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Astronomy Hacks
By Robert Bruce Thompson, Barbara Fritchman Thompson
June 2005
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HACK
#25
Learn Urban Observing Skills
Observe the universe from your own backyard.
[Discuss (0) | Link to this hack]

Backyard astronomy isn't what it once was. As a teenager in the mid-60s, Robert was able to observe most of the Messier Objects and scores of other deep-sky objects (DSOs) from his urban backyard using his home-built 6" Newtonian reflector. Nowadays, alas, urban light pollution limits serious DSO observing to rural dark-sky sites.

TIP

Light pollution is the bane of DSO observers, and the only way to avoid light pollution is to travel to a dark-sky site. By "dark" we don't mean a suburban park, either. You'll need to travel at least 20 miles from even small towns and 50 miles or more from larger cities to find a truly dark site. But the view is worth the effort. You can see more with a small scope or even a binocular from a dark site than you can see with a large scope from a light polluted site. If you've never observed from a truly dark site, give it a try. You'll be shocked at the difference.

Fortunately, light pollution does little or nothing to hamper other types of observing. If your métier is Lunar and planetary observing, for example, you might as well stay in town. Luna and the planets are just as visible from the most brightly lit urban backyard as they are from the darkest rural site.

There is much to be said for urban observing. You can save yourself a drive, observe more frequently, spend more time observing, and always have a bathroom and a warm refuge available. On weeknights when you don't have time to drive to your dark-sky observing site, you can observe for an hour or two and still be finished in time for the 11 o'clock news. About the only thing you can't do from an urban site is observe galaxies and other dim DSOs.

Urban Observing Targets

Light-polluted urban skies limit your choice of observing targets in two respects. First, the background skyglow makes it impossible to see dim, low-contrast objects such as galaxies. The largest telescope in the world won't let you see such dim objects because the sky background is literally brighter than the object itself. Second, light pollution makes it difficult to locate objects because you can see so few stars. Trying to hunt down faint fuzzies from an urban site is an exercise in frustration. Fortunately, there are many other objects suitable for urban observing.

Luna

Luna (which we dedicated deep-sky observers think of mainly as an annoying source of light pollution) can be a very rewarding target. In fact, some amateur astronomers devote their careers to observing Luna, and seldom observe any other target. Luna is easily visible no matter how bad the light pollution, and under less than perfect viewing conditions. Luna is visible most nights of the month, can take high magnifications, and shows immensely more detail than any other astronomical object.

TIP

Although it sounds stupid, we sometimes observe Luna through haze and even light clouds. The atmosphere is often extremely stable on hazy nights, making them quite suitable for Lunar and planetary observing. We are generally limited by seeing (atmospheric turbulence) to about 300X, but on hazy nights we have run as much as 650X on Saturn, 900X on Jupiter, and 1,200X on Luna. No, you can't do that every night, and yes, you have to wait for short steady periods, which may occur only every few minutes and last only seconds, but when the atmosphere steadies down, it's amazing how much detail you can see at these "stupid high powers."

Planets

Although their presence in the night sky changes from season to season, the planets are a rewarding target for many urban observers. Only Pluto is nearly impossible for urban observing. (Not that you're missing much. At its best, Pluto looks like a 14th magnitude star [Hack #13].) The outer gas giants, Uranus and Neptune, show pretty colored discs, but little or no detail is visible. Although some people have logged the brighter Uranian moons from urban locations, we've never had any luck distinguishing these 14th and 15th magnitude objects from the background skyglow.

Mercury is possible near sunrise or sunset at some times of year, although it shows little more than a featureless disk. Venus is often quite high as the Evening Star or Morning Star at some times of year, but its heavy clouds conceal all surface features, leaving its current phase as the only detail of interest. We think Venus, like Mercury, is boring.

That leaves only three planets, but what planets they are. Jupiter, Saturn, and Mars (when it is near closest approach) are magnificent objects, worthy of extended study. We often delight in watching the dance of the Jovian moons, trying to tease out fine detail in Saturn's rings, and looking for the Martian seas and canals that Percival Lowell sketched in detail. (The canals really are visible. Honest. They're an optical illusion, but they are visible.)

Most planetary observers are also Lunar observers, and vice versa. If you focus your urban observing on Luna and the planets, you'll seldom be without something worthwhile to observe.

