What You Need to Be a Successful Deep Sky Observer
Yes, it's true. The first thing people consider when they want to take up deep sky observing is their telescope. This is often the most expensive part of the hobby as well. But it's the least important. Seriously.
If you gave me the choice between a 40-inch telescope located near a city or a cheap pair of binoculars to be used at a dark sky site, I'd choose the binoculars every time. Believe me--I have experience with both. The telescope is just one part of the picture.
Nonetheless, a telescope is still important. Choosing the right instrument comes down to two primary factors: price and portability. I'll leave the price issue up to you. As for portability, you may be obsessed with a new 20-inch Dob, but if it won't fit through your back door, much less in your sedan, it isn't going to be nearly as much use as a good pair of binoculars.
Another aspect of portability is the level of difficulty you face in setting up your instrument. That 20-inch Dob may fit just fine in your van, but if you balk at the idea of hauling it out, setting it up and tearing it down, you may be better off with a smaller instrument.
As I mentioned earlier it is an unfortunate fact that it is the ultimate fate of the vast majority of telescopes to end up in the attic or buried in the back of the garage. The best telescope is the one you will use regularly. That is why most experienced observers will suggest that beginners start with a good pair of binoculars. Heck, even before you purchase a pair binoculars try taking your plain old eyes to a truly dark site. In all the reports you may read of faint Hickson galaxy groups or Abell planetaries it is easy to forget to state the obvious--no matter the level of experience, we all still enjoy drinking in a truly dark sky with nothing but our own two eyes.
It is seldom mentioned, and you won't find it in any catalog, but the most magnificent edge-on spiral galaxy in the sky is our own--the Milky Way. Seen from a dark site, it stretches from horizon to horizon, with star clouds and dark nebulae plainly visible to the unaided eye. Binoculars bring these features to the forefront and reveal intriguing knots and star clusters.
Deep sky observing is like a good book. You could jump right to the big ending, but it is best enjoyed from the beginning. The last chapter may be what gets the most attention, but that doesn't make the first any less enjoyable. This is a life-long hobby. Approach the sky like you would an onion; forget about the core--take it one layer at a time, starting from the outermost--start by learning the constellations and movements of the sun, moon and planets.
But I digress. If you are ready to buy a telescope, I've put together the following guide to help you determine which telescope is right for you. I'm not going to go into specific manufacturers or brands. Instead I hope to give you a general idea of what you need, leaving the actual shopping up to you.
Many people think that the magnification of a telescope is the most important thing, but that's not true for astronomical instruments meant for deep sky observing. The most important factor here is aperture. The aperture of a telescope is how big around it is. This determines how much light it gathers. Most objects in the sky are very faint, so the more light your telescope can gather the brighter and better they will appear. In addition, the larger the aperture, the more objects there will be within your reach.
I've arranged the following list of telescope classes by aperture to help give you an idea of what telescopes of different sizes can do.
Telescopes and binoculars under 4-inches in aperture
Whether it be a basic pair of 8x50 binoculars or a 90mm ETX, these instruments excel at wide field views and portability. In general, as the aperture of a telescope increases the largest practical field of view diminishes. The Pleiades cluster may look great in a 2-inch scope, whereas a 20-inch will only show a narrow view of one or two bright stars. Advantage, small scope.
With this class of instrument you can observe star clouds in the Milky Way, dark nebulae, large open clusters and a handful of the brighter nebulae and galaxies, such as M42 and M31. Many, if not all, of the Messier objects are visible, although you may not see much detail.
You can keep these instruments in your trunk or toss them into a suitcase. Their portability means that even if you move to a larger instrument later you will want to hang on to this one.
Telescopes 4 to 6 inches
When it comes right down to it this class of telescope may be all that you will ever require. These scopes are still small enough to take just about anywhere, are in general easy to set up, and will provide you with decades worth of objects to observe. They are also relatively inexpensive.
With this class of instrument all of the Messier Objects are within your grasp along with much of the NGC catalog. Visible are thousands of galaxies and hundreds of diffuse nebulae, planetary nebulae and star clusters. Many globular clusters resolve into myriad stars. The brightest quasar, 3C 273, is visible in a 6-inch.
My primary personal instrument for over 20 years was a 6-inch Newtonian and I never ran out of things to observe.
Telescopes 8 to 14 inches
This is the workhorse class of telescopes. They will do everything that the Messier class will, but more so. More galaxies and faint nebulae will be within their reach, and some objects will reveal a bit more detail.
They are, however, less portable and more expensive. If neither of these factors are an issue for you, then a telescope in this class is a very good choice.
