I'm LinkedIn and Google-Plussed.

Mail and packages, use maildrop:
Norman Sperling
2625 Alcatraz Avenue #235
Berkeley, CA 94705-2702

cellphone 650 - 200 - 9211
eMail normsperling [at] gmail.com

Norman Sperling's blog

The Disappearance of Darkness

Norman Sperling
Adapted from an invited paper presented at International Astronomical Union Colloquium 112, Washington, DC, 1988. Originally published in: David L. Crawford, ed, Light Pollution, Radio Interference, and Space Debris. Astronomical Society of the Pacific Conference Series, Vol. 17.


Until the 1900s, virtually all humans knew the appearance of the dark night sky. Even unschooled urbanites knew some constellations and planets. By 1909, light pollution made authors admonish readers to do their skywatching from the countryside rather than the city. The warnings have escalated along with the light pollution. Light pollution's effect on professional and volunteer observational astronomy, along with telescopes' changing focal ratios, largely determine which kinds of astronomy are done in which institutions. In times and places where individuals perceive little possibility to change their culture, astronomers cope as best they can. When activism earns results in other cultural matters, astronomers sometimes become activists to fight light pollution. Despite winning some battles, the war against light pollution is still being lost, so a different approach is suggested.

First-World Light Pollution

Attention to light pollution depended, and still depends, upon local and cultural conditions. Geography, meteorology, energy consumption methods, economics, technology, politics, and demography all mold local circumstances, and generate objectionable levels of light pollution at different times in different places. Light pollution's interrelationships with popular astronomy, professional and amateur research, instrumentation, and observing sites demonstrate its strong influence.

Light pollution is high in the consciousness of those who suffer from it nightly, but writers used to dark skies rarely mention it. Writers in smoky cities bemoan smoke, and writers in electrically-lit cities bemoan electric lights – though writers in cloudy climates bemoan the clouds at least as loudly. So the writers talk about whatever interferes with their skywatching. And none ever hints that anything can be done to avoid it, except travel.

By 1866, and perhaps earlier, the first caveats about light pollution crept into the popular astronomical literature. Sir John Herschel (1792-1871) noted the problem (Crawford, personal communication). Amedee Guillemin (1866) wrote that the dimmest stars are effaced altogether "in the great centers of population, by the illumination of the houses and streets." The haze enveloping Paris and London, and the smoke filling the skies of many cities, was the primary obstruction, however. It was largely wood and coal smoke plus street dust; we would call it air pollution. It still wasn't too bad, because in 1869, Edwin Dunkin was still able to advocate urban skywatching: "It is of no consequence, therefore, in what part of London, or its neighborhood, the observer is located. It may be in the heart of the city …" But another Londoner, John A. W. Oliver, wrote of the Zodiacal Light in 1888, that "the less luminous portions cannot be well seen in a town where there is smoke illuminated by gaslight, or where the electric light is in use, as in the city of Boston, where Searle finds it no longer possible to observe the Zodiacal Light satisfactorily." Therefore, light pollution has definitely interfered with astronomical observing since the late 1800s.

From that time on, professional astronomers have almost always considered seeing conditions when locating new observatories. In addition to climate and altitude, they include light pollution as a prime consideration. While San Jose, Flagstaff, and Pasadena were still small, dim towns, Lick, Lowell, and Mount Wilson Observatories grew on nearby peaks – only to suffer terribly in the light of recent developments.

Light pollution became a pressing topic in British and American – and even Austrian – popular astronomy books and amateurs' observing manuals from 1909 on. The problem escalated, both in the skies and in print, as the 1920s yielded to the 1930s, with authors preferring more and stronger warnings.

Then World War II blacked out major cities. All of a sudden, generations of urbanites who had never seen the starfilled sky clamored for books about this splendid vision, and despite wartime paper rationing, England (among other places) generated volumes to explain the sky. These old-fashioned star-watching manuals addressed readers who were seeing the dark sky as a novel phenomenon.

