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Norman Sperling
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Telescope Triplets

© Norman Sperling, November 25, 2011
Part of a series on Educational Star Parties:
Star Parties Designed for Students (July 7, 2012)
7 Spectral Types in 1 Big Loop (April 15, 2012)
Trading Cards for Telescopes and Celestial Objects (September 20, 2012)

For decades, I have been proclaiming that focal ratio is one of the most important characteristics in choosing a telescope. Most authorities tout aperture instead. But none of us has ever conducted a true visual test, isolating the variables of focal ratio, aperture, and eyepieces.

I propose that 3 triplets of Newtonian telescopes be made to demonstrate the effects of focal ratio, aperture, and eyepiece. They can be used for classes and at star parties to teach about the properties of the telescopes themselves. Mount each triplet so that viewers can easily shift among all 3 eyepieces to instantly compare views.

The "focal ratio" triplet should consist of 3 telescopes, all with the same aperture and eyepiece. Make one f/5, another f/10, and another f/20. For this triplet, I think 3-inch (76 mm) apertures are best: even the f/20 would be a manageable 5 feet (1.52 m) long. Users will see that Jupiter looks best at f/20, and the Great Andromeda Galaxy best at f/5. Trying this battery of telescopes on the sky's enormous variety of targets will probably reveal very few objects that look best at f/10.

A second application of this same telescope set will use different eyepieces that all result in the same magnification: a long eyepiece on the long scope, a short eyepieces on the short scope, and a middling eyepiece on the middling scope. How different are the views of different targets?

The "aperture" triplet should consist of 3 telescopes, all with the same focal length (perhaps 4 feet = 1.22 m) and eyepiece. Make one 3 inches (76 mm) aperture, the second 6 inches (152 mm), and the third 12 inches (304 mm). Users may be surprised how much even the 3-inch shows.

The "eyepiece" triplet should consist of 3 identical middling telescopes, perhaps 4-inch (102 mm) f/8. Insert eyepieces of equal focal length but different optical designs (such as Huygens versus orthoscopic versus Nagler). A second application of this same telescope array will use eyepieces of equal design but different focal lengths (perhaps Plossls of 6 mm, 12 mm, and 25 mm ...).

Make each triplet so the scopes, and their eyepieces, can also swivel to allow 2, or even 3, different people to watch through one of the scopes at a time. This is because, perhaps once a decade, some sky event brings out throngs, and the host needs to move a whole lot of eyeballs through the scopes in minimal time.

These triplets could be built by amateur-telescope-making workshops, such as several clubs run, or perhaps by a veteran scope-maker. Most are quite small, only one is large. Try hard to hold all but one factor constant so they really test that single variable.

A whole metropolitan area probably needs only one set. Telescope triplets can be passed around among nearby colleges, astronomy clubs, planetaria, etc., to use at their classes, star parties, and member-events.

Novice Astronomy Over 50 Years

© Norman Sperling, July 5, 2011

A presentation I saw on how to get into amateur astronomy showed how much has changed in the half-century since I began ... and how much hasn't. Amateurs from the Phoenix and San Jose areas explained the ins and outs to science fiction buffs at Westercon.

Stars, planets, and humans are still the same, so the principal advice is still to go somewhere dark (away from light pollution), and learn the constellations and how the sky moves. That advice is absolutely identical to what I was told in 1957, and it's right. They mentioned some recent and classic beginner books, as well as the latest 'pod apps. Light pollution is now a lot worse, so getting to a dark place is much more difficult, but the advice is the same.

The second advice is still to not dive into buying a big, complicated, expensive telescope. After the naked eye, use binoculars. After binoculars, a useful beginner telescope is now available for as little as $50 or $60. That price is relatively lower (considering inflation) than in my youth - an advantage of modern design and production. Then and now, beginners must be warned away from flimsy, incompetent, disappointing telescopes from non-specialist merchants.

They still recommend Sky & Telescope and Astronomy magazines. (OK, the latter was founded in 1973.) They still recommend finding your local astronomy club and star parties, and using red-light flashlights to preserve night vision.

They still recommend studying the richest and most informative telescope catalog – though that used to be Edmund's and now it's Orion's. The lust generated by seeing all the glorious equipment used to be called "aperture fever" and is now "Telescope Porn".

Modern optical and electronic technology has outmoded the old equipment, and enabled whole new categories of activities.

