Astronomy Hacks | O'Reilly Media

Content preview·Buy reprint rights for this chapter

Getting started in amateur astronomy seems simple enough. Buy a telescope, take it out at night, point it at the sky, and you're good to go. Or are you?

Many thousands of people follow just this route every Christmas, and nearly all of them are disappointed. They overpay for an inferior scope at the mall or a big-box store. Once they get it assembled, they discover they can't figure out how to use it properly. They soon find that being outdoors with a telescope in wintertime gets cold fast, and decide they'd really rather watch television instead.

Even those who persist lose interest quickly. After they look at the Moon a time or two, and maybe Jupiter and Saturn, they decide there's really not much else to see. Perhaps they've bought a computerized go-to scope that claims to find objects for them automatically. If that's true, why are the objects invisible, even though the computer swears they're in the eyepiece? Where are all those brightly colored objects pictured on the telescope box? The new scope ends up gathering dust in the closet or for sale on eBay. It can all be very discouraging.

But it doesn't have to be that way. Astronomy can be a wonderful, life-long hobby, one that the entire family can enjoy together. Thousands of devoted amateur astronomers are outdoors on every clear night, observing the wonders of the night sky. You can join them, but you need to get started right. In this chapter, we'll tell you what you need to know to avoid the most common beginner mistakes.

It's harder than it looks, but doable.

The night sky initially looks inviting—a big, black picnic blanket spangled with shine. Stellar objects are brilliant, easy to see, and seem to organize themselves into recognizable patterns. Vast, yes, but easily interpretable, welcoming.

Appreciating the beauty of a starry sky is easy. It's the next step that's hard. The night sky is the very worst kind of bully—the kind who punches you in the stomach, steals your lunch money, and then laughs at you when you cry.

Additional content appearing in this section has been removed.
Purchase this book now or read it online at Safari to get the whole thing!

Content preview·Buy reprint rights for this chapter

Getting started in amateur astronomy seems simple enough. Buy a telescope, take it out at night, point it at the sky, and you're good to go. Or are you?

Many thousands of people follow just this route every Christmas, and nearly all of them are disappointed. They overpay for an inferior scope at the mall or a big-box store. Once they get it assembled, they discover they can't figure out how to use it properly. They soon find that being outdoors with a telescope in wintertime gets cold fast, and decide they'd really rather watch television instead.

Even those who persist lose interest quickly. After they look at the Moon a time or two, and maybe Jupiter and Saturn, they decide there's really not much else to see. Perhaps they've bought a computerized go-to scope that claims to find objects for them automatically. If that's true, why are the objects invisible, even though the computer swears they're in the eyepiece? Where are all those brightly colored objects pictured on the telescope box? The new scope ends up gathering dust in the closet or for sale on eBay. It can all be very discouraging.

But it doesn't have to be that way. Astronomy can be a wonderful, life-long hobby, one that the entire family can enjoy together. Thousands of devoted amateur astronomers are outdoors on every clear night, observing the wonders of the night sky. You can join them, but you need to get started right. In this chapter, we'll tell you what you need to know to avoid the most common beginner mistakes.

Additional content appearing in this section has been removed.
Purchase this book now or read it online at Safari to get the whole thing!

Content preview·Buy reprint rights for this chapter

It's harder than it looks, but doable.

The night sky initially looks inviting—a big, black picnic blanket spangled with shine. Stellar objects are brilliant, easy to see, and seem to organize themselves into recognizable patterns. Vast, yes, but easily interpretable, welcoming.

Appreciating the beauty of a starry sky is easy. It's the next step that's hard. The night sky is the very worst kind of bully—the kind who punches you in the stomach, steals your lunch money, and then laughs at you when you cry.

When you first start looking at the stars through a telescope, the blanket shrinks to a napkin. The obvious becomes elusive and the elusive becomes invisible. Of course, the finding of things is part of learning to observe, but that knowledge is small comfort when you are unable to find the Andromeda Galaxy night after night after night.

What to say, really? Using a telescope can be frustrating. First, don't give up. Or rather, give up, but only for a while. Learning to squeeze an expanse of night sky into the eyepiece of a telescope and then to comb it degree by degree takes not only patience, but practice. Temporarily flinging up your hands and packing up the scope for the night is not only acceptable, it's necessary. When you're tired and angry, the fluid motion required to scan the sky become jerky and unpredictable; you're unlikely to find anything and you risk damaging your equipment.

Don't let the bully keep stealing your lunch money, though. Get your confidence back. The best tactic is a battle plan. Select a piece of sky, pull out a simple star map, and find your way around. Stars in the sky will look different from stars on paper, but once you've identified a few landmarks, go back to the scope. Wend your way through the familiar, retracing your steps until each star is a recognizable signpost.

The process is slow—arduous, even. But, eventually, as the stars stop looking like little blobs of light and start looking like a set of directions, the size of the viewing field matters less and less.

Additional content appearing in this section has been removed.
Purchase this book now or read it online at Safari to get the whole thing!

Content preview·Buy reprint rights for this chapter

Meet others who share your interests, learn a few things from them, and maybe even play with their toys.

The first piece of advice newbies generally hear is to join an astronomy club. We couldn't agree more. Join an astronomy club. Join an astronomy club. Join an astronomy club. What we tell you three times is true.

Surprisingly few amateur astronomers belong to an astronomy club. For example, we live in Forsyth County, North Carolina, which has a total population of about 200,000. The Forsyth Astronomical Society has only about 50 members. The population of the United States is about 300,000,000. Credible estimates say that between 1,000,000 and 2,000,000 of them are interested in astronomy. On that basis, if all interested Forsyth County residents belonged to the club, it would have between 667 and 1,333 members, many times its actual membership. The same is generally true nationwide, which means a lot of people are missing out on one of the best resources available to learn and enjoy the hobby.