After the War, "the lights came on all over the world." The skies lit up again, urbanites lost touch with the stars, and books returned to their warnings to seek dark – typically rural – skies. Now, great numbers of city children attending planetarium shows cannot relate to the dark sky shown because they have never experienced such a sight in nature. Decisions made by generations unfamiliar with nature often seem to ignore it, with results ranging from regrettable to catastrophic.

And, now that most towns are light-polluters, observatories have been pushed farther away – Fort Davis, Kitt Peak, Mauna Kea, Tenerife, Las Campanas. A peak's isolation is now one of its prime astronomical assets, almost regardless of the difficulties imposed on construction and operation. Astronomers working on low-surface-brightness problems depend on such facilities. Observational astronomers working in light-polluted areas are restricted to high-surface-brightness targets, most of which are far from the forefront.

Fast Optics Accent Light Pollution

An often-overlooked element in the rising clamor has been the change in focal ratio of both professional and amateur telescopes. Most 19th Century telescopes were f/15 to f/20 refractors. Such instruments excel for positional astronomy, as well as with high-surface-brightness objects like planets and double stars ... and are relatively unbothered by diffuse skyglow because they operate at high magnification with small fields of view. That is why most remain in the cities and campuses where they were first set up, and why so few have been moved to remote mountaintops.

Since World War II, however, the overwhelming majority of professional and amateur observing has been accomplished with reflectors. It is relatively easy to make reflectors optically fast (a difficult problem for refractors), and preferable for cutting exposure time, and lowering the cost of mounting and housing. Fast optics concentrate diffuse light, so they excel for deep-sky observing of low-surface-brightness nebulae, clusters, and galaxies ("Of Pupils and Brightness", Sperling 1985).

With their large, fast reflectors, amateurs have waxed enthusiastic for deep-sky observing. Several people have called this a result of the aperture explosion, which is perhaps spurred by economics as well as technology. But the faster focal ratios have been at least as much a factor as the wider apertures. And it is just those fast focal ratios that yield the wide fields of view and low magnifications that accent light pollution. Thus, when amateurs poured huge sums into huge telescopes to see huge distances, they found light pollution glaring back at them, preventing them from enjoying the view they had invested so much to see.

Political Activism

In the United States in the 1960s and '70s, light pollution increased precipitously with population, more brightly-lit cities, and suburban sprawl; amateur focal ratios sped up greatly, amateur apertures exploded enormously – and political activism spread from the Civil Rights movement to opposing the Viet Nam war, and to popular causes in general. This national mood gave American professional and amateur astronomers the idea to become activist – actually fighting light pollution, instead of merely running away from it.

The struggle in the United States, Britain, and Canada is largely political. Mostly through amateur astronomy clubs, American and British hobbyists have repeatedly challenged offensive lighting, and have won several notable battles (itemized in Sperling 1978, 1980, and 1986).

The first major salvo came from Tucson-area professional observatories in December 1971. Subtitled A Guide for Businessmen and the General Public, it described the problem and the ordinance they proposed to cope with it (Steward 1971). The ordinance passed, and Southern Arizona astronomers have been leaders in the struggle ever since. They continue issuing assorted publications (such as Crawford 1985), monitor Arizona's slowed-but-still-growing light pollution, and undertake such other strategies as organizing IAU Colloquium 112 and the International Dark-Sky Association.

In 1973 and 1974, the United States endured a painful fuel shortage of political origin. Professional and amateur astronomers seized upon an anti-waste strategy for fighting light pollution, and the struggle took on its current aspect. Kurt Riegel published his survey paper in Science (Riegel 1973), and Kitt Peak National Observatory issued another, more comprehensive book explaining the problem and recently-passed laws restricting the growth of outdoor lighting (Hoag and Peterson 1974).

