Hack22.Learn to See DSOs

Hack 22. Learn to See DSOs

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 objectsnebulae, 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.

So how does an inexperienced observer learn to see what experienced observers call "faint fuzzies" or "lumpy darkness"? (Why one would bother is another question entirely, to which we reply, "because they're there.") Ultimately, the answer is to practice for as long and as often as you can. But there are several techniques that can help you along the way.

2.13.1. Learn Patience and Persistence

The sky varies from night to nightand even minute to minuteas do your eyes. If you are sure an object is in your field of view but you can't see it, keep looking. Even if the sky appears perfectly clear, there may be a slight haze or thin cloud obscuring the object. If the object doesn't reveal itself within a reasonable time, abandon that object temporarily and visit the next object on your list. Return to the missed object five minutes (or five hours) later, and you may be able to see it.

Even experienced observers have "bad nights" when for some reason nothing seems to come together. And we're not talking about nights with poor transparency or other problems beyond your control. We're talking about excellent nights when everyone else is doing very well, but for some reason you're just not able to see much of anything. We've had them. Every experienced observer has had them. When that happens, we just give up on the really faint fuzzies and start observing brighter ones, such as Messier objects. Often, the problem clears up as mysteriously as it developed.

2.13.2. Observe from a Dark Site

It's difficult to overstate the importance of a dark site for DSO observing. General light pollution brightens the background sky, making it difficult or impossible to separate DSOs with low surface brightness from the bright sky background. An object that is distinctly visible with a small scope or even a binocular from a dark site may be completely invisible in a large scope from a light-polluted site. Local lights are also a problem because they prevent you from becoming fully dark adapted.

2.13.3. Observe Objects When They Are High

The ideal time to observe any DSO is when it culminates (reaches its highest elevation). When an object is at low elevation, you're looking through a lot of air and haze, what we call muck. Also, the effects of light pollution are most severe at low altitudes. When the object culminates, it isas high as it gets and you're looking through the least possible amount of air, haze, and light pollution.

If possible, we avoid observing DSOs at less than 30° elevation. If the object is rising, we simply wait until it is high enough to provide a good view. Of course, some objects culminate [Hack #64] at less than 30° at our latitude, so we have no choice. Also, depending on the object and the season, the object may have set below 30° by the time it's fully dark. If so, we'll still observe it, but we'll make a note to observe it at another time of year when it is higher in the sky.

2.13.4. Dark Adapt Fully

For seeing dim DSOs, it's critical that you be fully dark adapted [Hack #11]. It can take as long as an hour to reach full dark adaptation, so an object that's invisible to you soon after full darkness may be clearly visible later in the evening when you are fully dark adapted.

2.13.5. Use the Largest Instrument Available

When it comes to seeing DSOs, APERTURE RULES. A larger aperture gathers more light, and more light is exactly what you need to see dim DSOs. We don't necessarily mean you should run out and buy the largest scope you canafford. There are other issues, not least transportability and convenience. (A scope so large and clumsy that you don't use it shows you less than a smaller scope that you actually use. Duh.) But if you have multiple scopes, use the largest one to do your DSO hunting.

Nor do we meanthat DSOs can't be observed with smaller instruments. Experienced observershave logged all of the Messier objects and even all of the Herschel 400 objects with only a 50mm binocular. Experienced golfers have also shot low rounds using a 2 iron as their only club. Just because you can do it that way doesn't mean you should.

2.13.6. Use Averted Vision

The center of your eye is very good for seeing detail, but very poor for seeing dim objects. When you observe a dim DSO, do not look directly atit. Instead, look off to one side. Objects that are invisible when you look directly at them become quite distinct with averted vision.

Using averted vision even on bright objects allows you to see more. For example, an inexperienced observer who locates a galaxy with a bright core often views the galaxy only with direct vision because, after all, it's visible that way. But if you don't use averted vision to view that galaxy, you're missing lots of available detail that falls below the threshold of direct vision.

2.13.7. Keep Both Eyes Open

When you look through the eyepiece of a telescope, it's almost instinctive to close the other eye. Don't do it. Squinting reduces your ability to detect dim objects and detail within those objects. Train yourself to observe with both eyes open, particularly when you look at dim objects. If there is enough local light to make that distracting, cup a hand over the eye you aren't using, wear an eye patch, or use a towel over your head to screen out the local lights [Hack #12].

2.13.8. Absorb Photons

Everyone knows that a camera can capture dimmer objects if you use a longer exposure time, but few people realize that the human eye works the same way. An object that is invisibly dim when you first begin to look at it may become visible after several seconds, as your eye accumulates photons. From our own experience and discussing it with other experienced observers, it seems that the human eye can benefit from "exposure times" as long as a few seconds. Beyond that, there is no additional benefit.

2.13.9. Tap the Tube

As predators, our eyes and brains are hard-wired to be very efficient at detecting motion. A very dim, low-contrast object may lurk just under the limit of visibility as long as it isn't moving. But if it moves, your predator brain kicks in. Aha! Motion! Pounce! But DSOs don't cooperate. They just sit there, unmoving, unless you help them along a bit. The secret is totap the telescope tube slightly, just enough to make the field of view jiggle. Often, an object you couldn't see will jump out at you as the view jiggles.