Joining your local astronomy club has many advantages, which usually include:

Help and advice from experienced members: If you're just getting started, getting advice from more experienced club members can save you a lot of time, money, and aggravation. Unlike magazines—and even some web sites—that must please their advertisers, club members tell it to you straight. If something is junk, they'll sayso, despite all the pretty full-color ads for it running that month in the astronomy magazines. And you can take that advice to the bank. Most astronomy clubs have a very strong focus on helping new members with everything from buying equipment to learning how to locate celestial objects. It's difficult to overstate the value of that help for a newbie.
A chance to try out equipment: One of the biggest benefits of belonging to a club is that you can look at other people's stuff. If you're thinking about buying a $300 Nagler eyepiece, for example, but would like to see one first, a club is the place to be. Chances are, another club member already has just the eyepiece you're thinking about buying and would be happy to let you look through it. If you ask politely, he may even let you try it in your own scope. Local astronomy specialty stores are an endangered species in most cities, which means most astronomy gear is bought mail-order nowadays. If you're uncomfortable buying expensive items sight unseen, get yourself to an astronomy club.

Additional content appearing in this section has been removed.
Purchase this book now or read it online at Safari to get the whole thing!

Content preview·Buy reprint rights for this chapter

Take precautions to keep small problems small.

The days when most amateur astronomers observed from their own backyards are long gone. Light pollution forces most of us to seek observing sites far from cities, often in the middle of nowhere. But with civilization comes safety. If you have a problem at home, you dial 911 and the police or paramedics or animal control officers arrive in a few minutes. If you have a problem at a remote observing site, it's up to you to deal with it until help arrives, which may be some time.

If you're prepared, small problems tend to stay small. If you're not, a small problem can rapidly escalate into a dangerous emergency. Here's what we recommend to prepare yourself:

Observe in a group: There really is safety in numbers. A lone observer may be victimized by two- or four-legged predators or may have a medical emergency. Bad things are less likely to happen when you observe with a group [Hack #2] , and if a problem does arise there are people there to help.
Never leave the last person alone: Use the buddy system. The last two vehicles remaining at the end of an observing session should leave together, particularly if the site is remote.If you have company, a flat tire or other breakdown is merely annoying.If you don't, it can be very inconvenient at best and dangerous at worst.

Call us sexist, but we never leave a woman or even a group of women alone, even at a regular club observing site near civilization. It's just too risky. Most women appreciate having men stick around to protect them. For those who don't, we just pretend we aren't ready to leave until they start to pack up.

Carry a cell phone: A cell phone can be a lifeline in an emergency. Make sure the phone is charged and verify that you have a usable signal from your observing site. Store local emergency numbers for your observing site, not just 911, but the direct numbers for the local police and fire departments, hospital or emergency clinic, paramedic/rescue squads, and so on. Know the exact location of your observing site, and how to direct emergency services to locate it.

Additional content appearing in this section has been removed.
Purchase this book now or read it online at Safari to get the whole thing!

Content preview·Buy reprint rights for this chapter

Hypothermia kills, and frostbite isn't much fun either. Learn how to protect your vital organs and keep all your extremities attached.

As odd as it sounds to non-astronomers, it's possible to become chilled while observing even during high summer. Observing is a sedentary activity, so you generate little excess body heat. The temperature of the night sky is very nearly absolute zero, and it serves as a gigantic heat sink that sucks the warmth out of you. Observing sites are often located at high elevations and unsheltered from the wind, which makes the problem worse.

When our astronomy club holds public observations at a local state park in July and August, it's easy to tell the experienced astronomers from the public visitors. The visitors are wearing the same t-shirts and shorts they wore during the heat of the day. The astronomers are wearing long-sleeve shirts and jeans, and often don jackets and caps as the night goes on. More than once, we've loaned blankets to visitors who were shivering, unable to figure out why they were cold. The night sky gets you every time.

For cold-weather observing sessions, the problem is much worse. The standard advice is to dress for temperatures 20°F to 30°F (11°C to 17°C) colder than the actual temperature, counting wind chill. For example, if the forecast low temperature is 30°F including the effects of wind chill, we dress for 0°F to 10°F, and are often none too warm at that.

Fortunately, there are steps you can take to remain warm even under severely cold conditions.

Even moderate alcohol consumption increases the risk and severity of hypothermia. Alcohol is dangerous in three ways. First, it increases the likelihood of hypothermia by increasing heat loss from your body. Second, it masks the effects of hypothermia by reducing shivering and other hypothermia symptoms. Third, and most dangerously, it gives you a false sense of feeling warm. Drinking alcohol before or during an observing session is always a bad idea, but it's a particularly bad idea for cold-weather observing sessions.

Additional content appearing in this section has been removed.
Purchase this book now or read it online at Safari to get the whole thing!

Content preview·Buy reprint rights for this chapter

Rules exist for good reasons. Keep these simple ones in mind when you are observing, and you won't incur the wrath of your fellow participants.

An observing event can be anything from a large, formal star party with hundreds of participants to a small informal observing session among club members and friends who enjoy spending an evening together under the stars. Whatever the size of the event, behaving properly at the observing site allows you and others to enjoy the session.

A set of (usually) unwritten rules for observing site etiquette has developed. Most of the rules are based on common sense, but some may not be obvious to inexperienced observers. The rules vary with circumstances. If you are observing with a small group of close friends, you can be a bit more flexible. But if you are observing at a large star party, particularly if there are strangers present, it's best to follow the rules closely. Break a few rules, and you'll hear others muttering about the newbie. Break too many rules, and you may be asked to leave and not be invited back.

If you remember just one rule, remember this one: do not show a white light after dark unless you have everyone's permission. It takes half an hour or so for one's eyes to become fully dark adapted, and a brief flash of white light can instantly ruin that dark adaptation for everyone [Hack #11] .