In Manassas, Virginia, at the 18 May 1974 Middle East Regional convention of the Astronomical League, an unprecedented array of leading amateur astronomers passed and published a battle plan. It was authored by Jack Betz of Harrisburg, Pa., and championed by the usually-reactionary Bob Wright, as well as several much more activist leaders, including myself. Betz advocated a succession of measures, from shielding the observing area, through contacting neighbors and utilities, to seeking governmental action (Betz 1974). Several regions of the Astronomical League (the American federation of astronomy clubs) have sponsored activist projects fighting light pollution since then, most notably the South East and Great Lakes regions. These efforts typically collect and distribute anti-light-pollution campaign literature, for use by local astronomy clubs. Of course, every amateur works on his own time with his own resources, so these efforts flare up and subside sporadically.

On 1 November 1974, the Toronto Centre of the Royal Astronomical Society of Canada's "Sky Brightness Programme" issued a manual telling how to measure light pollution photometrically. This was another salvo in amateur activism, taking a scientific-measurement stance (Berry 1974). It has been pursued professionally by Arthur Upgren and others.

One of my articles (Sperling 1980) pointed out that astronomers most often succeed when they exercise personal connections with government officials – a very depressing conclusion for societies so proud of their democracy. This may result from a cultural climate in which the kinds of people who become public officials rarely know much science, and astronomy is so far from their awareness that they don't sufficiently understand an astronomer with an odd claim. They listen much more closely to individuals whose personal credibility they already know. Whatever the reason for this effect, astronomers would be wise to take advantage of personal acquaintances among government officials to fight light pollution.

Some local governments force dark-sky advocates to radical positions, as has happened in defense of Palomar Mountain Observatory near San Diego. There, John and Stephanie Mood write and speak from a radicalized stance, having learned that their local politicians respond to nothing less (Mood and Mood 1985).

Politics also affects other aspects of the struggle. Sky & Telescope magazine frequently plants ideas with its readers. A major article fighting light pollution was delayed more than a year because of political considerations: an amateur astronomer who wrote it joined S&T's rival, Astronomy magazine, and certain S&T editors were exceedingly reluctant to publish his by-line (Pike and Berry 1978). This phobia delayed the American fight against light pollution.

Major differences remain between the professional and amateur astronomers' advocacies. Professional astronomers, who do lots of spectroscopy, emphasize narrow-spectrum lighting and early-morning turnoffs. Amateurs, whose work is usually broad-band in the evening, campaign for hooding lights.

The Developing World

Much of the second and third worlds have yet to follow the path to development and pollution. Even now there are many places where light pollution is still minimal – amateur observers I have visited in Arusha, Tanzania, and Faaa, Tahiti, are blissfully unaffected. Their towns are small and concentrated, and use relatively low technology. They have no history of light pollution.

Elsewhere, different political systems impose different attitudes. I visited a very nice amateur club observatory in the capital city of a military dictatorship. Street lights glare almost all the way up to their doorstep. But they adamantly refuse to approach officials because "it is best if the government doesn't notice you at all." There, too, the solution is political rather than scientific, but necessitates terminating the dictatorship – obviously a bigger problem than light pollution. I am greatly distressed to notice that many countries, in their rush to build American-style industry and infrastructure, seem determined to repeat, as well, every mistake we have made in our own development. Those countries should notice the fabulous price that America now pays to restore its environment, mostly to undo our previous mistakes. Among these mistakes are many which contribute to air and light pollution. I fervently urge other countries to learn from our mistakes and deliberately avoid them, on their ways to development. "Those who do not learn from history are condemned to repeat it."

A Recipe to Change the Future

Though this history of light pollution offers some inspiration, it also teaches that we are still losing, and losing badly. Present tactics win only limited, local victories, while light pollution increases, even where restricted by ordinances. Therefore, this history suggests that we must take other approaches.

