This is the heart of any reflecting telescope. This primary mirror will be the main light gathering surface of the 16 inch Cassegrain telescope MDOA plans for the proposed observatory.
The current focal length (which is the point above the surface of the glass where light comes to a point focus) is approximately 80 inches. This will make the telescope an F/5.
This number, F/5, refers to the focal length of the mirror, divided by its diameter (in this case, 16 inches). An F/5 system is somewhat typical for Cassegrain telescopes.
A Cassegrain is a special type of telescope, in that the light reflected from the primary is eventually redirected back through a central hole in the primary. The light will pass through an eyepiece for viewing by the observer. Cameras (both using film and electronics) can replace the eyepiece. This feature permits images to be recorded.
Most reflecting telescopes have two mirrors. The main mirror is called a primary, and the secondary is the smaller mirror that redirects light to an eyepiece. The mirror you see here is the secondary. Only 4 inches in diameter, it is another vital part of the Cassegrain system.
Light from the primary is reflected a second time by this mirror, and due to the convex surface of the mirror (it's curved outward) it redirects all light toward the central hole in the primary. That light is then focused by any eyepiece in the back of the telescope.
Once installed, the secondary will be in a special mounting, which will move the secondary mirror toward, or away from the surface of the primary. This will help to bring images into a very fine focus.
"Special Feature" Notice how the secondary appears sort of rusty orange? That's because it's a special type of material. This mirror is not glass, but a ceramic call Servet. This ceramic will virtually have no change in diameter or thickness over extreme changes of temperature.
The secondary mirror has an outward curving surface (convex). Measurements of a convex surface can be done several ways, but we chose to make an interference test.
The test plate is the same size (diameter) as the secondary. It also has the same focal length of the secondary mirror. The two oppose each other with just a small gap of airspace between them. Using a special lamp, and a grating, light and dark lines begin to appear between the surface of the test plate, and the secondary. By counting the number of light, and dark lines per inch, we can evaluate the quality of the secondary mirror. This is why the image of the test plate has marks on it. These marks are at 1 inch increments.
Other tests will be performed soon, to evaluate the quality of the polished surface in greater detail.