If you must show a white light—and by "must" we mean for something really important, not just a matter of your own convenience—announce your intentions and get everyone's permission first. Once you have permission (which may be a long time in coming if someone is doing long-exposure imaging), give a timed warning—"White light in 30 seconds…15 seconds…5 seconds…white light is on." Limit the brightness of the white light—for example, by filtering the flashlight through your fingers—and keep it directed downward rather than waving it around. Finish what you need to do as quickly as you can, turn off the white light, and announce, "White light is off. Thank you."

Additional content appearing in this section has been removed.
Purchase this book now or read it online at Safari to get the whole thing!

Content preview·Buy reprint rights for this chapter

Carry what you need so you're ready to observe at any time.

Our regular observing gear could fill an SUV. In fact, it does. We have an Isuzu Trooper—Barbara calls it Astro Truck—dedicated to astronomy. It's always packed and ready to go on observing trips. Although it's not always feasible to carry that much gear, that doesn't mean we're not prepared to observe at any time and on no notice. The trick, as the Boy Scouts say, is to Be Prepared.

The amount and type of equipment you can have available depends on the situation; here's what we recommend.

Weight and size limitations strictly limit how much equipment you can bring along. At a minimum, carry a 7X50 or 10X50 binocular, which is also useful for observing birds and other wildlife, and a planisphere. A planisphere, shown in Figure 1-3, displays the positions of the stars for any date and time you "dial in." (Planispheres are designed to work at a particular latitude, and they are generally produced in latitude increments of 10° The one shown is designed for use at 40° N, but is usable from 30° Nt 50° N. Choose the version that's closest to your own latitude.)

Figure 1-3: A planisphere

Better still, substitute a PDA with planetarium software installed for the planisphere [Hack #65] . The PDA weighs no more than the planisphere, is smaller, and packs a great deal more information. If you can afford the extra tonnage, substitute a giant binocular—12X60, 15X70, or so—for the standard binocular. Carry a couple of inflatable pillows that you can use while lying on the ground to brace yourself for stable views. A red LED flashlight and perhaps a narrowband filter, which you can hold between your eye and the binocular eyepiece, completes your kit [Hack #59] .

An 80mm or 90mm short-tube refractor weighs the same or less than a giant binocular, but is much more flexible. With a low-power eyepiece, the scope has magnification and field of view similar to that of a binocular. With other eyepieces, the scope can provide much higher magnification. Inexpensive short-tube refractors are generally of poor optical quality and are limited to perhaps 75X or 100X. Better short-tube refractors—such as models from StellarVue, Tele Vue, Vixen, and others—can realistically support higher magnification.

Additional content appearing in this section has been removed.
Purchase this book now or read it online at Safari to get the whole thing!

Content preview·Buy reprint rights for this chapter

Use an Allen wrench to match your instruments to your eyeball.

Your pupils constrict in a bright environment to limit the amount of light that reaches your retinas. In the dark, your pupils dilate to admit as much light as possible as part of the dark adaptation process [Hack #11] .Astronomers refer to pupil diameter as entrance pupil size because it determines how much of the light from a binocular or telescope can enter your eyes. Measuring your entrance pupil size when your eyes are dark adapted gives you a key piece of information to help you select binoculars and eyepieces that are best suited to your own eyes.

Maximum dilated pupil size varies with age and other factors. A child under 10 years of age may reach maximum dilation of 8mm or slightly more when fully dark adapted. A young adult's entrance pupil may be as large as 7.5mm. As we age, our eyes may no longer dilate as fully as when we were young. By age 35 or 40, we may be limited to 6.5mm or less dilation, by 50 or 60to only 6mm or less, and by 80 to only 5mm. (This is not invariably true; some 60-year-old eyes can still dilate to 7mm, and some younger eyes cannot dilate to a full 7mm.)

For maximum light gathering, you want the exit pupils of your binocular and telescope to be no larger than the entrance pupils of your dark-adapted eyes. This delivers the maximum possible amount of light and allows you to see the brightest possible image. If the exit pupil of the instrument is larger than your entrance pupil, you waste light.

To calculate the exit pupil of a binocular, divide the objective size by the magnification. For example, a 7X50 binocular delivers an exit pupil of 50/7=7.1mm, while a 10X50 binocular delivers an exit pupil of 50/10=5mm.

To calculate the exit pupil of a telescope, divide the focal length of the eyepiece in millimeters by the focal ratio of the scope. For example, a 25mm eyepiece used in an f/5 scope delivers an exit pupil of 25/5=5mm, while a 35mm eyepiece in the same scope delivers an exit pupil of 35/5=7mm.

Additional content appearing in this section has been removed.
Purchase this book now or read it online at Safari to get the whole thing!

Content preview·Buy reprint rights for this chapter

See more with both eyes.

Inexperienced astronomers often think of binoculars as poor substitutes for a telescope, something to use only if you don't have a scope. That's a mistake. In fact, a binocular is essential observing equipment even if you own a dozen top-notch telescopes. Experienced observers use binoculars to orient themselves within the constellation when they are locating objects they intend to view with their scopes—to place the object within its context of bright surrounding stars. A binocular is also useful for planning star hops [Hack #21] because the dimmest stars visible in a standard binocular are of similar magnitude to the dimmest stars visible in the optical finder of a typical telescope.

Figure 1-4: Barbara measures her entrance pupil using an Allen wrench

Binocular is properly a single noun. Calling a single instrument a "pair of binoculars" is incorrect. "May I use your binocular" is grammatically correct; "May I use your binoculars" isn't, unless you mean two or more instruments. (Actually, asking to use someone's binocular is an etiquette faux pas among astronomers—nearly equivalent to asking to borrow someone's toothbrush—because most people have their personal binoculars adjusted to their own vision. If you do borrow a binocular, don't change the diopter adjustment.)