Eliminating waste lighting will help all astronomers, and also the consumers who pay for lighting, while conserving the resources that would otherwise be used to generate the waste. Sometime, there will be another energy crunch, or economic setback, or major reverse in public approval of utilities. To be ready when that happens, our coalition should design a new type of luminaire to satisfy our own criteria along with the public's. I think these include:
no light above 15° below horizontal.
smooth spread of light over target area on ground.
target area easily tailorable, perhaps by adjustable side shields.
2 or 3 narrow emission lines (perhaps from separate tubes, phosphorescence, or by other means) that the human eye will accept for decent color rendition, but at wavelengths not critical to spectroscopy, dim to color-film emulsions, and easy and cheap to filter out.
easy and cheap to manufacture.
rights easy to license to everyone who will make them.
economical to operate (easy installation and replacement, low power consumption, long life).

We should develop this luminaire. When we have it ready, we should promote it with all manufacturers, utilities, government authorities, and the public. When we offer a luminaire demonstrably superior for the public's purposes, it should win widespread acceptance, especially in an energy crunch. Since we also tailor it to suit our own purposes, that is how we can achieve our eventual victory.


Baker, Robert H. When the Stars Come Out, Viking Press, revised ed 1954, p24.
Beet, Ernest Agar, The Sky and Its Mysteries, G. Bell & Sons 1957, p11.
Bernhard, Hubert J., Dorothy A. Bennett, and Hugh S. Rice, editors, Handbook of the Heavens, Whittlesey House 1935 (first ed) pp 83-84.
Berry, Richard, A Manual for the Study of Light Pollution, Toronto Centre, Royal Astronomical Society of Canada, 1 November 1974.
Betz, Jack, Dark Pollution, Middle East Region, Astronomical League, 18 May 1974.
Chant, Clarence Augustus, Our Wonderful Universe, World Book Co 1928?, p150.
Crawford, David L., Light Pollution: The Adverse Impact of Urban Sky Glow on Astronomy and on the Environment, National Optical Astronomy Observatories, Kitt Peak National Observatory, October 1985.
Crawford, David L. personal communication, August 1988.
Draper, Arthur L., and Marian Lockwood, The Story of Astronomy, Dial Press 1939, p24.
Dunkin, Edwin, The Midnight Sky: Familiar Notes on the Stars and Planets, The Religious Tract Society 1869? p3.
Guillemin, Amedee, Le Ciel, 3rd ed, L. Hachette 1866, pp384-385.
Guillemin, Amedee, ed J. Norman Lockyer, revised by Richard A. Proctor, The Heavens: An Illustrated Handbook, 4th English ed, Scribner, Welford 1871, p271-273.
Hale, George Ellery, The New Heavens, Charles Scribner's Sons 1923, p1.
Hinks, Arthur R., Astronomy. Oxford U Press, 1944, p11.
Hoag, Arthur A., and A. R. Peterson, Observatories and Outdoor Lighting, Kitt Peak National Observatory, April 1974.
Martin, Martha Evans, and Donald Howard Menzel, The Friendly Stars, Dover Publications, 1964, p82.
Mood, John and Stephanie, Palomar and the Politics of Light Pollution, Astronomy vol. 13 #11, November 1985, pp 6-23.
Neely, Henry M., A Primer for Star-Gazers, Harper & Brothers 1946, p19.
Olcott, William Tyler, and Edmund W. Putnam, Field Book of the Skies, G. P. Putnam's Sons 1929, p441.
Oliver, John A. Westwood, ed, Astronomy for Amateurs, Longmans, Green 1888, p295.
Pike, Robert, and Richard Berry, A Bright Future for the Night Sky, Sky & Telescope, vol. 55, #2, February 1978, pp 126-129.
Riegel, Kurt W., Light Pollution, Science, vol. 179, 30 March 1973, pp 1285-1291.
Riegler, Gideon, The Amateur Astronomer, T. Fisher Unwin 1910, pp 20-23.
Serviss, Garrett P., Curiosities of the Sky, Harper & Brothers 1909, p312.
Serviss, Garrett P., Round the Year with the Stars, Harper & Brothers 1910, p17.
Sidgwick, J.B., Introductory Astronomy: A Guide for Night Watchers, Philosophical Library 1944, p v.
Smart, W. M., The Sun, The Stars, and the Universe, Longmans, Green 1928, p143.
Smith, Clarence R., Earth and Sky: Marvels of Astronomy, University of Knowledge 1938, p69.
Sperling, Norman, How to Fight Light Pollution, Sky & Telescope, vol. 55 #2, February 1978, pp133-138.
Sperling, Norman, Licking Light Pollution, Sky & Telescope, vol. 60 #1, July 1980, pp17-20.
Sperling, Norman, Of Pupils and Brightness, Griffith Observer, vol. 49 #1, January 1985, pp14-19.
Sperling, Norman, Light Pollution: A Challenge for Astronomers, Mercury, vol. 15 #5, September-October 1986, pp 144-150.
Stetson, Harlan True, Man and the Stars, Whittlesey House 1930, p5.
Steward, Kitt Peak National, and Smithsonian Astrophysical Observatories, Light Pollution: A Threat to Astronomy in Pima and Santa Cruz Counties, December 1971.
Swezey, Goodwin Deloss, and J. Harris Gable, Boys Book of Astronomy, E. P. Dutton 1921, p63.

