is important to make your telescope as flexible as possible, given its intended purpose and its optical design.

Magnification is determined by the eyepiece you select. High power eypieces bring out the fine detail, but give fainter images and a narrow field of view. Low power works best for faint, extended objects that require a wide field. In general, astronomers use the low powers much more than the high, In fact, extreme high power is of limited utility, and usually reserved for a very few objects, high quality optics (magnification also exaggerates flaws in any optical system) and very narrow fields of view.

Eyepieces are little metal barrels crammed with lenses which are fitted at the eye end of a telescope. They are rated by their focal length, with the shorter focal lenghts yielding higher powers, and (of course) higher cost. A typical amateur has eyepieces for low, medium and high power. Determining the power is done by dividing the focal length of the eyepiece into that of the telescope (aperture is irrelevant). For example, my lowest power eyepiece is 32 mm, which in my 600mm focal length refractor yields just short of 19X. My highest power eyepiece is a 6mm, which I can double to 3mm with an accessory Barlow lens. This gives me a high magnification of 200X. This roughly covers the entire magnification range my objective can deliver–any higher or lower power is not allowed by the optics and the physics of light. Incidentally, any optical system can provide magnifications that differ by a factor of ten–from 0.2 to 2 times the aperture in millimeters, or put another way, from 5x to 50x per inch of aperture. In my scope, this translates to about 20 to 200 magnification. Much lower than 20x and the light cone will not fully fit in the dark-adapted pupil of the eye, and some light is wasted. Much higher, and the image deteriorates due to the wave nature of light. If you want higher (or lower) power at a given focal ratio, you need a different objective. No way around it.

So if you have a 6″ telescope, your allowable power range is 30x to 300x. The actual eyepiece focal lengths you’ll need to reach these magnifications will depend on the focal length of your objective. There are circumstances when you may want to go a little higher or lower, but you will pay a price in performance, one way or the other. Another factor is the atmosphere. No matter how big or high quality your telescope is, turbulence in the atmosphere will rarely let you go much beyond 300x.

My scope is a fast refractor, designed for deep sky observing (large, faint nebulae). Its optical system is designed to favor low magnifications, and even though I can push it to about 200X, I rarely go above half that, the image quality quickly deteriorates. On the other hand, that 19x eyepiece gives breathtaking views of the Milky Way and bright nebulae and galaxies. It’s like looking through a porthole on a spaceship.