But binoculars are more than just an adjunct to telescopic observing. While the narrow fields of view of telescopes limit you to seeing just the trees, the wide fields of binoculars let you see the whole forest. For viewing large open star clusters, Milky Way star fields, comets, and other large objects, binoculars are often the best choice. On more than one occasion, we've set up our telescopes only to find at the end of the evening that we'd never used them. Instead, we'd spent the entire observing session using our binoculars to study the heavens.

Additional content appearing in this section has been removed.
Purchase this book now or read it online at Safari to get the whole thing!

Content preview·Buy reprint rights for this chapter

Understand the advantages and disadvantages of popular scope types.

If you want to start a war, just ask a group of astronomers what the best type of scope is. Everyone agrees that "junk" scopes should be avoided, of course, but that's about the extent of the agreement. There are many types of scopes, all of which have advantages and drawbacks relative to the other types. Each type of scope has proponents and detractors, and the debate can become quite heated. In this hack, we'll attempt to provide unbiased advice about the strengths and weaknesses of each type of scope.

Just so that you're aware of it going in, we confess that we're "Dob bigots." We tried to get an "SCT fanatic" we know to help us with this hack, but he's not speaking to us. Neither is a "refractor maniac" we know. And they're not speaking to each other. We're only kidding, of course, but feelings do run high when people start debating the best scope type. The Coke–Pepsi, PC–Mac, and Linux–Windows wars are nothing compared to the scope-type wars.

Here are the three most important characteristics of a telescope:

Aperture: The aperture of a telescope is the diameter of its primary mirror or objective lens, which may be specified in inches or millimeters. Amateur telescopes have apertures ranging from 60mm (2.4") to 30" or more. Aperture determines the amount of light a scope can gather, the fineness of detail it can resolve, and the maximum and minimum useful magnifications for the scope.

Light gathering is proportional to the square of the aperture. For example, a 10" scope gathers four times as much light as a 5" scope. The amount of light gathered determines how "deep" the scope can go. Larger aperture allows you to see dimmer objects (and more detail in all objects) than a smaller aperture.

Resolution is proportional to the aperture. For example, a 10" scope can resolve detail twice as fine as a 5" scope (assuming equal optical quality and steady seeing conditions).

Additional content appearing in this section has been removed.
Purchase this book now or read it online at Safari to get the whole thing!

Content preview·Buy reprint rights for this chapter

Choose the perfect grab-'n-go scope for observing at a moment's notice.

You can, of course, use your primary (or only) scope for urban observing if you are limited to doing so by budget or inclination. But to take advantage of the spur-of-the-moment aspect of urban observing, many serious amateur astronomers keep a special telescope dedicated to that purpose. It's called a grab-'n-go scope or quick-look scope, which describe its purpose perfectly. A grab-'n-go scope is always set up and always ready to use on a moment's notice. You can carry it out the door and start observing immediately. Figure 1-13 shows our grab-'n-go scope, a 90mm long-tube refractor that we leave set up and ready in our library. (We caught it with its pants down; ordinarily, this scope has a Telrad unit-power finder attached.)

Figure 1-13: A grab-'n-go scope, set up and ready to go

Here are some considerations for choosing the best grab-'n-go scope for urban observing:

Portability: Portability is critical. The ideal grab-'n-go scope should be light enough to pick up with one hand and carry out the door, and should be small enough to be stored unobtrusively in any handy corner or closet. A light, portable scope will be used often. A heavier, less portable scope will sit in the corner gathering dust. The differences can be substantial. For example, our 90mm refractor came with an equatorial mount and weighed more than 30 pounds. We replaced the equatorial mount with an alt-azimuth mount, reducing the total weight of scope and mount to about 10 pounds. That difference may sound minor, but it's not, particularly after a long day at work.
Fast set up and tear down: Urban observing is often done on the spur of the moment. You take the dogs on their evening walk and notice that the sky is particularly clear that night, or driving home from a party, you notice that Mars is ideally placed for a quick look. If your grab-'n-go scope is set up and ready, all you need to do is carry it out the door, set it down, and start observing. Conversely, if you know it'll take 10 or 15 minutes to set up the scope, polar align it, align the finder, and so on, chances are you'll let that observing opportunity go by. The same holds true, in spades, for tear down time. When you've stayed up past your bedtime watching the dance of Jupiter's moons, the last thing you want to face is another 10 or 15 minutes of tearing the scope down and packing it away.

Additional content appearing in this section has been removed.
Purchase this book now or read it online at Safari to get the whole thing!

Content preview·Buy reprint rights for this chapter

Locating and observing astronomical objects requires developing a special set of skills and practices, most of which are not intuitive. It requires a detailed knowledge of the night sky and of specialized astronomical terminology and conventions. There are things you must know and be able to do if you are to be successful.

Just finding the object you want to view can be difficult. The night sky is huge, and many astronomical objects are tiny, dim things. Even after you have found the object and verified its identity, teasing out the maximum possible amount of visible detail is very challenging.

We've watched many beginning observers encounter the same frustrating problems—what we call the "newbie blues"—and we've helped more than a few of them over the hump. All of them, particularly those who have go-to scopes, hope there are shortcuts to learning to observe. There are no shortcuts. A go-to scope is no better substitute for learning the night sky than an automatic transmission is for learning how to drive. Learning to observe is a hard-won skill, but one you can be proud of achieving.

In this chapter, we tell you what you need to learn, know, and do to locate, describe, and observe astronomical objects.

Have you ever wondered why all cats are gray in the dark?

Our eyes function in two entirely different modes, depending on how much light is available. In daylight or bright artificial light, our eyes function in day vision mode. After dark, our eyes shift to night vision mode. The physiological changes that occur in our eyes during the shift from day vision to night vision are called dark adaptation. Dark adaptation occurs slowly, typically requiring 25 minutes for 80% adaptation and 60 minutes for 100% adaptation. That's why astronomers get upset when someone shows a bright light.