Double stars

Although we've never seen much point in it, many astronomers enjoy observing double-and multiple-star systems. Okay, we admit we're as impressed as anyone by the brilliant blue and gold Albireo pair or the beautiful ternary β-Monocerotis system, but how long can you look at just two or three stars? Long enough, apparently. We know of one amateur astronomer who's observed more than 1,000 double stars, all from his urban site. The Urban Observing Club of the Astronomical League publishes a good beginner list for urban double-star observing (http://www.astroleague.org/al/obsclubs/urban/urbanls.html). This list includes a baker's dozen of the most impressive multiple stars and variable stars.

Bright open clusters and emission/reflection nebulae

Although urban sites are not optimum for observing deep-sky objects (DSOs), many of the brightest open clusters and emission/reflection nebulae are visible easily from all but the most light-polluted sites. The AL Urban Observing Club publishes an excellent beginner's list with dozens of bright DSOs suitable for urban observers (http://www.astroleague.org/al/obsclubs/urban/urbanld.html).

Planetary nebulae

Although planetary nebulae often have high magnitudes, many have relatively high surface brightness. Locating these tiny objects from an urban site is challenging—the smallest planetaries often appear stellar even at medium magnifications—but if you can find them, you can often see significant detail in them.

Although in our experience the so-called "light pollution reduction" filters are of minimal help in reducing the effects of light pollution, they can be quite helpful in locating and viewing planetary nebulae from an urban site. If you have a larger scope, at least 8", an Oxygen-3 (O-III) line filter is probably the best choice for bagging planetaries in-town (just as it is from a darker site). If you have a smaller scope, or if you just prefer to buy a more generally useful filter, a narrowband filter like the Orion Ultrablock or the Lumicon UHC is also quite helpful on planetaries.

WARNING

Don't buy into the manufacturer hype about broadband "light pollution reduction" or LPR filters like the Orion Skyglow or the Lumicon DeepSky [Hack #59]. These filters are usually represented as being useful for reducing "moderate light pollution." If you buy one expecting to improve your urban viewing, you'll probably be disappointed. The visual improvement, if any, will be very small. Before you buy a broadband filter, see if you can borrow one to test for yourself.

Conversely, narrowband filters like the UltraBlock and UHC do indeed cut down background skyglow, but they do so at the expense of making the entire image quite dim. We consider them to be "nebula filters" rather than "light pollution reduction filters," but they are as useful for increasing nebular contrast from an urban site as they are from a dark site.

Globular clusters

Like planetary nebulae, globular clusters often have high magnitudes but relatively high surface brightness. Also like planetaries, globs are sometimes difficult to find. Small globs may appear stellar in a low-power eyepiece, so the best plan is to use the low-power eyepiece to get the scope into the right general vicinity and then use a higher-power eyepiece to verify and view the object. Unlike planetaries, globs do not benefit from a narrowband filter.

Urban Observing Tips

To make the most of your urban observing time, use the following guidelines:

Know what to look for, and when

Impromptu urban observing sessions are by definition not planned. But it's easy to take a lack of planning too far, and end up just standing outside waving your scope around, hoping to see something. You'll make better use of your observing time if you have some idea of what to look for ahead of time. Dedicated urban observers often make lists of the objects they would like to log, and then organize those lists by when the objects are best placed. For example, you might create a consolidated list of 200 objects and then divide them into monthly or bi-monthly lists. That way, if an opportunity for a quick observing session arises in mid-July, you can simply check your 1 July, 15 July, and 1 August lists to determine which objects are high in the sky that night. Well-organized urban observers take the moon phase into account, generating lists of only bright objects during times when Luna is up, and DSOs for times when Luna is not up.

Let Luna be your guide

As with all observing, the state of Luna determines what it's possible to see. During the new moon, or when the moon has already set or not yet risen, you have at least a chance to observe DSOs. The views won't be as good as those from a darker site, but the brighter DSOs can still provide satisfying views on clear, dark nights. If Luna is up, well, observe Luna.

Take the weather into consideration

Weather always determines the success of an observing session, but this is particularly true for urban observing. Cool, dry nights are usually good for observing. When a cold front moves through the area, it pulls air pollution, dust, and haze with it, often providing excellent viewing conditions with very stable air. Periods immediately after a heavy rain are often best of all; the rain does a wonderful job of clearing dust from the air, which improves atmospheric transparency, stabilizes the air, and reduces the effects of light pollution. (It doesn't matter how much light escapes into the air if there's nothing for it to reflect from.)

