Astronomical Games: February 2000

Ten Myths

Bones of contention held near and dear by experienced amateurs

You can fool some of the people all of the time, and all of the people some of the time, but you can't fool all of the people all of the time.

—Abraham Lincoln (who may have said "please" instead of "fool")

ONE OF the most popular sections of Terence Dickinson and Alan Dyer's respected book, The Backyard Astronomer's Guide, is a chapter on ten widely held myths about telescopes and observing. In it, they tried not to tackle not the easiest prey, but rather those beliefs that even relatively experienced amateurs held near and dear to their heart. That single chapter received many accolades and has done much to disabuse incorrect or at least insufficiently corroborated notions.

But astronomy, as does all of science, brooks no authority, and there are parts of even Dickinson that stimulate vigorous disagreement, among amateurs generally and myself in particular (of course). In this month's column, I have collected another ten myths. Some of these myths are just plain wrong; others are sometimes right but are stated as unqualified generalizations cast in stone; and still others are simply a matter of the times a'changin'. Only a couple are actually from Dickinson and Dyer; I wouldn't want to give the impression that their books are riddled with errors (they aren't).

Of course, there's always the risk that someone will write later, "There are parts of even the Astronomical Games columns that I find totally objectionable," and proceed to list all of the faults therein, but that's a risk I'm willing to take.

1. The best beginner's telescope is a 6-inch or 8-inch dob.

Don't get me wrong; the trusty old 6-inch or 8-inch dob is a very good scope for many or even most beginners. For the money—typically between $400 and $500 U.S.—you cannot get better views. These are the apertures where serious work can actually be done, particularly on the planets, if the optical quality is there. And generally speaking it is: if there is any place where dobs do not cut corners, it is likely to be the optics.

Optical quality is only one factor, however, although it is probably the most important. One particularly important factor for beginners is ease of use, and that is one place where the dob is likely to fall short for some. The utter simplicity of the Dobsonian mount is exemplified by the way the scope is moved around the sky—you just grab the front of the scope and drag it wherever you want to go.

Some beginners take to this direct manipulation like a fish to water. Others, on the other hand, may be at a loss as to where to start, once the planets and the Orion Nebula have been exhausted. For them, a GOTO scope may be better suited to keep them in the hobby. But…

2. GOTO is only for advanced astronomers. (Also known as, "You aren't getting the maximum enjoyment out of amateur astronomy unless you learn to star-hop and gain a full appreciation of the night sky.")

The argument, so it goes, is that beginning the hobby with GOTO is like getting into pianos with a player piano. Other than the draw of live, automated music, what can the player piano provide in the way of cultural nourishment?

There are a couple of flaws with this argument. First of all, you don't have to be passive with a player piano—typically, you can play the instrument manually. So it is with GOTO telescopes; generally speaking, there is nothing preventing you from pointing the instrument yourself, although a few telescopes—notably the Celestron NexStar models—require you to use the hand controller to move the scope. You can, at your own pace, learn to copy what the GOTO is doing.

The second flaw with this reasoning is the implicit belief that beginners will get more out of some non-GOTO scope. Depending on the beginner, they may end up staying in the hobby longer if they have a GOTO (see Myth #1). If they had to rely on their own nascent star-hopping skills to find objects, they'd learn sufficiently slowly to want to give the whole thing up; if they had GOTO, they might pick it up. Of course, it's not as good as having an experienced amateur at your side, but it can be a big help. And star-hopping is not a requisite for enjoyment; some people really do just enjoy the pretty sights in the sky.

This problem is especially apparent in using larger scopes. Sometimes, optimistic beginners buy large scopes as first scopes. Large scopes are heavy, naturally, and that will be an impediment to setting one up if the amateur isn't absolutely dead set on observing. Furthermore, they tend to have a smaller field of view, making star-hopping somewhat more difficult. Other aids, such as a finder or a Telrad, can make this a bit less cumbersome, of course.

Finally, I've also observed that advanced amateurs who persistently hammer home the point that GOTO is only for advanced amateurs are often perceived by beginners (and other advanced amateurs, for that matter) as withholding a great gift from the masses. That sort of resentment can't be good for the strength of the hobby—and numbers is a big part of that strength.

3. Deep-sky observing is best done at low powers.

The conventional wisdom is that deep-sky observing is best done with exit pupils from 5 to 7 mm (depending on the maximum pupil opening of the observer). This means a magnification of anywhere from 3.5 to 5 times the aperture in inches, or 1/5 to 1/7 of the aperture in millimeters. Thus, for a 5-inch scope, it means a magnification of 18x to 25x, or for a 90 mm scope, it means a magnification of 13x to 18x, and so forth. High power observing is alleged to be good solely for planetary observing.

