Magnification is the most misunderstood aspect of telescopes, but not only by beginners. New telescope users often assume that more magnification gives a better view. They quickly learn that this is rarely true, and that on the contrary, lower power almost always yields a better image. Check out the Magnification Calculator to determine the power of any eyepiece/telescope combination.
Why Higher Power Is Not Always Better
There are several reasons why increasing magnification might not be preferable. The usual assumption by new astronomers is that since we are trying to observe objects that are very far away, we want to magnify them quite a bit to bring them in closer. But most objects in the night sky, despite being very far away, appear very large. For example, the Orion Nebula appears more than twice the size of the full moon, and the Andromeda Galaxy appears six times larger. Even though Andromeda is 70 trillion times farther away than the moon, it is also 420 trillion times bigger! A high magnification yields a small field of view, meaning a large object may not fit into the view.
Above: The view on the right is at a higher magnification, but the entire Andromeda galaxy can only be seen in the low-power view on the left
Another reason for keeping the magnification low has to do with image brightness. An unfortunate law of physics dictates that when the magnification is doubled, the image gets four times dimmer. Most celestial objects are very faint, so making them any dimmer than necessary is not recommended. This is why the most important thing with a telescope is the aperture rather than the magnification. Brightness is the key to astronomical observing.
Above: The image of the Orion Nebula on the right is more magnified but also much dimmer
Some objects, however, are small and bright and therefore hold up well to high magnifications. The planets especially fall into this category. Jupiter, despite being the largest planet in our solar system, is far enough away (400 million miles) to appear only 1/36th the size of the full moon, or about the size of a quarter at a distance of 350 feet--pretty small. However, Jupiter is bright, brighter than any of the stars in the sky. So high magnifications work well on Jupiter, Saturn, Mars, and other bright objects like the moon.
How Much is Too Much?
So why not just magnify Jupiter as much as we want? If it looks better at 200x than it does at 50x, shouldn't it look better yet at 600x or 1000x? Not usually, and there are two reasons why. The first has to do with the telescope itself. The brightness of an object is a function of the size of the telescope and the magnification. The more light you have to begin with (the bigger the scope), the more you can magnify before the image becomes too dim. Also, the resolution, or finest detail that can be seen, is a function of the telescope size as well. This means there is a theoretical upper limit to how much a telescope can magnify before the image becomes to faint and too blurry. This is determined by a very simple equation:
For this equation, the aperture is in inches. For example, a 3" telescope has a maximum theoretical magnification of 150x. A 6" telescope can magnify up to 300x, and an 8" telescope up to 400x. However, this is strictly a theoretical maximum, because the primary limiting factor is not the telescope itself.
The usual limiting factor in maximum magnification is Earth's atmosphere. Since we have to look through the atmosphere to see anything in space, the more we magnify the celestial objects we're looking at, the more we magnify the atmosphere. And if the atmosphere is turbulent, that turbulence will tend to blur the image. The steadiness of the atmosphere is called the seeing conditions. When the seeing is good, the atmosphere is steady and the image looks very sharp. When the seeing is poor, the atmosphere is very turbulent and the image appears blurry. On nights of poor seeing, even a good telescope cannot give a detailed view.
Above: On the left, Jupiter in excellent seeing conditions; on the right, Jupiter in poor seeing
A realistic upper limit to magnification, no matter how large the telescope, on an average night would be about 250x. On a bad night, you might not be able to exceed 100-150x. Note that seeing conditions and transparency (the clarity of the atmosphere) are not the same. Often very dark, clear nights will have poor seeing conditions, while hazy nights of low transparency often produce great seeing.
Okay, If Too Much is Bad, What About Not Enough?
The corollary to the misconception that more magnification is better is that, if that's not the case, then less magnification must be better. Less magnification gives a wider field of view and a brighter image. However, just as there is such a thing as too much magnification, there is such a thing as a minimum magnification as well. The minimum magnification is determined by the exit pupil of the telescope system. Exit pupil is the diameter of the beam of light coming out of the eyepiece. The larger this beam is, the brighter the image will appear. At least up until the point where the exit pupil of the telescope is larger than the pupil of the observer's eye.
Above: Different size exit pupils. The large exit pupil on the right is wider than the pupil of the observer's eye.
If the exit pupil is wider than the pupil of the observer's eye, there is wasted light. The effect is exactly the same as restricting the telescope's aperture. The size of an observer's pupil depends on whether the observer is dark adapted and how old the observer is (maximum pupil size decreases with age). A typical dark-adapted pupil will be 7mm in diameter. Older observers' eyes may only open to 5mm or 6mm. Assuming the standard 7mm size, there is a simple equation for minimum magnification:
Again, this is for aperture in inches. For aperture in millimeters, simply divide the aperture by 7. For example, a 4" telescope has a minimum magnification of 14x. An 8" telescope has a minimum of 29x, and a 14" telescope has a minimum of 50x. Below the minimum magnification, the effect is to stop down the aperture. So, a 14" telescope used at 40x gives an exit pupil of 9mm. This is 2mm larger than the typical dark-adapted human eye. The effect is the same as stopping down the telescope's aperture to 11". While the field of view will get wider at lower powers, the brightness will not increase beyond a 7mm exit pupil (unless you are blessed with unusually big eyeballs).
A related problem is that below a certain minimum magnification, the central obstruction (due to the secondary mirror) in a Newtonian or Schmidt-Cassegrain may become visible as a dark spot in the center of the field. This limits the minimum magnification on these scopes to a higher value than the equation above would suggest. Refractors, not having any central obstruction, do not suffer from this problem and thus make ideal wide-field instruments for sweeping along through the star clouds of the Milky Way or for viewing large clusters like the Pleiades.
A second concern is that decreasing magnification reduces image scale and detail. The human eye's best resolution comes when less than the full pupil diameter is used. Observational experiments usually find that, for deep-sky observing, the best detail can be seen with an exit pupil of between 2mm and 3mm. This would be a magnification of around 35-50x on a 4" scope, 70-100x on an 8" scope, and 120-175x on a 14" scope. A lower magnification may be necessary to encompass an entire large object, but when trying to observe fine details in a smaller galaxy or nebulaor globular star cluster, these medium magnifications may prove ideal.
For planetary viewing, higher powers may be used. Of course, each object, telescope, and observer are unique, so certain magnifications may be better for certain combinations. Most astronomers own three eyepieces--one high power, one medium, and one low--to cover various observing conditions. Usually these are in the range of 50x to 250x, since this covers everything from wide field to high power. A higher power may be useful for excellent nights, but will likely be an eyepiece that rarely gets used. A lower power might be good for wider fields of view, but only if the telescope can accept such a low magnification.
For a more thorough discussion of ideal magnification, see the Observing Theory page.