Telescopes 16-inches or greater
These are the monster scopes with the monster price tags. Many of these instruments aren't portable at all, housed in their own permanent observatory building. As Dobsonians they can still be portable, but are much heavier, take up more space, and require more effort to set up. The eyepiece on most Dobsonians up to 18-inches can be reached with the aid of a simple step stool. Larger scopes will require a cumbersome ladder that you will have to clamber up and down. These scopes are a poor choice for the majority of beginners because finding your way about the sky is more difficult due to the smaller field of view. My advice to beginners is to start with a Pleiades or Messier class instrument and keep the Abell class as a dream for some dark night down the road.
With these scopes you will find galaxies
just about everywhere you look; hundreds of thousands of them. Many Hickson
and Abell galaxy groups will be observable. Many distant quasars will be
visible and for those with the necessary skill even a gravitational lens
or two. Globular star clusters that appear as fuzzy balls in lesser instruments
will become swarms of brilliant stars. Telescopes of this class really
excel at planetary nebulae--fuzzy blobs will begin to show details reminiscent
of photographs and many lesser known planetaries will become visible for
the first time.
Another decision you may need to make regards the type of telescope mount. These break down into two main classes, equatorial and alt-azimuth.
Equatorial mounted telescopes are aligned with the earth's axis of rotation such that they only need to move in one direction to follow targets in the sky. They usually have a clock drive motor that keeps objects centered in the eyepiece for long periods of time. This can be a great advantage when trying to carefully study an object at high power, take photographs, or show other people things in your scope.
These mounts are heavier, more bulky, more difficult to set up and not as easily pointed. They are also more expensive. To accurately follow the sky they require alignment to the celestial pole each time they are set up.
The most common alt-azimuth telescopes are Dobsonians. These are mounted like artillery; they move up and down (altitude) and around the horizon (azimuth). It is possible, but more difficult, to motorize these scopes to follow objects in the sky but most do not. Objects will continually move across the field, particularly at high magnification, requiring constant nudging to keep them centered. On the other hand, alt-azimuth mounts are much easier to set up and to point around the sky.
The eyepieces of your telescope are analogous to the speakers of your stereo. A poor eyepiece can ruin the view in even the most expensive, optically perfect telescope. Don't skimp.
The eyepiece you insert into your telescope determines two main things--the magnification and field of view. For deep sky observing you will want a good range of magnifications, from something under 100x for a good wide field of view, to something close to the theoretical magnification limit of your scope.
There are two numbers which describe an eyepiece; the focal length and apparent field of view. The actual numbers will depend on your telescope, but in general the smaller the focal length, the higher the magnification. For a given telescope and eyepiece focal length, an eyepiece with a larger apparent field of view will deliver a larger actual field of view when you look though it. Eyepieces with small apparent fields of view are like looking through a narrow paper tube. All of the information is there, but it isn't spread out very much. A larger field generally gives a more pleasing view. Eyepieces with larger apparent fields of view are also important at higher magnification when the telescope isn't motor driven to follow the stars. The object will take longer to drift across the field of view and therefore will remain in view much longer before you need to nudge the scope again to put it back.
One way to cheat with eyepieces is to use a Barlow lens. A Barlow slips into the telescope before the eyepiece and increases the effective magnification, typically by a factor of two. This is a good way to get two magnifications out of a single eyepiece. When choosing eyepieces, it is best to choose them in focal lengths that do not increase by the same factor as your Barlow. For instance, if you have an eyepiece that gives you 50x, you don't really need an eyepiece that gives you 100x, because the Barlow will already do this with the eyepiece you have. It would be better for your next eyepiece to be say, 150x or 175x.
There are three filters that can be very useful for deep sky observing, the UHC, OIII, and H-Beta. The basic principle behind these filters is to pass only certain colors of light, carefully chosen to pass the light of most nebulae while blocking much of the background skyglow. The results are twofold; improved contrast between the nebula and sky, and subtle differences in the detail observed. These filters may bring out more detail, or make the entire nebula more apparent. For some obejcts, such as the Horsehead or faint Abell planetaries, a filter is typically necessary to see anything at all.
The OIII filter is often the filter of choice for observers of diffuse and planetary nebulae. As compared to the UHC it often produces greater contrast. It is particularly useful when applied to the Veil and Crescent nebulae in Cygnus. The UHC filter allows more light to pass, making it more useful for many fainter objects. It can also reveal subtle differences in the nebula compared to the OIII. The Veil looks more lacy in the UHC, for instance. The H-Beta filter is of much more limited use, but it is almost essential for seeing the Horsehead and a handful of other objects.
The use of these filters comes with a caveat; since they throw away some light they are generally more useful in conjunction with larger aperture instruments.