Time for a physics lesson. I picked up a little 3″ diameter concave makeup mirror and looked at myself up close. It enlarged my image and looked upright. I extended my arm slowly while looking at myself and noticed my image got bigger and blurry. As I moved the mirror farther, my image inverted and started to get smaller again. I wondered how my image would look if the mirror was across the room. I stood it on the mantle in front of the clock. I was barely visible.
In a recent post about Maine lighthouses, I included two photos of the Fresnel lenses used to project the bright light beam across the water. One of the readers is a man I’ve enjoyed working with before in the blog world. He suggested I add a post with some description of how the Fresnel lens works. Here it is.
Basics of Converging Lenses
The converging, or convex lens, is able to bring parallel rays of light toward a focal point. As a child, I played with a magnifying glass lens to burn leaves, grass, and other things.
The lens can also be used in a different way to project light rays parallel to each other in a beam. Simple projectors work on this basic principle. A lighthouse is designed to do this.
The previous post Big Telescopes | Examples of Three Types, describes the refractor, Newtonian reflector, and Cassegrain reflector. Of these, the Cassegrain design is built into the largest diameter and most powerful telescopes in the world. In this post, I focus on the process used to form the mirrors of the Giant Magellan Telescope scheduled for completion in 2019. Each mirror is 8.4 meters across. Those seven mirrors will form a telescope objective of 24.5 meters, or 80 feet. Note the human figure at the lower left in this artistic rendering.
The mirrors need to be as light as possible. They cannot be made of a thick piece of solid glass. Instead, they are built in a honeycomb hollowed out structure. It is a remarkable story of engineering and technology.
This four minute video from the Univ. of AZ Mirror Lab explains some of the rationale for this telescope and the large mirrors being cast for it. More details are found below.