The Dobsonian Revolution made far larger telescopes affordable to serious amateurs, and they can observe deep sky objects spectacularly better than 50 years ago. Today's top Schmidt-Cassegrains, Maksutovs, and refractors deliver markedly better images than you could buy 50 years ago. Some astronomers love automatic object-finding telescopes because it's easier to observe what you want; purists consider it cheating if you don't point the telescope correctly yourself.

Electronic imaging has popularized incredible tools like webcams. Commercial mounts now mate phone-cameras to telescopes. Software now lets photographers stack multiple exposures using more skill and time than money. The best amateur astrophotography of 2011 far surpasses the best that the big professional observatories could do just 30 years ago. These tools enable amateurs to study, and make discoveries about, far fainter objects than before.

One aspect that hasn't changed is the mindset that "amateur astronomy" = observing. That wasn't true 50 years ago and it's less true today, but it's what springs to mind. Lots of non-observational aspects are wide open – history, education, tourism, and telescope making are just a few popular options. Data-mining now combs and analyzes enormous amounts of data, usually gathered by professionals. Anyone competent with a computer and an internet connection can do this. Some such projects are called "Citizen Science".

Overall, getting to a dark sky is markedly harder nowadays. Learning the sky and climbing above beginner status are about the same. But optical as well as electronic technology have improved spectacularly. Far greater viewing and computing power are affordable, and projects to use them multiply very fast. Nowadays the limiting factor isn't telescope size, or imaging skill, or computing talent, but the creativity to think up a new project. Go for it!

Astroscan Memories

© Norman Sperling, January 15, 2011

A recommendation by Sky & Telescope magazine last month, following a [.pdf] review last July, rekindled an old glow. The Astroscan telescope - my first big project - was once again named one of the 3 best inexpensive telescopes ... 34 years after it was introduced!

I remember its development clearly.

It was meant to be a superior first telescope, and it is. It has also proven to be a superior second telescope: folks keep it after they graduate to something bigger, and use it for a quick session, and as a convenient portable. Because people keep their Astroscans, remarkably few are offered on the used market.

Robert Edmund was taking over Edmund Scientific Company leadership from his father Norman. Norm has enjoyed retirement in Florida ever since. Robert had studied business management and knew how to run a going concern in changing markets. His telescope line was not doing well. Telescope leadership belonged to Criterion, Unitron, Questar, and Celestron, and Edmund Scientific wanted to earn its way to the top tier. The Astroscan was his opening salvo.

Robert Edmund hired me as a consultant in 1975, when I was 28. I was planetarium director at a private school, an hour's drive north of Edmund's. I was young and unknown and had even rougher edges than now. My ideas were unconventional, and entirely untested in the market. I contributed to a lot of Edmund's smaller astronomy projects, too.

I had observed observers observing in amateur, public, and school settings, and discovered that some of the wisdom of my elders wasn't wisdom. Telescope setup took frustratingly long, mountings were clumsy and shaky with narrow pivot points and long overhangs, eyepieces were tough to squint through, and views were underwhelmingly faint and dull. To improve on those, I preferred quick setup with minimal moving parts, stubby bodies, wide fields of view with wide exit pupils and bright contrast, lightweight and cheap. Those all shouted "Rich-Field".

Dr. Harvey Davis of the Lansing Astronomical Society introduced me to the principles of rich-field telescopes in the late 1960s. He was a friendly young math prof at Michigan State, where I was an undergrad. In the early '70s my friend - everybody's friend - Roger Tuthill made an RFT with an optical window (the success of which spurred us to do the same with the Astroscan). Roger's scope had a conventional cylindrical tube with a simple handle, so the only characteristics in which it was a predecessor of the Astroscan were the window and being an RFT. It didn't sell well at all.

No one in all history had ever gotten Americans to buy a LOW-power telescope, and we knew this was a huge hurdle. I assured Edmund that the telescope would please its users, but I explicitly never promised that anyone would buy it, and I wondered whether the expensive project would ever turn a profit. When Marketing VP Jack Sharff claimed that people would buy it, I thought that was bravado more than business sense. Sharff assured me that making it "popular" was his task, not mine. A good thing, because I understood almost nothing about marketing back then.