When we move from dim light to bright light, our eyes undergo physiological changes called light adaptation. But while dark adaptation occurs slowly, light adaptation occurs quickly, in two phases. During

Additional content appearing in this section has been removed.
Purchase this book now or read it online at Safari to get the whole thing!

Content preview·Buy reprint rights for this chapter

Locating and observing astronomical objects requires developing a special set of skills and practices, most of which are not intuitive. It requires a detailed knowledge of the night sky and of specialized astronomical terminology and conventions. There are things you must know and be able to do if you are to be successful.

Just finding the object you want to view can be difficult. The night sky is huge, and many astronomical objects are tiny, dim things. Even after you have found the object and verified its identity, teasing out the maximum possible amount of visible detail is very challenging.

We've watched many beginning observers encounter the same frustrating problems—what we call the "newbie blues"—and we've helped more than a few of them over the hump. All of them, particularly those who have go-to scopes, hope there are shortcuts to learning to observe. There are no shortcuts. A go-to scope is no better substitute for learning the night sky than an automatic transmission is for learning how to drive. Learning to observe is a hard-won skill, but one you can be proud of achieving.

In this chapter, we tell you what you need to learn, know, and do to locate, describe, and observe astronomical objects.

Additional content appearing in this section has been removed.
Purchase this book now or read it online at Safari to get the whole thing!

Content preview·Buy reprint rights for this chapter

Have you ever wondered why all cats are gray in the dark?

Our eyes function in two entirely different modes, depending on how much light is available. In daylight or bright artificial light, our eyes function in day vision mode. After dark, our eyes shift to night vision mode. The physiological changes that occur in our eyes during the shift from day vision to night vision are called dark adaptation. Dark adaptation occurs slowly, typically requiring 25 minutes for 80% adaptation and 60 minutes for 100% adaptation. That's why astronomers get upset when someone shows a bright light.

When we move from dim light to bright light, our eyes undergo physiological changes called light adaptation. But while dark adaptation occurs slowly, light adaptation occurs quickly, in two phases. During αadaptation, which requires about 1/20th of a second, the sensitivity of the retina drops by 50% or more. During βadaptation, which requires from one to several seconds, the sensitivity of the retina drops more gradually, and we recover full color vision and visual acuity.

There are many misconceptions about night vision and dark adaptation, even among astronomers. To understand the process of dark adaptation, you need to understand something about the physiology of the human eye. Our eyes have two types of light sensors, called rods and cones. Rods provide monochromatic vision, but are very sensitive to light. Cones provide full color vision, but are relatively insensitive to light.

Cones and rods are unevenly distributed over the surface of the retina. Cones predominate in the fovea, the center of the retina, where they are densely packed. The fovea contains about 200,000 cones in an area of about one square millimeter, and thus provides acute resolution of fine detail. The entire retina contains about only 7,000,000 cones. That means cones are very sparsely scattered outside the fovea, just enough to show brightness and color with little detail in your peripheral vision. Rods predominate outside the fovea. The entire retina contains about 130,000,000 rods. They are less densely packed—at about 90,000 per mm

Additional content appearing in this section has been removed.
Purchase this book now or read it online at Safari to get the whole thing!

Content preview·Buy reprint rights for this chapter

Stay dark adapted and keep stray light from the eyepiece (and keep your ears warm).

Night vision is all-important when you observe DSOs. For those fortunate enough to have access to a truly dark observing site, it's not difficult to preserve night vision using standard methods—red LED flashlight, covering your notebook computer screen with red film [Hack #44] , and so on. But for many astronomers, the only sites within easy driving distance are, at best, semi-dark. The problem with these sites is often not so much general light pollution as local light pollution—the presence of streetlights and other nearby bright light sources.

For example, our regular "dark" observing site routinely offers mag 5.5+ skies, and on good nights mag 6.0 or better [Hack #13] . In terms of general light pollution, that's a respectable DSO observing site, at least by Eastern U.S. standards. Unfortunately, there are half a dozen mercury-vapor lights within a few hundred yards of the site. Their combined light makes it impossible to become fully dark adapted. In fact, it's bright enough to read a newspaper on the observing pad, literally. Because the site is on private property, it is impossible to install permanent screens against the local light pollution. Portable screens are impractical for various reasons.

Fortunately, there is a cheap, easy solution to such local light pollution problems, as long as you don't mind looking like a complete idiot. All you need is an old towel and a pirate's eye patch. Figure 2-1 shows Robert working at the chart table, looking like an idiot, but with his night vision intact.

Figure 2-1: Robert, fully equipped with eye patch and towel at the chart table…

Dark adaptation occurs individually for each eye. That means you can keep one eye completely dark adapted by covering it with the eye patch whenever you are not using it to look through the eyepiece. The other eye is never fully dark adapted, but that doesn't matter. You use it for other purposes, such as locating objects with your notebook computer or charts, or recording observations on your log sheet. For that matter, Robert sometimes uses his "regular" eye to locate objects in the finder and Telrad, for which full dark adaptation is less important.

Additional content appearing in this section has been removed.
Purchase this book now or read it online at Safari to get the whole thing!

Content preview·Buy reprint rights for this chapter

When speaking of brightness, magnitude and surface brightness are two terms you'll hear a lot. Understand what these terms mean and how they relate to one another.

For astronomy, the primary purpose of a binocular or telescope is to gather light, allowing you to see dimmer objects than are visible to the naked eye. Before you begin observing, it's essential to understand the limits of your viewing apparatus, be it the naked eye, a binocular, or a telescope. Otherwise, you could spend hours looking for objects so dim you'll never be able to see them.

Figure 2-3: Robert, smoking while preserving his night vision

The magnitude of an object is a numerical quantification of its brightness. Ancient observers first categorized stars by their brightness, describing the brightest stars as being "of the first magnitude." Slightly dimmer stars were grouped as second magnitude, and so on, down to the dimmest stars visible to the naked eye, which the ancients described as sixth magnitude. Dimmer objects accordingly have numerically larger magnitudes.

