140503 From the Workshop #5...by Tim C
Starlight comes to our telescopes from afar – essentially from infinity. This means that light comes to our scopes as parallel light. So, if we finish with a spherical mirror, will this be good enough? It turns out that, since we need this parallel light to converge to a point, we need our finished mirror to have the shape of a parabola. In the figure below the solid line represents a sphere, and the dotted line, a parabola. Imagine that the parallel rays of light fall onto the surface of these two figures. What happens to them after reflection? For the sphere, these rays do not come together at one point. But, in the case of the parabola, they do. Recall that in a previous article, you were asked to stand at the center of a sphere, holding a candle and to watch what happens to the reflected light. Since this light emanates from a single point (the radius of curvature of the sphere), it returns to this point after reflection. Starlight doesn’t enter our telescopes from one point. So, ultimately, we have to change our mirror’s figure to that of a parabola. How do we do this in our optical shop?
It turns out we achieve different shapes for our mirror’s surface by using different grinding patterns as we fashion our mirror. There are many resources you can go to that demonstrate these principles. One great one is the Stellafane website at www.stellafane.org. Let’s get back to business: we have our 8" flat blank and our grinding tool. For simplicity let’s assume our tool is a piece of plate glass covered with tiles glued to the glass. Begin by placing the tool face-up on the work surface. Spray a little water on it and sprinkle its surface with 60-grade Silicon Carbide. To preferentially grind the middle of your mirror, start with the chordal stroke. The mirror is always on top for this stroke. And the tool is always on the bottom. Much of the mirror “hangs over” the edge of the tool so that the center of your mirror will frequently pass over the outside of your tool. This will cause more wear on the center of your mirror and, likewise, more wear on the outer regions of your tool. Your mirror becomes concave as your tool becomes convex. Walk around your work as you grind. As you move around your work in a clockwise fashion, turn your mirror counter-clockwise. This “organized randomness” is actually the secret to mirror grinding. To me, it is one of the processes that, although simple, makes this process so elegant.
Starlight comes to our telescopes from afar – essentially from infinity. This means that light comes to our scopes as parallel light. So, if we finish with a spherical mirror, will this be good enough? It turns out that, since we need this parallel light to converge to a point, we need our finished mirror to have the shape of a parabola. In the figure below the solid line represents a sphere, and the dotted line, a parabola. Imagine that the parallel rays of light fall onto the surface of these two figures. What happens to them after reflection? For the sphere, these rays do not come together at one point. But, in the case of the parabola, they do. Recall that in a previous article, you were asked to stand at the center of a sphere, holding a candle and to watch what happens to the reflected light. Since this light emanates from a single point (the radius of curvature of the sphere), it returns to this point after reflection. Starlight doesn’t enter our telescopes from one point. So, ultimately, we have to change our mirror’s figure to that of a parabola. How do we do this in our optical shop?
It turns out we achieve different shapes for our mirror’s surface by using different grinding patterns as we fashion our mirror. There are many resources you can go to that demonstrate these principles. One great one is the Stellafane website at www.stellafane.org. Let’s get back to business: we have our 8" flat blank and our grinding tool. For simplicity let’s assume our tool is a piece of plate glass covered with tiles glued to the glass. Begin by placing the tool face-up on the work surface. Spray a little water on it and sprinkle its surface with 60-grade Silicon Carbide. To preferentially grind the middle of your mirror, start with the chordal stroke. The mirror is always on top for this stroke. And the tool is always on the bottom. Much of the mirror “hangs over” the edge of the tool so that the center of your mirror will frequently pass over the outside of your tool. This will cause more wear on the center of your mirror and, likewise, more wear on the outer regions of your tool. Your mirror becomes concave as your tool becomes convex. Walk around your work as you grind. As you move around your work in a clockwise fashion, turn your mirror counter-clockwise. This “organized randomness” is actually the secret to mirror grinding. To me, it is one of the processes that, although simple, makes this process so elegant.
[from May, 2014 SBAU newsletter]
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