But my experience contradicts that, whether you mean by deep-sky observing the detection of faint fuzzies, or the teasing out of detail in those objects. My experience with M1, the Crab Nebula, in a light-polluted suburban environment with my 5-inch SCT, is that the object is best seen at about 100x. An Orion SkyGlow filter (a broadband filter) improves matters somewhat, so that it is best seen at about 62x. Note that these yield exit pupils of 1.25 and 2 mm, respectively.

Then there is the issue of detecting detail in deep-sky objects. Even if you live under dark night skies and an object can most easily be seen at low magnification, the details in those objects may be too small to be seen readily at those powers. Only when, perhaps, those details are magnified to the same size as the entire object is at low power, only then can you see those details as well. Under reasonably dark skies (limiting magnitude about 5.7), the best power for my observation of M1 through that same 5-inch SCT…52x, yielding an exit pupil of about 2.4 mm.

The only method that works for sure in determining the best magnification for a given object is to try them all out. Sometimes objects will surprise you.

4. Spur-gear drives are inferior for long-exposure astrophotography.

A worm-gear drive consists of a large plate gear, driven by a worm whose threads run almost perpendicular to the plate. As the worm turns at a certain slow rate, the gentle slope of the worm threads turns the plate gear at an even slower rate—for example, the sidereal rate. (Many drives can be set to operate at one of three or more speeds.) These drives often exhibit what's called periodic error, as small imperfections in the mesh between the worm and the large plate gear cause fluctuations in the instantaneous drive rates, in a pattern that repeats with each turn of the worm.

A spur-gear drive, on the other hand, drives the large gear with a smaller, parallel gear with much fewer teeth. It's generally contended that because of the greater variance with which the spur-gear teeth meets the large plate gear, these drives are unsuitable for long-exposure astrophotography, where accurate guiding is of paramount importance.

They don't have to be. They are generally more tedious to use for long-exposure photographs, because they have a relatively large component of random error along with the periodic error, for which there is no cure but manual guiding. There is also a somewhat greater tendency for abrupt changes in right ascension; these, however, manifest themselves mostly with overloaded spur-gear drives. Especially with lighter scopes, spur-gear drives are reasonable for long exposures.

That's not to say that worm-gear drives aren't better. According to amateur astronomer Clive Gibbons, however, Meade and Celestron were engaged in an advertising battle in the early 1980s, in which Meade proclaimed the superiority of their worm-gear drives and suggested that the Celestron drives (which used spur gears) were unsuitable for long-exposure photographs. And generally speaking, worm-gear drives are more dependable from a steady motion point of view. But many wonderful astrophotos were taken through both brands of scopes before either introduced worm-gear drives. (Celestron eventually did introduce worm-gear drives on their premium 8-inch and larger SCTs.)

5. Once you get a Telrad, you'll want to throw your finderscope away.

There is no question that a Telrad is a very intuitive tool to use. All you have to do is point the scope until the object you want is in the bullseye—you look through the eyepiece, and there it is! Since it's a simple reflex sight, there is no issue with image scale, or inversion, or a right-angle mirror to reverse the image. It's a completely natural way to find objects. If you can point out in the sky with your finger where to find the object, then the Telrad will put in the eyepiece.

However, there are still places where the old reliable finderscope will outperform the Telrad (or similar reflex device), at least for many observers. I recently tracked down M93, a 6th-magnitude open cluster, in magnitude 3.3 skies. (It was low, near the horizon.) There was no easily seen naked-eye star within about 10 to 15 degrees of my target. No matter what method I used, I couldn't point out reliably where to point the scope. But through the finder, there were just enough stars of 4th and 5th magnitude to track down the right place. When I finished, the cluster was right in the eyepiece, although the light pollution was so pervasive that I didn't even recognize it as a cluster at first!

Another point in favor of finderscopes is their weight. The traditional 6x30 finder is much lighter than a Telrad, and can more easily be mounted on small scopes. There are lighter reflex sights, though, like the Rigel and the Tele Vue models, which can be mounted on small scopes without unbalancing them.

6. Glass flows, causing large mirrors to deform over a lifetime.

Not if you mean a human lifetime.