Of Pupils & Brightness

by Norman Sperling

Copyright © 1985, Griffith Observer magazine, January 1985

reprinted by permission

All you pupils in this short-course want to select telescopes. Great! You have an idea that skywatching can be a nice hobby. In fact, it is sensational. And you think that a telescope will show some impressive sights. In fact, the views can be awesome. If you pay moderate attention you can learn enough to buy acceptable telescopes. But it's a much brighter idea to learn the ins and outs and get a telescope tailored to your wants.

Selecting a telescope can bewilder the beginning astronomer. There are so many types -

"reflectors," "refractors," and "compound catadioptric" systems like Maksutovs and Cassegrains. Manufacturers cunningly select specifications to make systems sound too good to be true. So how do you choose?

At First Glance

You're likely to look first in a book or magazine. Most of these blithely recommend getting the fattest telescope - that is, the greatest aperture, or width - you can afford. That advice indeed achieves a lot of very useful light-gathering power. Unfortunately, it also limits portability, and it is heavily biased toward Newtonian reflectors that are not optimal for some uses. Other sources proclaim the unexcelled view through refractors, although that's true mostly for planets and double stars. Through the 1950s, those were the most popular targets for amateurs, but no longer. Still other authorities tout the benefits of Schmidt-Cassegrains and Maksutovs, the compound catadioptric types, especially for astrophotography. This advice, too, should be restricted, mostly to those needing extreme portability.

What a Telescope Does

Think of a telescope simply as a tool to funnel light. There are just 2 basic things the funnel can do: It can spread light out, or it can concentrate it. To spread light out is to magnify the image. This enlarges the object in view, which is usually good, but it dilutes the brightness, which isn't. High-power images also have tiny fields of view, and this makes targets hard to find. The other alternative is to concentrate light, to shrink the object in view. This is usually not so good, but it also makes the image bright and contrasty, which is. Low-power images have wide fields of view that take in lots of stars. In fact, telescopes designed for this are nicknamed "rich-field" telescopes.

The steepness of the funneling is the focal ratio. Many optical instruments, especially cameras, express this as an f/number. This is simply the focal length (how far away light focuses) divided by the diameter of the opening where light enters. For example, if the diameter is 100 mm, an f/4 reaches focus at 400 mm, while an f/15 reaches focus at 1500 mm. For most refractors and Newtonian reflectors, the focal length determines the tube length. That, in turn, affects the height of the mounting and, therefore, its weight.

