If you’re interested in buying an astronomical telescope, either for yourself, or as a gift for someone else, there are a few things you need to know. You really need to talk to someone familiar with astronomy and optics to avoid making a costly mistake. Really good scopes are available at very reasonable (in my opinion) prices today, I encourage you to get into the hobby. But there are a lot of places where you can go wrong. I will talk about just one of them: power.

Power, or magnification, is the amount of times an object will appear larger, or closer, through the scope compared to viewing it with the naked eye. So for example, if you have a 10 power scope
(abbreviated 10x) it will make an object 1000 yards away appear as if it is only 100 yards away. Or to put it another way, it will look 10 times bigger (the apparent size of an object is an inverse linear function of its distance from you.

Power is important, after all, we expect distant objects to appear bigger and closer through a scope than to the naked eye. But magnification is not the most important feature of a telescope. The primary purpose of an astronomical instrument is not to make small things look bigger, but to make faint things look brighter. And sometimes magnification and visibility work against one another.
Furthermore, there are limitations on the magnification of any telescope, even one of excellent quality. These limitations depend on the laws of optics, the properties of light, the clarity and turbulence of the atmosphere and even the anatomy of the human eye. So when you see the garish advertising on the box of a telescope tell you how powerful it is (usually next to a gorgeous Hubble photograph of some distant galaxy) don’t believe a word of it.

The single most important specification of a telescope is its aperture, or the diameter of its objective lens (in a refractor) or mirror (in a reflector). Regardless of the engineering, the physics works the same for both. The larger the aperture, the more light the scope can collect, and the fainter an object it can detect. My telescope, for example, has a 4″ (100mm) lens. Light gathering power goes up as the square of the objective diameter. My old telescope used to be an 8″ reflector, so with double the aperture, it could scoop up four times as much light as my refractor does. Remember, light gathering power goes up as the square of the aperture; while a good rule of thumb for price is that cost goes up as the cube of the aperture, all else being equal.

So what is the power of a telescope of a given aperture? The answer is that it can be any power! You can change the magnification of the optical system by using different eyepieces, (the little lenses that go at the end of the scope), what you actually look into. Eyepieces are rated by their focal length, and the magnification of a given eyepiece is determined by simply dividing the focal length of the eyepiece into the focal length of the objective. So for example, my 4″ refractor has a focal length of 600mm, so my lowest power eyepiece (32mm f.l.) yields a magnification of 600/32 = 18.75x. Lets just say 19x, to keep it simple. My highest power eyepiece is a 6mm, which gives a magnification of 100x. You will note that only the focal lengths are needed for the calculation of power. Aperture is irrelevant.

Both my refractor and reflector were “fast” (to borrow photographer’s lingo) objectives. That is, their focal ratios, or “speeds” (focal length/aperture) were the same, or “f/6″. The reflector had a focal length of 1200mm or exactly twice as much. So in my reflector, an object would appear twice as big and four times as bright as in my refractor, using the same eyepiece.

Actually, the previous paragraph is a bit of an oversimplification. If we are looking at a point source of light, say a distant star, it will not look any bigger regardless of how big a telescope or eyepiece you use, because stars are so far away they always look like points, in any telescope at any power. But for an extended source, an object with a size and shape like a nebula or a planet, It will look twice as big and four times brighter in my reflector than in my refractor. But, and here’s the rub, the object may be four times brighter in the reflector, but since its twice as big, its light is spread out over four times the area. So the average surface brightness of the object is the same in both scopes! If you should use an eypiece in the reflector with twice the focal length as the one in the refractor, the objects would appear the same size (equal magnification), but they would be brighter in the reflector.

