Telescopes: Observing Techniques & Affecting Factors

There are many techniques and tricks that experienced observers use when viewing objects through a telescope to get the most out of the image. Here is a quick description of each:

Averted Vision: This may be the easiest and most important technique that an amateur astronomer can master, especially when observing deep sky objects like nebulae, galaxies, or even globular clusters! Averted vision will also help reveal dimmer stars and increased detail on closer objects like planets. Simply put, averted vision is looking off to the side of the object being observed (using "the corner of your eye"), while continuing to concentrate on the object. Doing this often helps bring out detail you otherwise would miss by looking directly at an object. The reason for this lies in how our eyes are constructed. Basically, the portion of your eye's retina that best detects dim light is located all around the edges rather than the center. Practice averted vision by placing an object at the center of the field of view, then look at different areas in the field while still concentrating the object. You'll soon find pacticular spots on your eye where you can see the most detail.

Scope Rocking: A lesser known technique that I sometimes find useful is slowly (or quickly!) moving the telescope tube very slightly back and forth while viewing a deep sky object. This works best if you have slow motion controls on your telescope, but you can also gently push the tube back and forth with your hand. Like averted vision, this technique will also help you to notice detail or even an entire object that you otherwise wouldn't notice. This works because our eyes are naturally drawn to objects in motion, and rocking the telescope tube while looking through the eyepiece creates the illusion of putting the field of view (and the objects in it) in motion.

Aperture and Optical Quality

Exactly which objects are visible in your telescope is often first determined by the aperture and optical quality of the instrument. The aperture of your telescope is just a fancy name for the diameter of the objective lens or mirror. Both light gathering and resolving power increase as the aperture of a telescope is enlarged. The obvious result of more light gathering power is that the viewer will be able to see dimmer objects. But it also means that a given object will appear brighter in a larger aperture scope than in a smaller aperture scope. You might think that a 4-inch telescope has double the light gathering power of a 2-inch scope (2 x 2 =4). But that isn't correct! It's important to remember that light gathering power of a telescope increases in proportion to the surface area of the primary mirror, not the diameter. Therefore, a 4-inch telescope (with about 12 in.2 of mirror surface area) has four times the light gathering power of a 2-inch telescope (which has about 3 in.2 of surface area). Resolving power represents how well a telescope can present fine detail or close double stars.

But there are many different factors besides aperture and quality that will affect what you can see in your telescope. Weather conditions, atmospheric turbulence, and light pollution can all combine to limit your view. In fact, you'll probably notice that what you are able to see varies from night to night, even minute to minute.

Atmospheric Turbulence

Atmospheric turbulence has a serious effect on visibility through your telescope. Earth's atmosphere is a dynamic system in which different layers and cells of the atmosphere have different temperatures and pressures. In a never-ending effort to reach equilibrium these layers and cells are set in motion by the laws of thermodynamics, causing the atmosphere to "ripple" like water in a pool. What does this have to do with your telescope? Well, air in motion degrades images seen through a telescope! You can tell how turbulent the atmosphere is very easily with your naked eye at night -- just look at a star. A lot of twinkling means a lot of turbulence. Similarly, on any given night a star close to the horizon will almost always twinkle more than one directly over head. That's because there's much more atmosphere to look through towards the horizon, and therefore more turbulence. If the stars near the horizon twinkle very little, you are in for an exceptionally steady night!

The idea of image degradation by air in motion can be extended to other observing practices as well. You should avoid observing through an open window in your house because the temperature difference between the air in your house and the air outside will cause enough turbulence in the air around the window to harm the view through your telescope. You need to allow time for your telescope to cool down to outside temperatures when you first bring the telescope out of the house to observe, otherwise the difference in temperature will cause air currents inside the telescope tube.

Weather Conditions

Weather conditions play an important role in the quality of observing on a given night. Obviously if you have any sort of cloud cover, viewing will be limited if not impossible!