How to see very faint things: for dim nebulae, star clusters, and galaxies, the weakest part of your telescope is your eye. Here's how to improve your reach.
You move over to look into the main telescope's eyepiece. What will you see?
Probably less than you were expecting--but more than the disappointing first look leads you to believe.
Let's say you've targeted Messier 101 (M101), a big spiral galaxy off the handle of the Big Dipper. At the eyepiece, with luck, you'll barely see a roundish gray smudge among the pinpoint stars. It may be so faint that you only occasionally glimpse it.
You've come up against a hard fact: our eyes evolved to work best in the daytime. At very low light levels, we can't see nearly as well as a camera does. Your eyeball view of a galaxy will never approach the galaxy's spectacular color photos.
This would even be true even if you were right there next to the galaxy, looking at it from the window of a starship.
But here lies the challenge. Many deep-sky objects--nebulae, clusters, galaxies--do reveal details when studied long and well. Hundreds of these ghostly glows and subtle spatterings make fascinating targets for amateur telescopes. Many of us pursue an ever greater collection of them for a lifetime.
After all, as Robert Burnham Jr., wrote in Burnham's Celestial Handbook:
Considered as a collector of rare and precious things, the amateur astronomer has a great advantage over amateurs in all other fields. ... Only a few mineralogists could hope to own such a specimen as the Hope Diamond, and I have yet to meet the amateur fossil collector who displays a complete Tyrannosaurus skeleton. In contrast, the amateur astronomer has access at all times to the original objects of his study. ... and there is no privilege like that of being allowed to stand in the presence of the original.
Of course, the bigger your telescope the better. The larger its aperture (the diameter of its main lens or mirror), the more dim light it gathers and sends to your eye.
Beyond that, though, deep-sky observing involves some tricks. Many of them are aimed at helping your daytime-animal's eyeball to work better in the dark.
Countering Light Pollution
For most of us, the biggest problem for deep-sky observing is light pollution--the artificial skyglow overhead from all the poorly designed and improperly aimed outdoor lights for miles around. For deep-sky objects, a dark sky matters at least as much as a big scope.
But don't despair! From a city rooftop or a suburban yard, you can take pleasure in the things you can see through the skyglow. It's a matter choosing your nightly projects to it your conditions.
New York City observer Jenny Worsnopp logged nearly the entire Messier catalog of 109 deep-sky objects with a 6-inch reflector from her rooftop in lower Manhattan. Sky & Telescope's Tony Flanders has done the same with a 7-inch from the city park in Cambridge, Massachusetts, pictured on page 13. Even when you determine that you cannot detect something despite examining its exact spot, that's worth an entry in your observing notebook. Having found its location once, it'll be easier to try for again on some dark country getaway.
Experienced observers know several ways to beat the sky brightness a bit:
* Work nearly overhead. The higher you look, the darker the sky. So when planning your deep-sky observing indoors, plan for objects that will be high overhead when you go out.
* Watch the daytime sky. The deeper and purer the blue of the sky is in the afternoon, the cleaner and more transparent the air is likely to be that night.
* Haunt the witching hours. Light pollution diminishes a bit after 11 or midnight, when many businesses and buildings turn off their lights and fewer car headlights are on the road.
* Switch to moderately high power once you've centered your object. High power penetrates light pollution better.
Your eye takes time to adjust to the dark. Your pupils expand to their full nighttime size within seconds. But the most important part of dark adaptation involves chemical changes in your retina, and these require many minutes.
After spending 15 minutes in darkness you might think your night vision is fully developed. But in fact your eyes may gain another two magnitudes of sensitivity--a factor of six in brightness!--during the next 15 minutes. After that, dark adaptation improves very slightly for up to 90 minutes more. So don't expect to see faint objects at their best until you've spent at least a half hour in the dark.
In practice, complete darkness is unattainable. Light pollution aside, you need some light to see what you're doing. For this astronomers use a dim red-light flashlight, because red light affects your night vision the least. You can use a rubber band to attach red paper or plastic over the front of an ordinary flashlight. Much better is a small red LED light, with its purer, deeper red.
Another trick is to observe with one eye and read your charts with the other. Keep your observing eye closed or covered with an eyepatch when not in use. Some observers dark-adapt one eye indoors by wearing an eyepatch for at least a half hour before going out.
Use Averted Vision
When you look directly at something, its image falls on your retina's fovea. This spot in the back of your eye is optimized for sharp resolution in bright light. But in dim light, the fovea is practically blind. So to see something faint, you have to look slightly away from it. Doing so moves the image off the fovea and onto parts of your retina that are more light-sensitive.
To see this for yourself, stare straight at a moderately faint star. It will disappear. Now shift your gaze just a bit; there it is again.
Practice concentrating your attention on things a little off to one side of where your eye is looking. This technique is called averted vision. You'll use it all the time when observing deep-sky objects.
How far to avert your vision is a matter of trial and error. Not enough and you don't get the full beneit. Too far and you lose the ability to resolve detail.
Your peripheral vision is also highly sensitive to motion. Under certain conditions, wiggling the telescope makes a big, dim ghost of a galaxy or nebula almost pop into view. When the wiggling stops, the object disappears again into the vague uncertainty of the sky background.
Use High Powers
You'll often read that low magniication works best for deep-sky viewing. After all, low power concentrates all of an object's light into a small area, increasing its apparent surface brightness (the amount of light per square millimeter on your retina). But this low-power-is-best assumption is usually false. Medium or high powers often work better on many faint deep-sky objects.
