DSO Locating Devices

Jack Kramer

When you set up your scope to observe deep sky objects (DSOs), there are two opposite problems that you might face, depending on sky conditions at your site:

Problem 1: When I try to locate DSOs from my light polluted backyard, I can never see enough stars to starhop to the objects.

Problem 2: In a really dark sky, there are so many stars visible that I have trouble finding the constellations and stars to use as guides in locating objects.

The issues are similar in that both involve using a series of stars that point the way to galaxies, nebulae and star clusters that are too faint to see with the naked eye. In one case, you have too few stars and in the other case too many. I've encountered both situations. The solution will require effort on your part to learn the constellations, but knowing your way around the sky is only part of the answer.

With regard to problem 1, the stars are there, but you just can't see them because of the competition from skyglow. This is a very good reason for having a high quality finder scope on your main telescope. Through a good finder, many more stars will appear than are visible with the naked eye, even where light pollution prevails. That is why deep sky observers usually recommend a larger finder, such as an 8x50, which has enough light grasp to catch many more stars. A well-made finder will provide sharp images and a wide field of view. Unfortunately, some telescopes come equipped with such small finders and/or finders of such poor quality that they are virtually worthless. If you have one star to begin your star hopping journey, plus a good finder, the battle is almost won. In fact, you may be able to spot the DSO right in your finder scope!

The problem of light pollution also makes a compelling argument for having a computerized go-to mounting or digital setting circles. Many observers who use this technology point to the absence of visible stars that are convenient for star hopping. And of course, go-to is simply faster than starhopping. Plain old fashioned setting circles can also be used, though I find them more trouble than they are worth. You need an equatorial mount for them to work; however, the setting circles on many mounts are so imprecise that they're worthless.

As for problem 2, it may be hard to feel much sympathy for someone lucky enough to enjoy really dark skies. But you may very well find yourself in that fortunate situation. Probably nothing tests your knowledge of the sky like a dark night filled with stardust. The best method is to locate a single bright and familiar constellation, then with the naked eye, step off to find adjacent constellations until you arrive near your destination.

In the case where you have a profusion of stars, that large finder that helped you in a light polluted area may end up only adding to the confusion as you star hop in a dark sky. When you look into the finder, all you see is even more stars, many of which are difficult to differentiate based on their brightness. In this case a 1x finder may help. This is a sighting device, such as a Telrad or Rigel Quick Finder, that does not provide any magnification at all, but seems to project a spot or reticle pattern on the sky to help you aim your scope. And looking through a 1x finder, you won't see the fainter stars that can be the source of confusion in a finder scope.

Of course, a good finder in a dark sky can still be an asset. You can often see many DSOs right in a finder scope. This is a distinct advantage in that you can simply aim the telescope about where you believe your target is located, and sometimes there it is, staring right back at you in the finder. Moreover, many observers who have straight-through finder scopes (without a star diagonal) have adopted the practice of looking through the finder with one eye while looking at the sky with the other. This helps greatly in aiming the finder. The field in the finder seems projected on the sky while, in effect, your "free" eye acts as a 1x finder.

Finally, selection of the right star atlas can be a great help. Some new observers take to the field with atlases that are not nearly detailed enough for locating anything beyond the brightest objects. A good atlas ought to show stars a couple of magnitudes fainter than what you can see naked eye. That way, there are some additional stepping stones to use with your finder. The Sky Atlas 2000 is one of the most widely used star charts because many observers find its stellar magnitude limit of 8.5 and complement of galaxies, nebulae and star clusters to be just about right.

You'll often see experienced observers with two or more aiming devices on their main telescope...perhaps a finder scope and a Telrad, or a finder and digital setting circles, or some other combination. Regardless of the sky conditions at their site, they come prepared. There is no magic solution to the problems of finding deep sky objects in different environments. But there are several tools that can help.

Published in the August 2002 issue of the NightTimes