The Greek astronomer Hipparchus created the first known star catalog in about 120 BCE. This catalog contained 1,080 stars visible to Hipparchus from his latitude. He organized these stars into constellations, described the position of each star relative to other stars, and rated their brightness from first to sixth magnitude. In about 125 CE, the Egyptian astronomer, cartographer, and geographer Ptolemy updated the Hipparchus catalog in his famous book Mathematical Syntaxis, usually called The Almagest. Ptolemy added a few northerly stars that Hipparchus had missed, and added more southerly stars that were visible from his Alexandria observatory at 31°15'N but had not been visible to Hipparchus in his observatory at 36°15'N on the island Rhodes. The magnitude system used by Hipparchus and Ptolemy is still in use today, with only minor modifications.

Additional content appearing in this section has been removed.
Purchase this book now or read it online at Safari to get the whole thing!

Content preview·Buy reprint rights for this chapter

Learn common star names and how they're pronounced so the other kids won't laugh at you.

From remotest antiquity, every culture has given names to the brightest stars. Such names are called the common name or proper name of a star. Several hundred of the brightest stars have common names, but most amateur astronomers know and use only a few dozen.

One star may have many common names. Vega, for example, the brightest star in the constellation Lyra, is said to have more than 50 known names.

Hundreds more, no doubt, are lost in the mists of time. Many stars have similar names because they share common root names. For example, the syllable "al" is Arabic for "the" and appears in many common star names, as does "deneb" for "tail."

The same common name is sometimes used for more than one star. When used without qualification, that practice is fortunately limited to less prominent stars. For important stars that share a name, that name is qualified for at least one of the stars. For example, the name Rigel used alone refers unambiguously to the brightest star in Orion. Another bright star named Rigel exists in the constellation Centaurus, but that star is always referred to as Rigel Kentaurus to avoid confusion.

Some common star names—including Sirius, Procyon, Castor, and Pollux— originated with ancient Greek astronomers like Hipparchus and Ptolemy and have come down to us unchanged. The Romans, great engineers but poor scientists, also contributed a few common star names, including Arcturus, Bellatrix, Regulus, and Vindemiatrix. A few common star names, like Polaris and Cor Caroli, are Latin but of relatively recent origin. But the vast majority of common star names come from the Arabic.

The pronunciation and even the spelling of many common star names varies. Original pronunciations and spellings have often been lost or corrupted beyond recognition. For example, the star Almach in Andromeda may be spelled Almaak, Almaach, Almaak, Almak, or even Alamach, with similarly differing pronunciations. Vega is properly pronounced WAY-guh, but if you say it that way people think you're strange. The common pronunciation is VAY-guh, with VEE-guh also sometimes heard.

Additional content appearing in this section has been removed.
Purchase this book now or read it online at Safari to get the whole thing!

Content preview·Buy reprint rights for this chapter

Learn about the stellar catalogs used by amateur astronomers.

Of the thousands of stars visible to the naked eye—and millionsvisible with optical aid—only a few hundred of the brightest have proper or common names, such as Vega or Sirius. Although proper names are convenient for referring to bright "guidepost" stars, astronomers also need unambiguous designations for dimmer stars.

It is possible, although awkward, to specify a particular star by giving its coordinates [Hack #17] . For example, we could say, the star located at 14h15m40.35s right ascension and +19°11'14.2" declination, but that gets old fast. As a more convenient alternative, astronomers have developed star catalogs, which assign each star a unique short identifier, such as α-Boötis or HIP 69673. (All three of these designations specify Arcturus, the brightest star in the constellation Boötes.)

Early astronomers began the work of cataloging the stars visible to the naked eye, dividing the stars by constellation and then assigning unique identifiers to each. Modernday astronomers have continued that practice, but they now treat the sky as a contiguous whole and catalog stars without respect to constellation. Surprisingly, the star naming syntax of some catalogs produced 300 to 400 years ago remains in common use. Here are the stellar catalogs you need to be familiar with.

The German astronomer Johann Bayer (1572–1625) published Uranometria, the first comprehensive star atlas, in Augsburgin 1603. Uranometria predates telescopes, so it contains only stars that are visible to the naked eye. Uranometria was unique for its time because it mapped far southerly stars, including those in south circumpolar constellations. In Uranometria, Bayer introduced his system of labeling the bright stars in each constellation with lowercase Greek letters, a system that is still used today.

Bayer designated the brightest star in each constellation alpha (

Additional content appearing in this section has been removed.
Purchase this book now or read it online at Safari to get the whole thing!

Content preview·Buy reprint rights for this chapter

Although stars are important, constellations place those stars in context: constellations are the things you can find in the sky most quickly. And once you've found the constellation that's your point of reference, you can go on to find what you're really looking for.

The first step in learning the night sky is to know the constellations. Before you attempt to identify constellations in the night sky, you should know the names of the constellations you are looking for and how to pronounce those names. There are 88 official constellations. Fortunately, you can learn them in groups because only some of them are visible, according to your latitude and the time of year.

The best way to learn the constellations is to buy a planisphere [Hack #6] , which allows you to dial in the date and see an accurate representation of the night sky for that date. Alternatively, the monthly sky charts in Astronomy and Sky & Telescope magazines provide similar whole sky views for the current month.

Table 2-4 lists all 88 modern constellations, including the following data:

Name: The official name of the constellation, as assigned by the International Astronomical Union (IAU), the only body authorized legally to name celestial objects.
Pronunciation: The pronunciations shown are those commonly used by amateur astronomers. Although many are incorrect, these pronunciations are used so commonly that there is little point in attempting to correct them. For example, Orion is correctly pronounced oh-REE-un rather than oh-RYE-un, and Virgo is WIR-goh rather than VUR-goh, but saying oh-REE-un or WIR-goh will draw strange looks from your observing buddies.
Abbr: The official International Astronomical Union (IAU) abbreviation for the constellation name.