This is a persistent urban legend, stemming primarily from claims that glass windows in medieval buildings are found to be thicker at the bottom than at the top, often by as much as millimeters. Since the flow cannot be depended on to be uniform, and since mirrors require tolerances at least 10,000 times smaller, mirrors can deform to the point of unusability. The effect is said to be especially dangerous for large mirrors, due to their added weight.

Proponents of this theory cite the correct claim that glass has some properties that mimic that of a liquid—in particular, that glass has no crystalline or quasi-crystalline structure. It's true that many don't. It's also true that glass does not have a well-defined freezing point like crystals such as quartz.

The problem is that we have no reliable evidence that the old windows didn't start out that way, with the thicker end down, and we have at least circumstantial evidence that they did. These old windows were not made the same way as they are today, and uneven windows could and did crop up rather frequently. Whenever they did, the installer had the choice of putting either end down. For the sake of the integrity of the glass, if you had a choice, would you put the thicker (and therefore heavier) end up or down?

And while it's true that glass shares some properties with liquids, they share a lot more with crystalline solids. They break, for example. There is a rather sharp dividing line between breaking and elastic deformation for glasses, too. And although they don't have a well-defined freezing point, they do have two "quasi-freezing points." Below these temperatures, creep does occur, but on geological time scales. The same telescope that works today will definitely work thousands of years into the future. Otherwise, given the claims of millimeters of creep within hundreds of years, large mirrors cast earlier this century would already have been rendered worthless.

7. Light pollution filters are designed to block out light pollution.

Well, to some degree, they are. But the most effective filters are designed primarily to block out everything but specific emissions of astronomical objects. For example, narrowband filters let in the light of emission nebulae (like M42, the Great Orion Nebula) and block out just about everything else—not only streetlights, but stars, planets, the moon, reflection nebulae, airplanes, satellites, etc. These remain visible, but only by the light they emit within the narrow frequency band (hence the name) passed by the filter, so they are a lot dimmer. The emission nebulae remain just as bright and hence appear brighter and more distinct by contrast.

As a result, narrowband filters continue to enhance the views of emission nebulae even when you take them under dark skies; in fact, most experienced amateurs argue that they help even more under dark skies than under light polluted ones. Therefore, they should by all rights be called nebula filters or something like that, but light pollution filters they are called by vendors and that they shall remain. After all, even the rankest amateur learns the deleterious effects of light pollution almost immediately upon starting the hobby, and that market must not be neglected.

There are so-called broadband or wideband filters that do let in many more light frequencies and are designed primarily to block out sodium lights that are prevalent in many urban areas. These could more accurately be considered light pollution filters, but that is still what they block out rather than let in, and they are only "light pollution filters" to the same extent that a Wratten 80A filter (which appears blue to the unaided eye) is a "yellow-orange-blocking filter." (Incidentally, the view through a broadband filter appears vaguely pinkish to my eyes, but with some blue-green tints mixed in. Apparently, my eyes have trouble making sense of the particular mix the filter lets through.)

8. Eyepieces with fewer elements will perform better than eyepieces with more elements.

Well, all other things remaining equal, that's probably true, but all other things are rarely equal. If they were, then no one would buy Naglers, Panoptics, and Radians, and Tele Vue would go out of business.

For one thing, the bad thing about lots of glass is not the number of elements necessarily, but rather the number of air-glass interfaces. The latter are really the prime opportunities for incoming light to get scattered away from the view. This scattering not only takes light away from the right parts of the image, but also distributes it indiscriminately around the wrong parts of the image. Scattering makes you lose both ways. So it's really the number of air-glass interfaces that count. For example, an orthoscopic eyepiece has four elements in it, but three of them are jammed together in one hunk, meaning that there are only four air-glass interfaces in it. In comparison, a Huyghenian eyepiece has only two elements, but they are separated, meaning that they too have four air-glass interfaces.

So why do people universally prefer orthos to Huyghenians? Because the other things are not equal, and one of these is correction of aberrations. Huyghenians exhibit a great amount of off-axis coma and astigmatism and huge chromatic aberration, all of which orthos do better on. Huyghenians also exhibit distortion off-axis, which is not necessarily bad (Naglers have their pin-cushion distortion, for example), but just the same it can make your head swim and you'd rather not do that if you don't have to in order to get something in return. And Huyghenians don't give you much in return. They used to be made for fields as wide as 50 degrees, but they were so poor off-axis that this is no longer the case. They are only sold with the cheapest of beginner telescopes and don't work well unless the scope is small (60 mm or smaller) and has a focal ratio of at least f/15 or so.

