Using High Powers

Conventional wisdom holds that low magnification (low power) works best for deep-sky viewing. After all, low power concentrates an extended object's light into a small area, increasing its apparent surface brightness (the amount of light hitting a given square millimeter of your retina). But, as Roger Clark has documented, this assumption is usually false. High powers should do better on many faint deep-sky objects. The reason is subtle but important, so we'll go into some detail.

Unlike a camera or other purely mechanical lens system, the eye loses resolution in dim light. This is why you can't read a newspaper at night, even through you can see the newspaper, and even though your large nighttime pupil should theoretically resolve the letters even more sharply than in daylight. Studies show that the eye can resolve detail almost as fine as 1 arcminute (1/60 of a degree) in bright light but can't make out features smaller than about 20 or 30 arcminutes (1/3° to 1/2°) wide when the illumination is about as dim as the unpolluted night sky. This is almost the size of the Moon as seen with the naked eye. So, details in a very faint object can be resolved only if they are magnified until they appear tens of arcminutes across. In many cases, this can require using extremely high power!

Why does the eye work this way? The explanation lies in how the visual system has adapted to cope with night. Photographic film records light passively, but the retinal nerve system contains active computing power. In dim light, the retina compares signals from adjacent areas. A faint source covering only a small area — such as a small galaxy in the eyepiece — may be completely invisible at the conscious level. But it is being recorded in the retina, as evidenced by the fact that a larger galaxy with the same low surface brightness is visible easily. In effect, when rod cells see a doubtful trace of light they ask other rods nearby if they're seeing it too. If the answer is yes, the signal is passed on up the optic nerve to the brain. If it's no, the signal is disregarded.

When an image is magnified by high power, its surface brightness does indeed grow weaker. But the total number of photons of light entering the eye remains the same. (A photon is the fundamental particle of light. Experiments show that most people can detect as few as 50 to 150 photons per second.) It doesn't really matter that these photons are spread over a wider area; the retinal image-processing system will cope with them. At least within certain limits. A trade-off is needed to reach the optimum power for low-light perception: enough angular size but not too drastic a reduction in surface brightness.

What does all this mean for deep-sky observers? Simply that it's wise to try a wide range of powers on any object. (A judiciously chosen, high-quality zoom eyepiece makes this a breeze.) You may be surprised by how much more you'll see at one power than another.

One more point: There is a folk belief among observers that a telescope of long focal length (high f/ratio) gives a cleaner, higher-contrast view of dim objects than a short focal-length scope. But f/ratio is not the issue. A long-focus telescope is simply more likely to be used at high power! (It's also more likely to have high-quality optics, because "slow" mirrors and lenses are easier to manufacture well than "fast" ones.)