2.13.10. Use Different Magnifications and Fields of View

Conventional wisdom about viewing DSOs says you should use only low power, such as 15% or 20% of the aperture of your scope in millimeters. Wrong. The best magnification to use is the one that shows you the most detail in the object. It's true that low power (and a correspondingly large exit pupil) works well for many objects. But it's equally true that many DSOs are best viewed at relatively high power. For example, we often use 125X (50% of our 250mm aperture) to 270X (108% of our aperture) to view small, dim galaxies because that high magnification reveals details that aren't visible at low power.

Magnification and exit pupil are directly related. A magnification equal to the diameter of your primary mirror or objective lens in millimeters yields an exit pupil of 1mm. For example, if your primary mirror is 250mm in diameter, a magnification of 250X yields an exit pupil of 1mm (250/250 =1). In the same scope, 125X magnification yields a 2mm exit pupil (250/125=2).

Field of view is also important, particularly for large extended objects such as open clusters and some nebulae. Ideally, you want a field of view that frames the object in context. For example, if you view a tiny planetary nebula at low power, it looks just like another star in the field. But if you put some magnification on it, it is visible as an extended object within the star field. Conversely, if you view an open cluster with a narrow field of view, you may not be able to see the forest for the trees. The cluster stars look just like field stars. Using a wider field of view allows you to see the cluster as a cluster, set among a sparser group of field stars.

2.13.11. Focus Carefully

When an object is out of focus, its light is spread, becoming dimmer. An object that is on the edge of visibility when in focus may be dimmed sufficiently to be invisible if it is even slightly out of focus. Critical focusing is difficult, particularly under unsteady seeing conditions, where an object that is in focus at one moment may be slightly out of focus the next. It's worth tweaking the focus constantly as you observe an object to keep it in the best possible focus. You'll see more details that way.

2.13.12. Defocus Slightly to Locate Tiny Objects

Tiny objects, such as some planetary nebulae and galaxies, may appear stellar at low magnification. Using more power will reveal them as extended objects, but figuring out where they are in the star field can be difficult. One trick we use to locate such objects is to defocus slightly. With the image slightly out of focus, stars appear different from even small extended objects. Once you locate the extended object, you can apply more power to see the details.

2.13.13. Use Nebula Filters to Increase Contrast

Narrowband filters (such as the Orion Ultrablock or Lumicon UHC) and line filters such as the O-III and Hydrogen Beta can reveal detail in nebulae and comets that's invisible without filtration. In the most extreme cases, an object that is invisible without filtration in a telescope may be clearly visible naked eye with a filter. But even in less extreme cases, a filter can reveal detail that's invisible without the filter [Hack #59].

Don't limit yourself to one filter. Try different filters on each object, whether or not they're supposedly suitable for that object. For example, an O-III filter is usually thought of as a "planetary nebula filter," but there are planetaries for which a narrowband filter works better than an O-III. Also, different filters reveal different aspects of the object. For example, when we use narrowband and O-III filters on the Great Orion Nebula(M42), the narrowband filter shows a larger extent of nebulosity than does the O-III, but the O-III reveals details in the nebulosity that aren't visible with the narrowband filter.

An O-III filter is also handy for locating planetary nebulae, as opposed to just viewing them. Planetaries can be difficult to find because many of them are so small they appear stellar at low magnification. To locate a planetary quickly, once you are sure that it's somewhere in the eyepiece field of view, just move the O-III filter in and out of view between your eye and the eyepiece as you look through the eyepiece. The O-III filter dims most or all of the stars to invisibility, but it passes the light from the planetary nebula. The planetary nebula remains visible as the stars blink on and off.

2.13.14. Keep Looking

Even if you are able to see the object, you won't see all of the available detail in the first five seconds, or even the first five minutes. Keep examining the object, trying to tease out more detail. Once you have seen all the detail you can see in the object as a whole, focus on separate parts of the object, such as a dust lane or one arm of a galaxy, the fringe of a nebula, or the core of a globular cluster. When we view an object as a whole, our eyes and brain eliminate fine detail in favor of providing a gestalt of the object. When we direct our attention to a small part of the object, the object as a whole disappears, and we can see finer detail in that smaller area. It's almost fractal in its effect.

Being able to see fine detail in dim objects is an acquired skill, and one that must be practiced constantly if you are not to lose it. When you are first learning to see, it often seems that it's a matter of one step forward and two steps back. It's frustrating to hear an experienced observer describe an object as "bright" when you can barely see it. Keep at it, though, and you'll soon realize that you're seeing more and more detail in dimmer and dimmer objects. Before long, the newbies will be wondering how you can possibly be seeing details that are invisible to them.

    Astronomy Hacks
    Astronomy Hacks: Tips and Tools for Observing the Night Sky
    ISBN: 0596100604
    EAN: 2147483647
    Year: 2005
    Pages: 112
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