The question is a little misleading because the answer depends on what you are looking at. When you are simply looking at a distant object, such as a mountain or even an airplane, you should keep both eyes open. When you are trying to focus on a closer object, such as a bird in your yard, it is best to close the eye that is not looking through the telescope. That gives your other eye a chance to relax since it is not doing any work at that point.