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Properties of Light
In general, the length that the light travels to a focal point can impact the image that is observed. Similarly focusing the light several times repeatedly can work to create higher resolution images. This is achieved by basically extending the focal point by the distance that the light travels after the reflection. Due to the properties of light, some angles and lengths have been found to be more effective at creating quality images.
Refracting Telescope Basics
If we think of the internal workings of the mirrors and lenses of a telescope in terms of the path of the light that enters the telescope, the process is fairly simple to understand. In terms of directness, the refracting telescope is among the simplest. In refracting telescopes of this sort, the shape is a direct cylinder. You look directly into one end of the cylinder to see what is beyond the other end.
Refracting Telescope Lenses
Most telescopes have a transparent lens or cover piece at the end. This is little more than a translucent seal that light enters the telescope through. It is typically durable, easy to clean and may be replaceable. Beyond that, the light passes down into the telescope. Here it passes through what is termed an objective lens. The objective lens is convex (larger in the middle than at the edge) on one side. The other side of the lens is flat. A lens of this shape focuses the light that passes through it to a single point. The point where light from the edge meets is the focal point. Here is where the eyepiece is located. The knob that one turns on the eyepiece adjusts the distance of the eyepiece from the objective lens to match the focal point.
Reflecting Telescope Basics
These types of telescopes involve the use of mirrors as the name implies. Again light passes through a translucent cover into the telescope. There is no actual lens though. The light passes through the telescope to the other end where it strikes a concave (wider at the edges than at the center) mirror instead.
Reflecting Telescope Mirrors
Again it is the shape that focuses the light to a focus. In this case, however, the light is now traveling back toward its entrance point. A smaller mirror is used to reflect the light out of the side of the telescope to a focal point in the Newtonian version of a reflecting telescope. This is one of the more common versions and is often easily identified by the eyepiece position. The angle and placement of the second mirror are essential to the quality of the image in reflecting telescopes.
Catadioptric (Mirror-Lens) Telescopes
These telescopes use a combination of both mirrors and lenses. The light passes into the telescope through the translucent cover. It then passes through a lens that is concave on the side that faces the light's entrance point and is convex on the side that faces away from the entrance point. This shape works to direct light out to the edges of the telescope. Around the edge at the focal length from the lens there is a mirror. The mirror is concave and reflects the light back to a center mirror. This mirror is located on the back of the lens and is concave in shape on its reflecting side. It reflects the light back to the end of the telescope opposite of the light's entrance point. There an eyepiece is located at the focal point.
The curves of the lens and mirrors are related. The primary intent though is to focus the light on the next point in the path. As a result, the length of the telescope and the diameter of the telescope are factors. It may go without saying that these and reflecting telescopes have a blind spot in the light's entrance point.