The higher-end Tele Vue eyepieces have even more elements than the orthos: six, seven, or even eight elements. Some of the high price of these oculars goes into superlative coatings, so that the air-glass interfaces don't hurt you as much as they would in cheaper, less well-coated eyepieces. Furthermore, they correct aberrations even better than orthos, over a much wider field. That's why people prefer them and are willing to pay a premium for them.

9. The Cassini division is sharp black and you could drive a Mack truck through it! (Or other similar claim about observing equipment and prowess.)

Such claims are often made using telescopes whose resolving power isn't great enough for the Cassini division to appear sharp black. The division is only 0.5 arcseconds wide at its widest—at the extremities of the rings—and considerably narrower elsewhere, even when the rings are wide open, as they will be in a couple of years as I write this. The Airy disc of a 10-inch unobstructed telescope is about that wide, and even such a telescope would not be sufficient to make the Cassini division sharp black all the way across, but only a deep black at the very center of the division.

In other words, a 10-inch telescope is the minimum required to make even two points in the Cassini division sharp black; perhaps a 20-inch telescope would be sufficient to make a sharp black line all the way around the rings when fully wide open. Yet claims such as the above are made on telescopes barely 6 inches in diameter. It's enough to give a beginner the heebiejeebies about their own skills and/or equipment quality. What gives?

For one thing, the rings are bright. The B ring, the one on the inside of the Cassini division, is brighter even than the disc of the planet, since it is largely composed of highly reflective ices. The A ring, though dimmer than the B ring, still rivals Saturn's disc in brightness. This helps make even a greyish Cassini division appear black to the eye.

In fact, Voyager showed that the Cassini division is not truly empty but actually has microringlets scattered throughout it. What's more, photometric studies showed that the C or crepe ring, on the inside edge of the B ring, is actually intrinsically darker than the Cassini division. That means that even through a perfect, 200-inch telescope (say), the Cassini division should appear no darker than the crepe ring which the same amateurs can quite rightly see in those same 6-inch scopes.

Another factor is that linear features such as the Cassini division are easier to see than point features of the same dimensions. For example, the Airy disc of the unaided daytime eye is perhaps one arcminute across, about 1/30 the width of the full moon. That is about the angular width of a two-inch circle at 1/8 mile, and in fact a two-inch object at such a distance is just about at the edge of being discerned by the unaided eye. And yet telephone wires—which are about two inches wide—can be easily seen at distances far above 1/8 mile. Apparently, the ability to draw even the hazy pattern of the wires, diffracted by the eye's aperture, into a coherent arc aids in the detection of the wire. Something similar is likely happening in the observations of the Cassini division.

Finally, the eye-brain combination is capable of internally inverting the diffracted image, because it knows what should be there, except on point sources where the Airy disc at high powers is unavoidable. I've written about this previously, calling it deconvolution, and it's a skill that becomes better over time. So the claim really does speak to observing skill, but it's important that novices don't look through their scopes, find a barely detectable dimming encircling the planet, and conclude that something is wrong with either them or their equipment.

10. Always buy a telescope at a telescope store. Never buy it at a department store.

Actually, you should distrust a rule that says "always" or "never." It's almost never always true.

In my trips to various telescope stores in California, where many amateurs live, I've found both good and bad telescopes. One store had the Bushnell 8-inch Dobsonian, which is by many accounts the worst-executed commercial Dobsonian on the market. On the other hand, they carried a Tele Vue Bizarro, a specialty bino-view short-focus refractor, which gave drop-dead beautiful views of the moon and planets.

Over in the opposite corner, at a department store, at Christmas time (of course), I found the usual assortment of "department-store scopes": 60 mm refractors of reasonable optical quality mounted on rickety alt-azimuth or, even worse, poor equatorial mounts, finders worse than gun sights, terrible eyepieces, overpowered and undercorrected Barlows, and naturally, deceptive advertising. (Imagine a Voyager picture of Saturn cropped in a circular "field of view.") But I also found an ETX, a good, popular GOTO scope, and of all things, a nice 80 mm Celestron alt-az refractor.

However, some things remain true: you get better service and knowledge at a telescope store, it's not quite as hectic at the holidays, and as always, try before you buy.

Well, there you have it, my version of The Ten Myths Most Likely To Provoke Strong Discussion At Your Astronomy Club Meeting. I make no claim to infallibility; if you disagree with any of my conclusions, I'd love to hear about it, and I promise I'll mention it in a future column.

Copyright (c) 2000 Brian Tung