I wanted to make the eyepiece's exit-pupil an enormous 6 mm, because that's about the widest a dark-adapted human eye can take in. So, figuring from that, I championed a 4 1/4" f/4 (which the company nudged to f/4.2 for manufacturing convenience). Astroscan's richfield view - 3 degrees wide - means that finding things is easy, and keeping them in view is easy. It also means that hundreds of deep-sky objects are unusually contrasty, making them more obvious to beginners. The tradeoffs are minor: no astrophotography (which we wouldn't wish on novices anyway), planets look too tiny, and only a few double stars would look good. But any novice scope would only show pleasing detail on Jupiter and Saturn, the other planets being too small, featureless, and/or faint. So we swapped decent views of 2 objects (Jupiter and Saturn) to get superior views of hundreds of deep-sky objects.

I expounded on telescope design, exit pupils, and surface brightness in "Of Pupils and Brightness", Griffith Observer, January 1985.

At least as important as the optics, I wrote Astroscan's behavioral specifications. I remember blathering on and on for maybe 2/3 of a page singlespaced that I could have shortened enormously had I known the term "user-friendly". I didn't have the term, but I did have the concept. In beginner telescopes, it meant minimizing adjustments to fiddle with, and shortening the setup time (competitors, then and now, often take 15-20 minutes). Our setup time target was 3 minutes. We got it down to 10 seconds, and NO user's attention-span is too short for that.

While I did the optical and behavioral design, a brilliant young optical engineer, Mike Simmons, created the mechanical design that satisfied our needs. Simmons figured out that pushing the tube into the mounting made sense, and Simmons figured out that the ball-in-socket would work best. He was right. He advocated a very large sphere, with just the focuser-end of the tube sticking out. However, manufacturability, aesthetic appearance, and the awkwardness of a large-diameter sphere pointed the company to a smaller sphere, with more of the cylinder sticking out. This, however, is top-heavy, so to balance it, 2 semicircular slugs of cast iron surround the mirror. The extra weight, and the need for it, offended Simmons, and he left Edmund's soon after. I haven't seen him since the early '80s.

The shell satisfied all my specifications, including being nearly student-proof (it's meant to be checked out by students and carried home on a school bus). An industrial designer did the detail work. It's cast in 2 pieces of ABS plastic (one with the focuser insert, one without) and glued together.

In the fall of 1976, just before the first ads came out, I asked Robert Edmund what amount of sales he'd consider successful. He said 800 units by Christmas. Privately I thought that unlikely. Well, they sold 3,000 Astroscans in those first 3 months, which taught me another business lesson: there are DISeconomies of scale, as well as economies of scale. For example, the company couldn't produce the telescopes fast enough, and had to add shifts. Part of the optical design was meant to use an excellent, but slow-selling eyepiece that Edmund had a thousand of. They ran out, and had to scramble, buying every eyepiece on the world market that could possibly work - some Astroscans were shipped with Clave Plossls worth almost as much as the entire scope! Robert Edmund soon had Penn State's Dr. David Rank design the RKE eyepiece line, stimulated by the need to make a new eyepiece for the Astroscan. I'm happy that the company has sold in the neighborhood of 100,000 of them.

It was Robert Edmund who selected and hired and coordinated all the various people whose work combined to make Astroscan a success. He paid for all the work and assumed all the risk. He paid me quite well. In addition, the Edmund family and company ALWAYS treated me exceptionally well, and very often did me favors far beyond a conventional business relationship. Then and now, I regard my relationship with Edmund as one of the best I have ever had. I consulted for them for 9 years, 1975-84, but I have been a customer of theirs for 50 years, and endorse them as a fine set of people.


Nobody since then has hired me to design a telescope, and such a project is beyond my personal resources. But I still get ideas.


Parts of this post appeared on the Old Scope list in February 2002.

Great Book Sale

I'm moving into an RV and simply can't keep the library I've built over 50 years. (What I do next is described at www.everythingintheuniverse.com/node/76.)
* Thousands of books, mostly <$10.
* These are the best copies I ever got, the ones I kept for myself.
* Many scholarly, lots of popularizations at all levels.
* A few hundred are from the 1800s.
* Over 100 are autographed by their authors.
* Runs of many science periodicals.
* Posters.
* Miscellaneous clippings, brochures, pamphlets ...

Cash preferred. Checks and time-terms accepted from people I know, and people they vouch for personally. PayPal possible, but I'm not set up for credit cards.