I prefer “fast” optics in my scopes because I like to look at faint extended objects like nebulae, clusters and galaxies, which require lower powers. Astronomers specializing in planetary studies or double star work go with slower optical systems, that is, longer focal lengths for a given aperture.
I work at lower powers, they use higher ones, so we each buy objectives optimized for the application. Another consideration is cost. All else being equal, a fast objective is more expensive to make than a slow one, so for a given cost and aperture, a slow objective gives a sharper more detailed image. I look at faint fuzzies, they look at distant planetary surfaces or close binaries, So given my budget and requirements, I go with fast scopes. Another consequence of the “speed” of an objective is that you need shorter focal length eyepieces for a given power in a fast scope, and short focal length eyepieces are more expensive, are harder to use and tolerances are more critical than long fl ones. So in general, fast scopes are optimized for low power, low detail, wide field work.

And of course, in any telescope, you spend much more time observing at lower powers than at higher magnifications. Short focal length eyepieces are less clear and have fuzzier images, you have to move your eyeball too close to the lens (leaving a hard to clean greasy spot), they have a tiny field of view which makes it harder to locate objects and they cost more. They are also harder to make and more expensive. Unless you’re trying to see detail on planetary surfaces, or split a close double, you go with a higher fl if you can. In general, when observing, use the lowest power you need to see the object, in context with its surroundings. It will give you a brighter, clearer and more beautiful image.

There is one exception. The human eye tends to pick up faint objects much easier if they are against a dark background. This is why astronomers prefer dark skies to light polluted skies–all else being equal you can see more detail in faint nebulae and galaxies if the sky background is black. Unfortunately, the sky background is never totally black, no matter how remote your observing site or how clear the atmosphere. The upper atmosphere glows faintly, and this skyglow can hide faint objects because it decreases the contrast. Every tiny patch of sky gives off a little bit of light, and if you boost the magnification enough, you can spread this light over a larger area and make the sky appear darker, bringing out those really faint features that are right at the limit of your scope and your vision.

So other than cost, and light collecting power, what is the relative advantage of larger aperture over smaller? Not that much, actually. Although a larger objective is physically capable of a higher resolution image, (image resolution goes up with aperture linearly) the clarity of the atmosphere makes it difficult to push the scope to its theoretical full potential magnification. Regardless of how big your objective lens or mirror, you will rarely be able to go above 300x, even on the clearest sharpest skies. So if magnification alone were an issue, there really wouldn’t be much point in buying a scope of more than 6″ (150mm) aperture, whose theoretical maximum power is 300x. Of course, we are looking for faint, not tiny, and there the bigger a light bucket you can afford the better.

So what are the physical limits to magnification? What are the powers the optics of your eye and telescope can handle? Some amateurs say 3.5X per inch of aperture at the low end. and 60x per inch at the high end. Other authorities use 0.2 to 2.0 multiplied by the aperture in millimeters to define the useful range–a span of about an order of magnitude. Too low a power and the light cone formed by they eyepiece will not fit in your pupil. Too high and diffraction and interference effects will deteriorate the image to the point where further information cannot be extracted from it. It will just be too blurry. In my refractor, this means about 20x to 200x. This means a high power eyepiece of 3mm effective focal length (actually, a 6mm eyepiece with a 2x Barlow lens, nobody makes a 3mm) and a low power wide field 32mm eyepiece with an effective power of about 19x.
And I rarely push my rig to over 100x. Image quality deterorates rapidly past that point The only time I go to the max is to pick out faint detail on planets or the moon, they are bright objects and can stand considerable magnification without appearing too dim. I have seen the Martian polar caps, .the bands on Jupiter and two of Saturn’s moons.

My eyepiece collection: 32mm, 25mm (the one I use most often), 18mm (my favorite, sharpest, clearest performer), 12.5mm, 9mm and 6mm, plus a 2x Barlow lens which I can use to double the magnification of any of them. Over the years I’ve accumulated several more, but those are my best and most often used. You will note there is some duplication, the 25mm (24x) used with the Barlow is equivalent to the 12.5mm. But I like to experiment. In general, the longest fl eyepiece, and the less glass between you and the universe, the better. Different objects and conditions respond differently to different combinations.