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What You Need to Be A Successful Deep Sky Observer

Knowledge and Desire


Attics and garages around the world are full of unused telescopes. This is the unfortunate fate of the majority of instruments sold. Yet there are so many interesting observing projects that I couldn't begin to list them here. There is something for everyone in the night sky, it's just a matter of finding out which things are for you. 

The primary motivations of the deep sky observer typically fall into two categories. We'll call the first wonder. Deep sky objects are anything but flashy in the eyepiece. They typically aren't colorful, don't move, and most are so subtle that they are challenging to see at all. If it is flashy and colorful you desire, the planetarium gift shop has plenty of pictures for you. Seeing a deep sky object for yourself is like attending a sporting event versus seeing it on TV. There is a connection with the universe we inhabit that cannot be made through a photograph. The photons that enter your telescope from M31 traveled over 2 million years just to be recorded by your eye. An entire galaxy's worth of light, an incredibly vast area full of stars and gas and dust and planets, and it's all there waiting just for you. There is incredible beauty in the night sky, but it isn't the kind you find in photographs. It's the kind of beauty that is apparent only to those who understand the grandeur they behold. People are born, they mate, raise their children and they die. What a shame it is that more don't take the time to open their eyes and have a look around them while they are here. Observing is about taking the time to look; and the beauty lies in knowing what it is you are seeing.

We'll call the second the hunt. There are billions of people who inhabit this planet. How many of those can find M31 all by themselves? How many can even identify Andromeda, its constellation in the sky? Hunting down deep sky objects isn't easy. It requires knowledge and skill that most people don't posses. Finding anything in the sky is something to take pride in, whether it be the Great Nebula in Orion with binoculars or the Twin Quasar in a 20-inch Dob.

Knowing the Sky

The deep sky observer should know the constellations and brightest stars. These are your guideposts. You should also be aware of where north, south, east, and west are and how the sky moves over the course of a day or year.

The Dark Period

The moon is the natural enemy of the deep sky observer because it lights up an otherwise dark sky, rendering many objects invisible in the sky glow. You must learn the motion and phases of the moon in order to avoid its light. You must also be aware of the length of twilight at your location at the current time of year. There are calendars, handbooks, and software available that can tell you when it will be truly dark.

Recognizing and Predicting Good Observing Conditions

Not all dark skies are alike. We observe through an often turbulent and cloudy atmosphere. The two important factors are seeing and transparency.

Seeing describes how steady the atmosphere is. As light passes through air of differing temperatures it can be refracted, acting like a large lens. The effect is to bend the path of the light to and fro, causing the stars to appear to twinkle, distort, swell, and even dance about. Poor seeing has the effect of blurring and dimming your view, making faint objects and intricate detail more difficult to see. To quantify the seeing astronomers often use Seeing scales.

Transparency describes how much light is passing through the atmosphere. The journey of light through the atmosphere can be a perilous one; clouds and dust may keep some or all of it from ever reaching your scope. The transparency of the atmosphere is a very complicated subject, but it is clear that moisture plays a significant role. Even if the sky appears cloudless, there can still be a uniform haze that may keep you from seeing all that you might otherwise.

The Proper Time and Place

Take the sun. When it is high in the sky it is bright and can give you quite a sunburn. Yet when it is setting it is much dimmer and redder. This is the result of the amount of atmosphere the sunlight passes through. When an object is low in the sky, near the horizon, it passes through many times more air than when it is directly overhead. Astronomers quantify this with the Airmass unit. The amount of air light must travel though from directly overhead is termed an airmass of one. As you observe an object lower in the sky the airmass increases until at about 30o above the horizon you are looking through twice as much air as you would overhead, or two airmass. At this point the effects of the earth's atmosphere on your observation is doubled.

For this reason, in general it is best to observe an object when it is at its highest point in the sky, whenever possible. At the very least it should be more than 30o above the horizon.

Dark Adaption

Deep sky observers count on their eyes to see faint detail. In order for your eyes to perform at their best they must be completely dark adapted. It takes time for this adaption to take place--at least 30 minutes. You must shield your eyes from any light for this amount of time if you expect to see faint objects. Using a red flashlight to read your charts can help, but even a red light can do harm. If you observe in the presence of lights you may need to drape a dark cloth over your head at the eyepiece. Another useful item is an eye patch, available at any pharmacy. Keep the eye patch over your observing eye while reading your charts or in the presence of any lights. I highly recommend a pair of AstroGoggles as well. These goggles will shield your eyes from most of the light which causes them to react, allowing you to even go into a lighted room yet preserve much of your night vision.

Knowing What to Look For

I can't emphasize this point enough. You will be much more successful if you have a good idea what the object will look like ahead of time. Knowing how large it will appear in the eyepiece and how bright it will be are the two keys to success. Much of this comes via experience, which is why you should start with the brighter, easy objects first and work your way down to those more difficult.

Find out the true field of view of your eyepieces. One simple way to do this is to find a star near declination zero and time how long it takes to drift across the field of view (with the drive off if you have one). Your field of view in arc seconds will be the time in seconds times 15. Divide this by 60 to get the field of view in arc minutes.

Knowing What You Are Looking At

You will become bored with deep sky observing quickly if you don't take the time to learn about what you are seeing. Visit your local college bookstore and look for an Astronomy 101 textbook. These texts are written for freshman astronomy courses, use no math, and are appropriate for anyone interested in astronomy. I recommend a subscription to Sky & Telescope magazine as well.

next --> Skill and Patience

1. A Dark Observing Site
2. Knowledge and Desire
3. Skill and Patience
4. Good Finder Charts
5. The Choice of Instrument