Additional content appearing in this section has been removed.
Purchase this book now or read it online at Safari to get the whole thing!

Content preview·Buy reprint rights for this chapter

Orient yourself to the night sky.

Celestial coordinate systems are used to specify the locations of stars and other astronomical objects. Four celestial coordinate systems exist, but only two of them are used commonly by amateur astronomers. Celestial coordinate systems are analogous to the geographic coordinate system of latitude and longitude used to specify locations on the earth's surface, but they are used instead to specify locations on the celestial sphere. The four coordinate systems differ only in what they designate the fundamental plane, called the equator, which bisects the sky into two hemispheres along a great circle.

The horizontal coordinate system, also called the altitude-azimuth or alt-az coordinate system, uses the local horizon as the equator, with the two poles straight up (the zenith) and straight down (the nadir). The location of an object is specified by two values called altitude and azimuth. Each of these is denominated in degrees (°), minutes ('), and seconds ("). One degree is divided into 60 minutes, and one minute is divided into 60 seconds. Accordingly, one degree contains 3,600 seconds.

Altitudes and azimuths are sometimes specified in decimal degrees. For example, 63°30' may also be written as 63.5°, because 30' is equal to 0.5°. Using decimal degrees allows you to specify the location of an object with very high precision, for example 63.5342°. Locations specified in degrees/ minutes/seconds use decimal fractions of a second to achieve the same fractional precision. For example, a decimal value of 63.5342° can be converted to 63°32'3.172" by multiplying the fractional degree 0.5342 by 60 to yield 32.052', and the fractional minute 0.052 by 60 to yield the fractional second 3.172".

Objects above the horizon have positive altitudes, and those below the horizon have negative altitudes. An object exactly on the horizon has an altitude of 0°. An object directly overhead (at zenith) has an altitude of +90°. An object that is straight down (through the earth, at nadir) has an altitude of 90°.

Additional content appearing in this section has been removed.
Purchase this book now or read it online at Safari to get the whole thing!

Content preview·Buy reprint rights for this chapter

Make the most of your observing time by taking along the charts you need.

Observing time is precious, particularly for DSO observers [Hack #22] , many of whom must drive several hours to a dark-sky site on a new moon weekend and hope for clear skies when they arrive. Many DSO observers have half a dozen or fewer opportunities per year to observe under optimum conditions, so it's critical to make the most of that limited observing time.

The best way to do that is to be prepared before you set up your scope. Know which objects you plan to observe, and know exactly how you're going to find them. By "exactly" we mean not just knowing the general location of the object, but having a detailed plan, including Telrad [Hack #53] and finder circles plotted and, if necessary, a detailed star hop [Hack #21] worked out.

Many observers, including experienced ones, make the mistake of hauling their star atlases and a notebook computer to the observing site, intending to use those resources to locate objects on-the-fly. While you can do it that way—and we would certainly never be without star atlases and our note-book computer—it's not the best use of your observing time. With everything pre-planned, we can usually locate one of our target objects in a minute or two at most. If we have to figure out how to locate the object as we go along, it may take as much as half an hour to locate the object through trial and error and then to verify that we've in fact located the object we were looking for rather than some other nearby object. Instead of using your formal charts and planetarium software routinely, reserve them for special needs that arise while you're observing.

In general, for formal observing trips, we try to devote at least two hours of planning time for each hour of expected observing time, which is to say 12 hours of planning time for a typical six-hour observing session. (It gives us something to do on those cloudy nights, and planning an observing session is the next best thing to actually observing…) For

Additional content appearing in this section has been removed.
Purchase this book now or read it online at Safari to get the whole thing!

Content preview·Buy reprint rights for this chapter

Refrigerator magnets aren't just for refrigerators any more.

Think back to the last time you tried to find a difficult object. You probably sat at your chart table, trying to memorize the relative positions and magnitudes of several field stars near the object. You then returned to your scope and scanned around a bit, trying to locate that pattern of stars in the eyepiece. Before long, you weren't entirely sure what you were looking for. Back to the chart table. Back to the eyepiece. Back to chart table. Rinse and repeat.

If you have one of the incredibly popular steel-tube Chinese or Taiwanese Dobs, there's a better way. Print out (or copy) the chart, take it with you to the eyepiece, and stick it right on the tube using refrigerator magnets, as shown in Figure 2-14

Figure 2-14: Using a chart right at the eyepiece

In fact, nothing limits you to having just one chart. A strong magnet will hold half a dozen or more sheets of paper, ready for immediate use.

Additional content appearing in this section has been removed.
Purchase this book now or read it online at Safari to get the whole thing!

Content preview·Buy reprint rights for this chapter

You can find things quickly by reckoning from the easy-to-find objects, imagining some lines between them, and going in for the kill with a bulls-eye finder.

The human eye and brain are superbly evolved for detecting patterns. We see lines, angles, and patterns even where none exist, particularly when we look at bright stars against the velvety black background of the night sky. You can take advantage of this by creating imaginary lines and patterns on the celestial sphere, and using those lines and patterns to locate objects geometrically. Geometric navigation allows you to jump directly to objects in seconds rather than spend minutes tracking them down by following a path of dim stars.

A unit-power finder [Hack #53] like the Telrad (shown in Figure 2-15) is an essential aid to geometric navigation. The Telrad works like the heads-up gunsights in WWII fighter planes. When you look through the Telrad, you see a dim, red bulls-eye target against the background sky. The Telrad circles are 0.5°, 2°, and 4°, which means you can use a Telrad to locate almost instantly any object within 4° of a bright star or other easily identifiable object.