Telescopes have dozens of qualities to optimize. But no telescope is best at all the things telescopes can do. You can optimize some but inevitably at the cost of others. Principles of optics and physics extract a price for every gain. So telescope design is an art of tradeoffs. To get the most-desired qualities, others must be sacrificed - preferably ones you can live without. The "3 Laws of Telescope Design" are:

1. Every time you gain something, you lose something.

2. Every time you gain much, you lose more.

3. There's no such thing as a free lunch.

If these look familiar, it's because they seem to be the laws of everything else, too.

So the first step is to define what the telescope must do. The dominant question is: What kind of objects do you most want to view? Other important questions include: Where will you observe from? What about carrying the telescope by car, or by muscle? How perfect must the system be? And, of course, how expensive?

The objects you want to see should determine the focal ratio. A behavioral look at optics provides a few quick answers. It turns out that for solar system observing, long-focal-ratio refractors are superior. For the galaxies, nebulae, and clusters - deep sky observing - use the shortest and fattest system possible, and that usually means a stubby Newtonian when other factors are accounted for.


And if you want to observe both within the solar system and beyond it, compromise. Get 2 telescopes: One each for the different types of viewing. If, however, it must be just one single telescope, there are choices to weigh. Only one configuration is both long and short - the Newtonian/Cassegrain - but only one small US producer has made them.

Or select a compromise focal ratio. Instead of the f/15 to 18 that shows the best detail on planets, or the f/4 to 6 that gives nebulae and galaxies the best contrast, try around f/10. Unfortunately, such systems usually deliver less-than-optimal images. To achieve the right magnifications for planets or for deep-sky objects, they require rather extreme eyepieces - either very short (less than 5 mm) or very long (more than 40 mm). Pushing optics to an extreme means a lot will have to be sacrificed to achieve even a little. Enormously long eyepieces are both expensive and heavy. Incredibly short ones are both difficult to construct and notoriously stingy on eye-relief, the ease with which you can see through them.

Poking Around the Neighborhood

Classic, long refractors are the "spyglass" type that leaps to most people's minds any time the word "telescope" comes up. Refractors team up lenses of at least 2 kinds of glass - commonly crown and flint - in a way that minimizes the chromatic aberration (spurious color) around bright images. This works best with focal ratios longer than f/11. New designs may work well at shorter ratios, but they will probably cost a lot. And their exotic, new types of glass may suffer problems of their own. So practical refractors are optically long. They deliver high magnification from conventional-length eyepieces, because magnifying power is simply the focal length of the objective divided by the focal length of the eyepiece.

Refractors are optimal for viewing the planets, Moon, and Sun. Their unobstructed light paths deliver the crispest and sharpest images. Planets appear quite small in the sky, as do details on the Moon and Sun, so you want to magnify them a lot. High magnification spreads out the image of an object, and that dilutes the light. But planets appear quite bright, so there's no problem. The classic long refractor need not be, therefore, too wide. The aperture gathers light, and the Sun, Moon, and planets offer plenty. This keeps the width, bulk, and cost of the telescope down.

Peering Far Beyond

Since the 1960s, observers have been flocking to deep-sky objects. This is due partly to the aperture explosion: Amateurs can now afford telescopes wide enough to gather enough light to make faint star clusters, nebulae, and galaxies impressive. Another stimulus was the incessant "Deep-Sky Wonders" column in Sky & Telescope magazine, written by Scotty Houston starting September 1946. Readers who initially passed over it eventually read a bit, then more and more until they were hooked. The star clusters, nebulae, and galaxies sought by amateurs have a lot in common: most appear much larger than planets, but vastly fainter. They are notoriously elusive, too. Some are so pale that it can take a long time to search them out.

The stubbier a telescope's focal ratio, the lower its magnifying power, so the better it concentrates the diffuse light of these objects. At first, it seems contradictory to use the lowest power on the farthest objects. But high magnification would produce a tangle of problems. The high-power field of view is tiny, and this makes it hard to locate and identify the right place in the sky. When you finally find it, only a small portion of the object may fit in at a time. And its light is so diluted, the image is washed out. You can scarcely tell anything is there at all. A short focal ratio delivers low power. The large field of view accommodates both the target and enough stars to facilitate identification. Also, it concentrates the diffuse light, enhancing contrast. This leads to an important supplementary principle to the laws of telescope design:

The bright ones are short, fat, and wide.

