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.
The following table lists the theoretical magnitude and resolution limits of several common small telescope apertures. They are theoretical limits because the values don't consider such things as atmospheric conditions and light pollution. These effects are difficult to quantify exactly, but they can make a drastic difference. I'll discuss them further below!).
| Aperture, inches (mm) | Faintest Star Magnitude* | Max. Resolving Power, seconds of arc* |
|---|---|---|
| 2.4 (60) | 11.5 | 2.0 |
| 3.1 (80) | 12.2 | 1.5 |
| 3.5 (90) | 12.5 | 1.3 |
| 4.5 (114) | 13.0 | 1.0 |
But there are many different factors besides aperature 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 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 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! More to Come!
More to Come!