Figure 2-15: The Telrad unit-power finder

For example, let's say you want to locate Messier Object 79 (M79), a globular cluster in the constellation Lepus, shown in Figure 2-16. You could track it down the hard way, by star hopping [Hack #21] from the star 9 β-Leporis (Nihal), following a trail of dim stars (not shown in the graphic) in your optical finder until you eventually end up with M79 in your eyepiece. Although doing it that way might give you a sense of accomplishment, it might also take 5 or 10 minutes that could be better spent looking at the object.

Figure 2-16: Locating Messier 79 with a Telrad

Additional content appearing in this section has been removed.
Purchase this book now or read it online at Safari to get the whole thing!

Content preview·Buy reprint rights for this chapter

When a Telrad is not enough, find and focus on patterns, and then hop pattern by pattern to the target.

Although the Telrad allows you to locate many objects with amazing speed [Hack #20] , it's not a panacea. Some parts of the sky are simply devoid of the bright stars that you need to orient your scope with a Telrad. To find an object in those barren parts of the sky, you need to star hop.

Star hopping is the process of locating an object by beginning at a bright "guidepost" star and then using your optical finder to follow a trail of dimmer stars until you arrive at the object. The secret to star hopping is to plan the hops so that each hop provides a distinct pattern of reasonably bright stars in the finder. When the pattern is right, you know that the finder is pointed exactly where you think it is, and you can then move the finder to locate the next pattern of stars.

Before you attempt to star hop, you need to know the field of view of your optical finder and how objects are oriented in it. You can calculate the field of view precisely by drift testing [Hack #57] or by using the finder to look at star pairs with known separations [Hack #56] . A correct-image finder (rightangle or straight-through) provides an image that is correct top-to-bottom and left-to-right. A traditional straight through finder provides an image that is correct left-to-right, but inverted top-to-bottom. If you are using the latter type of finder, simply invert your star charts to make them correspond to the view in the finder.

You can plan a star hop using either printed charts or planetarium software on your computer. We much prefer using planetarium software because it lets us print out custom charts set to whatever limiting stellar magnitude [Hack #13] we want, and it prints finder circles directly on the charts.

There are two ways to plan a star hop using printed charts. The first method, although it is more commonly used, is less desirable:

Additional content appearing in this section has been removed.
Purchase this book now or read it online at Safari to get the whole thing!

Content preview·Buy reprint rights for this chapter

Seeing a black cat in a coal bin at midnight is easier than seeing some of these dim objects.

One of the major challenges that every beginning astronomer faces is learning to see. That sounds stupid, we know. After all, you've been using your eyes every waking moment all your life. How hard can it be?

Very hard, as it turns out. In daily life, you look at brightly lit, colorful, high-contrast objects that are familiar to you. When you observe DSOs (deep-sky objects—nebulae, galaxies, and so on) you look at unfamiliar, gray, dim objects with almost no contrast. You have to retrain your eyes, your brain, and your way of thinking if you want to see dim astronomical objects.

Imagine a gray scale that runs from 0 (pure black) to 255 (pure white). In daily life, you see objects that span most of that range. In DSO observing, you may be trying to tease detail from an object with an average brightness of perhaps 2 or 3 against a sky background of 1 or 2. Under light-polluted skies, it's even worse because the background sky may be nearly as bright (or brighter) than the objects you are trying to see. That's why you need to get to a dark site to observe galaxies, for example.

Beginners are often flabbergasted at just how dim and low contrast many DSOs are. Even under dark skies, newbies often literally cannot see a "bright" DSO even if an experienced observer has centered it in the eye-piece. For example, one very clear, dark night we were with a group of several experienced observers and one relative newbie. Robert had the bright Messier galaxy M51 framed in the eyepiece of our 10" scope. The experienced observers were discussing the structure visible in M51, including knots, nebulosity, dust lanes, and the connector between NGC 5194 (M51) and its connected companion galaxy, NGC 5195. The newbie thought we were making it all up. He could see only a few stars in the eyepiece, but not even a hint of M51. We weren't making it up. All of that and more was visible to us, but the newbie had not yet learned to see.

Additional content appearing in this section has been removed.
Purchase this book now or read it online at Safari to get the whole thing!

Content preview·Buy reprint rights for this chapter

Things that go zip, zoom, phizz, DOH-DEE-DOH, and BOOM in the night.

For a long, long time, the stars were referred to as "The Fixed Stars" because it was believed that they were perfect and unchanging. Well, I'll forgive the ancients for thinking they were perfect. Given the lack of light pollution they had and their dependence on stars for navigation, time and date calculations, and religion, they had great knowledge and appreciation for the night sky. But they made one crucial mistake: they ignored things that didn't fit their idea of how things worked. Because it is plain to see, if you look closely enough, that the stars do change. There is a fair bit of action out in the universe.

It is possible to take note of this from your backyard (or, better, from your friends' backyards in the country). The most obvious mobile objects are the Sun and Moon. You may not even think about the Sun being a mobile astronomical object but it is a fine one (always use a safe solar filter when observing the Sun) and, of course, its motion in the sky gives us the day. The Moon, also, you've no doubt seen tooling around the sky, changing phase as it does so. And, whether you've ever explicitly noticed it or not, if you're reading this book, you're most likely aware that the planets move relative to the stars in our sky. The ancients knew about all of these objects moving in the sky and had nice, if incorrect, explanations for all this motion. But there is a lot going on they didn't know about and it makes for an interesting tour of the changing sky. You'll have to look carefully and occasionally take some notes.

Shooting stars have long intrigued people, and you've almost certainly wished upon one (how did that come about, anyway?). These meteors are little bits of fluff—bits of sand, flakes of dust—that hit our atmosphere and burn up in a flash. Every now and then, a bit more substantial chunk enters the atmosphere and burns as bright as the full Moon. These fireballs are impressive and almost completely random. The brightest one I ever saw I didn't actually see. I had my eye at the eyepiece when I noticed that the world was lit u

Table of Contents