Novices find deep sky objects much more readily in short-ratio telescopes, and experienced amateur astronomers notice more detail through them.

Short-ratio telescopes are almost always Newtonians. That's mostly by elimination: It is difficult and expensive to build short refractors unless chromatic aberration grows objectionably. Compound telescopes gain most of their advantage by being compact. Compared to an already-compact reflector, they add little convenience, but they do cost a lot more. The remaining alternative is the Newtonian. There has been a gratifying flood of stubby Newtonians since the "Astroscan" appeared in 1976 and demonstrated that people would, indeed, buy a low-power telescope.


All this hints at limits to telescope capabilities that are only incidental to the optical pattern used. These limits are so remote from the beginning telescope purchaser that they are undreamt of. The truly limiting factors in designing a telescope for amateur skywatchers are not in the telescope itself! Instead, they result from phenomena beyond its ends.


On the top end, the limiting factor is the surface-brightness of the object viewed. Surface brightness is its apparent brightness divided by its apparent area. Nature provides surface brightnesses only in 2 radically different families: "high" - in the Sun, Moon, and planets, and "low" - for nebulae, clusters, and galaxies. There is virtually nothing in between. Only fleetingly will a bright comet straddle that interval.

The gap in surface brightness results from a void in distance: Our star brilliantly illuminates only its local neighborhood, so nearby planets appear bright. Then there's a huge gap to stellar realms beyond. Between us and the next-nearest system (alpha Centauri) yawns an abyss of more than 4 light years. Since light's intensity diminishes with the square of the distance, light from beyond the solar system invariably appears radically fainter.

For example, compare 2 popular targets for amateur astronomers' telescopes. The planet Jupiter shines at about magnitude -2. Because its diameter is about 2/3 arcminute, its angular area is about 0.35 square arcminute. By contrast, the Dumbbell Nebula, M 27, is much larger and dimmer. At magnitude 8, it is 10,000 times fainter than Jupiter. M 27 spans 8 arcminutes by 5 arcminutes, or 40 square arcminutes. This is 115 times larger in area than Jupiter. The Dumbbell Nebula's surface brightness is, therefore, about 1,150,000 times less than Jupiter's. No wonder different optical systems are needed to show each at its best.

So, on the top end, telescopes are constrained by the surface brightnesses of their targets.


On the bottom end, the limiting factor is not so much eyepieces as the human eye itself. The dark-adapted eye's pupil is rarely much over 6 mm wide. In young people the pupil can stretch to 7 mm, but the pupils of older folks don't exceed 5 mm. Also, smoking shrinks the pupil's ability to open widely. The telescope-and-eyepiece combination must be tailored to this. Any light that arrives wider than the pupil cannot enter the eye, and is thus sheer waste. So the telescope's exit-pupil must not exceed about 6 mm. The exit-pupil is simply the objective's diameter divided by the magnification. For any given telescope, the exit pupil enlarges as the power shrinks - that is, as the eyepiece lengthens. For a nice long eyepiece with low power and wide-field, contrasty views of deep-sky objects, the exit pupil must be large. Up to about 6 mm that's fine; beyond, there is no gain. The longest common eyepieces, used with conventional telescopes, deliver exit pupils around this size.

Therefore, in determining what telescope to make or buy, the paramount considerations are not in the telescope itself. The limiting factors are the surface-brightnesses of the objects you observe, and the entrance pupil of your dark-adapted eye. Tailor a telescope - a light-funnel - to fit between those objects and the eye, so that the second will receive the optimal view of the first.

The Journal of Irreproducible Results
This Book Warps Space and Time
What Your Astronomy Textbook Won't Tell You

Your Cart

View your shopping cart.