If I use a telescope to observe a plane in the sky, how can I find the altitude of the plane(altitude of the plane with respect to the ground)?
If you know the dimensions of the plane then it is possible to calculate its distance. One way of doing that is with a stadiametirc rangefinder. If the airplane dimensions are unknown, you can also find the distance by using two telescopes and calculating the distance by triangulation. After you have established the distance, you can calculate the altitude by taking the cosine of the angle the telescope made with the ground and multiplying by the distance.