TIP

Atmospheric transparency is always important, but particularly so for urban observing. Use the Clear Sky Clock (http://cleardarksky.com/csk/) transparency forecasts to plan your urban observing sessions.

Observe late (or early)

Light pollution is greatest during the period from full dark until midnight or so. After midnight, some (by no means all) businesses have turned off their exterior lights, there are fewer cars on the road, and so on. The atmosphere is also often significantly clearer late at night. Dust has had a chance to settle, water vapor and some of the air pollutants have dispersed, and so on. If it fits your schedule, one of the best times for urban observing is often the hours just before dawn, which are usually the clearest and darkest times available in urban environments.

View near zenith

You can minimize the adverse effects of light pollution and poor transparency by observing objects when they are as high as possible. When your scope is pointed at zenith, you are looking through the minimum possible amount of dirty air and light pollution.

TIP

Of course, not all objects culminate anywhere near zenith, and observing near zenith with a Dobsonian or other alt-azimuth-mounted scope is problematic. When we observe with our Dob from our moderately to severely light-polluted suburban skies, we generally restrict ourselves to objects that are at least 60° and no more than 80° elevation. At much less than 60° the light pollution and atmospheric grunge makes it difficult to see objects; at much more than 80° Dobson's Hole becomes a problem.

Use planetarium software to orient yourself

One of the problems with urban observing is that few stars are visible, which may make it difficult to locate your target objects. At times, it can even be difficult to locate familiar constellations visually because you don't have the context of the other constellations. Using planetarium software [Hack #64] gives you a real-time map of where the various constellations are located in your night sky. If you are looking for the globular cluster M13 in Hercules on a July evening, for example, your planetarium software may tell you that M13 is located nearly dead west at an elevation of about 70°. Once you know that, it's much easier to locate Hercules. Once you've located Hercules, finding M13 is easy.

Master star hopping with finder and eyepiece

As much as we like unit-power bulls-eye finders like the Telrad, they are often of limited use under light-polluted skies. Using a Telrad effectively requires being able to see more stars than may be visible under urban skies. Fortunately, the same star-hopping skills that are so important under dark skies are also helpful under bright skies. The star hops are often longer because you must start from a brighter star farther from your object, but with patience and skill, it's possible to locate most objects via star hops.

At dark sites, star hopping with your finder allows you to locate nearly any object because the finder shows more than enough stars to allow you to map a course to the object. Under very bright urban skies, even the finder may not suffice. When that happens, use the light gathering power of your telescope with your widest-field eyepiece to take your star hops down to the eyepiece level. It takes longer to locate an object that way, certainly, but it can be done.

Enlist your neighbors' help

As annoying as the skyglow of general light pollution is, local light pollution is even worse. There's not much you can do about streetlights and similar light sources. Well, we do know many amateurs who turn them off temporarily by keeping a laser focused on the sensor, and at least one who claims to turn them off permanently with his .22/250 rifle, but in general you're stuck with them.

You can do something about neighbors' porch lights and flood lights, though. Sometimes, just asking them to turn off their exterior lights suffices. But if you want to convert them from grudging cooperation to enthusiastic participation, invite them over periodically to share the view. Once you've done that and they see for themselves how much difference the absence of local lights makes, at the very least they'll turn off their lights any time you ask them to. They may even invite you to come over and turn them off yourself any time you want to observe.

Reduce the effects of local light pollution

It's also worth the effort to improve your observing equipment and accessories. Make sure your optics are clean. Even a slight film of dirt or small smudges can reduce image contrast significantly under light polluted conditions. Install a dew shield or baffle to screen your objective from stray light. Flock your scope to reduce the effect of off-axis light. Use an eye patch to allow one eye to remain fully dark adapted, and use a towel or dark cloth to cover your head and eyepiece while viewing [Hack #12].

WARNING

Make sure people know what you're up to. More than one urban observer has been reported to the police as a Peeping Tom. One poor guy was setting up his scope in a public park when several police cars showed up with sirens blaring, responding to reports of a terrorist setting up a missile launcher. The best way to avoid misunderstandings is to tell everyone you see what you're doing and offer them a quick look through your scope.


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