The reason is that your eye, unlike a camera, loses resolution in dim light. This is why you can't read a book at night even through you can see the book, and even though your large nighttime pupil theoretically resolves the letters on the page even more sharply than in daylight. Because of how the retina works, you see details in a faint image only if they are magnified to appear large--despite the fact that the surface brightness gets dimmer with higher magnification.
Up to a point, anyway. The ideal power for viewing a very dim object is a balance between making it appear large enough, and not reducing its surface brightness so much that it becomes totally invisible.
What does this mean for deep-sky observers? Simply that it's good to try a variety of powers (eyepieces) on any object.
For one particular class of object --nebulae of glowing gas--a special narrowband nebula filter can be a big help. These ilters block nearly all light except the narrow wavelengths emitted by certain nebular gases, boosting contrast.
Every deep-sky observer, even with a computerized telescope, needs to become good at poking around with detailed star charts. Standards are the Pocket Sky Atlas (which shows stars to as faint as magnitude 7.6), the larger Sky Atlas 2000.0 (stars to magnitude 8.5), or better still, the even larger Uranometria 2000.0 (stars to magnitude 9.75). Digital charts going even deeper are available for your laptop or phone.
This matters--because if you can tell the exact spot to examine, you can detect objects less than half as bright as you could otherwise. That's like widening your telescope's aperture by about 40%. A 6-inch scope becomes as powerful as an 8- or 9-inch! That's what good charts do.
Of course, you have to know how to use the charts to work from a known starting point to the correct spot. Read how in the box above.
Another deep-sky tip: Breathe deeply. Low blood oxygen begins to reduce night vision in just a few seconds. Breathing deeply can be easy to forget.
Most of all, be patient. If at first you see nothing where that treasured target is supposed to be, keep looking. Then look some more. You'll be surprised at how much more glimmers into view with prolonged scrutiny--another faint little star here and there, and just possibly the object of your desire. After you glimpse your quarry once or twice, you'll glimpse it more often. After a few minutes you may be able to hold it almost continuously where at first you thought there was nothing but blankness.
Your observing skills, like all abilities, improve with practice. Pushing your deep-sky vision to its limit is a talent that you will develop the more you do it.
The more detailed the sky charts you use --and the more you get to know them--the better your telescope will work.
Here's the bowl of the Big Dipper in three widely used sky atlases. In the top left of each frame, the dashed red circle shows the size of the 1[degrees] view you see in a typical 50x telescope eyepiece. Pretty tiny!
The lower-right bright star of the Dipper's bowl is Beta (P) Ursae Majoris, or Merak. Near it, notice the little galaxy symbol labeled M108 and the planetary nebula M97. The larger the chart's scale, and the more faint stars you have, the easier it will be for you to narrow in on the exact points to examine for these two dim little glows.
Above: On the Bright Star Atlas, with stars plotted only as faint as magnitude 6.5, there's little to guide you from Beta to M108 and M97, which are more than a field of view away. You have to move there pretty much blindly.
Right: On the Pocket Sky Atlas, with stars to magnitude 7.6, three faint stars now appear along the way to help guide you.
Below: On the larger Sky Atlas 2000.0, with stars to magnitude 8.5, there's room for M108 and M97 to be plotted with more realistically sized symbols.
Using a Chart at the Telescope
To find deep-sky targets using a detailed star atlas, you need to know two things.
1. HOW BIG is your finderscope's field of view on the chart?
The finderscope is the little sighting scope on the main instrument's side. You need a good one. A typical finder has a view that's 5[degrees] or 6[degrees] wide. Compare this to the declination scale, in degrees, along your chart's sides.
To determine the exact size of your finderscope's view, aim at a small naked-eye star pattern you know, such as Orion's Belt or the front stars of the Big Dipper's bowl. Find two stars that fit just inside the edges of the finder's view. See how far apart they are on your chart.
It helps to make a wire ring this size, as shown here, and keep it with your sky atlas. Slide the ring across the page to see just what star patterns to use to work your way to a target.
2. WHICH WAY are celestial north, east, south, and west in your view? You need to know this in order to turn the chart around to match what you see.
On detailed star charts, north is up. In the sky, north is the direction toward Polaris, the North Star, no matter what odd angle this may be. Nudge your scope slightly toward Polaris while looking in the eyepiece. You'll see new stars enter the view from the north edge. Turn your chart around to match.
East is left of north on star charts. In your eyepiece view, east is 90[degrees] counterclockwise from north if the view is correct-reading--such as you see in a straight-through finderscope or a Newtonian reflector.
But if you're looking at a mirror-image view (as usually comes from using a right-angle star diagonal at the eyepiece), east is 90[degrees] clockwise from north, and you'll have to flip the chart left-for-right to match what you see. Some people can do this in their head. Or, turn the paper over and shine a light up through the paper to read the printing through the back side. Or scan the chart on a scanner, flip it left-for-right in software, and print out this mirror-image version to use at the scope.
* ANOTHER TIP: To star-hop across a chart sure-footedly, look for little triangles and quadrilaterals of stars to compare with your view. Individual stars look alike--but every triangle is unique.
That's basically it! If you'd like to read more, including a guided tour walking you through an illustrated example in Gemini, see skypub.com/charts.
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|Title Annotation:||Visual Observing|
|Date:||Jan 1, 2013|
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