Low-Power Lessons

Why would you want to employ your maximum pupil size, anyway?

By doing so you get the lowest power (minimizing the problems of an unsteady mount or lack of a clock drive), the widest possible field of view with a given eyepiece design (making objects easiest to find), and also what's called the "richest field." This means the most stars are packed into the view. A richest-field view has the maximum surface brightness — the density of light for each square degree of your field of view — that your eye can possibly receive when looking at a given scene.

This does not mean the stars themselves get brighter, contrary to misconceptions that have been spread by sloppy wording in books and tricky wording in ads. The amount of light you get from a star or any other object is governed by the instrument's effective aperture — not its f/ratio or exit-pupil size. Low power merely squeezes the same light into a smaller area.

For anyone who still isn't convinced of this (so entrenched are the misconceptions), consider that you use the maximum possible amount of your eye's pupil — all of it! — when you observe with the naked eye. Naked-eye viewing is thus the richest rich-field viewing possible. You see the greatest surface brightness on objects that you ever possibly can. No telescope of any size, power, or design can ever beat the naked eye in this regard — which puts the surface-brightness issue into its proper perspective.

If that's all you want, why bother with a telescope? In fact there are good reasons not to make much use of your telescope's lowest allowable power. For one thing, if the telescope's exit pupil is exactly the size of your eye's pupil, you must hold your eye rock-steady in exactly the right place or you'll cut off light. This may be practical if you arrange to clamp your head in a vise. Otherwise, a margin of a millimeter or so gives comfortable room for slight natural movements.

Another reason is that the optical quality of your eye is worst around the edges. This is why you'll discover that no eyepiece, no matter how perfectly designed, shows truly "pinpoint" stars at very low power. This is also why bright stars viewed with the naked eye have little points and flares on them. Whoever popularized the five-pointed "star" shape simply immortalized his or her particular eye aberrations.

In fact, the main reason our pupils open and close may not be to regulate light so much as to reduce aberrations by stopping down the eye's aperture whenever there's enough light to allow this. Alas, our eyes are so imperfect that nature has been reduced to a shoddy trick used by makers of the worst department-store telescopes — stop down the aperture to hide the aberrations.

There is no cure for this short of taking out your eyeballs and grinding and polishing a better optical figure on them, something we do not recommend. (However, vision can be corrected to a degree by laser "machining" of the cornea, and hard contact lenses are said to mold an irregular cornea to some degree.) At any rate, when you use a telescope or binoculars, a less-than-maximum exit pupil keeps light from passing through the eye's bad outermost zone.

This zonal problem may also help explain the so-called "scotopic Stiles-Crawford effect," whereby very dim light entering the edge of your pupil is not perceived as readily as the same amount of light entering near the center. Here is another reason to be conservative about using your lowest power, even when objects are too dim for your lens aberrations to show.

All this adds up to bad news for the 7x50 binocular as the traditional skywatching standard. The tradition should be rewritten more toward 8x50s or 10x50s, especially for folks who are no longer young.

With reflectors and Schmidt-Cassegrain telescopes, there's yet another reason to avoid the lowest possible power. These telescopes have a central obstruction — the secondary mirror — blocking their entrance pupils. The larger the scope's exit pupil, the larger is this black spot in the middle of it, and the more it blacks out the optically best zone in the center of your eye.

Lastly, of course, there's light pollution. When you increase the surface brightness to get a richest-field view, you increase the richness of the skyglow by an equal amount.

Some people love rich-field viewing despite all this. As for myself, the lowest power I use on my 12½-inch reflector is 60x, which gives an exit pupil of 5.3 mm. Even then stars show points and flares. (I know they're not the eyepiece's fault because they move when I rotate my head, and not when I rotate the eyepiece.) But since when is life perfect? In fact I usually find myself using 75x (a 4.2-mm pupil) as my basic "low" power.