11 AM to 4 PM
Saturday, August 11, 2012
413 Poinsettia Avenue, San Mateo, CA 94403
(enter left of the garage, through the courtyard)
near the Hillsdale exit off US-101
Landline: 650-573-7125 (expires about September 22)
Cell: 650-200-9211

Solar System
Celestial Mechanics
Historical astronomy
early NASA
and much, much more

Historical Astronomy
Histories of Science, and specific sciences
Heroes of Science

Earth Science
Science Fiction
Africana/Black Studies
and miscellaneous other interests

The family is also selling kids' bikes, a drum set, 1990 Ford van ($1990), and (closer to September 22) household furniture and stuff ... and then, of course, the house itself. I'll move about September 22, perhaps to Pittsburg, CA, for the fall, then Trek in the RV.

FFNs, LBBs, and LBMs

© Norman Sperling 2002. Excerpted from his book What Your Astronomy Textbook Won't Tell You, 0-913399-04-3.


When novices start to use their first telescope, they look at the sky's major showpieces, such as the Messier nebulae, clusters and galaxies. They're big and bright enough to show up in binoculars, and a beginner's telescope shows detail in many of them. In the background lurk many more faint objects.

Experienced skywatchers buy bigger and better telescopes, seeing ever-richer detail in more and more nebulae, clusters and galaxies. But always, in the background, there are even more objects, too small and faint to make out. Some irreverent amateur astronomers in San Jose call those background objects "Faint Fuzzy Nothings" – FFNs.

FFNs continue in the background as seen by big, professional telescopes, too. Look at a picture of a galaxy in your textbook. In the background you can often notice dim smudges. Each of those is a galaxy, too, but so much farther away that you can't make out as much detail. A 3-meter-wide telescope shows magnificent detail in objects that amateurs can barely glimpse – and in the background lurk uncountable thousands of more FFNs. A 6-meter telescope shows detail in those, and in the background, even more FFNs. A 10-meter telescope reveals detail in those objects ... and in the background, there are ever more FFNs. No telescope has ever been made that didn't find more FFNs in the background.


One day when I was visiting my brother, a bird-watcher, I noticed his log of sightings. Almost every entry included "LBB". He told me that LBB stands for "little brown bird". They are so common, so small, and so similar, that they're not worth examining to see which common species each one belongs to. They flock all over, they're usually there, and they're not the big or pretty or rare birds that bird-watchers prize.


The university's mycological society hosted a meeting about LBMs. Mycologists study fungi, and I didn't have to attend to figure out that "LBM" stands for "little brown mushroom". LBMs are so common, so small, and so similar, that they're not worth examining to see which common species each one belongs to. They're not the big or pretty or rare mushrooms that fungus-hunters prize.

There's more! In prospecting, ignore LGRs: "Little Grey Rocks". In wildflowers, ignore DYCs: "Darned Yellow Composite" flowers that fill meadows. Among stars, ignore MV red dwarves. Among meteorites, ignore L6 "ordinary" chondrites. Among galaxies, ignore dE dwarf ellipticals. In archaeology, ignore undecorated body shards (they don't have initials, but ignore them anyway). In entomology, ignore midges.

The same principle applies outside of science. In coin collecting, ignore small copper coins. In stamp collecting, ignore definitives. In antiquarian books, ignore textbooks. And in the serious study of literature, ignore science fiction.

This happens a lot in science. Beginners learn all the kinds of phenomena in the field, and quickly concentrate on certain ones, all but ignoring certain others. Sometimes practicality forces the distinction: some are available, others are too difficult to study. Often, though, it's about what's fashionable to study.

Technology advances at such a furious pace these days that it may be worth looking anew at common background items, using the latest devices. Most people don't pay attention to them. You just might recognize something interesting that no one noticed before.

Primary Use of Right Eye versus Left Eye by Members of the Public Observing Through Telescopes at Chabot Observatory

Norman Sperling. Originally published in The Refractor, vol. 73 #1, September 1996, p6.

Do people use their right eyes, or their left eyes, to observe through telescopes? If they predominantly use one, the design of telescope eyepiece areas might be specialized for that side.

On 5 public nights in March through July, 1996, tallies were kept of which eye was first used by members of the public who were observing celestial objects through telescopes at Chabot Observatory. The nights were selected for the following characteristics:
The sky was clear.
At least 30 members of the public were present
No other duties promised to distract from the tally.

In fact, answering questions from patrons did indeed distract from tallying approximately 10 observers. Also, fewer than 10% were noticed to try both eyes while at the telescope. Only the side first used was tallied.

Night Left Eye Right Eye
1 19 22
2 26 23
3 26 34
4 11 22
5 11 36
Total 93 137

Each side is used by large numbers of the public. Therefore, as expected, the design of eyepiece areas of telescopes for public viewing must accommodate both sides.

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.
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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.

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