Making a telescope mirror
To start, a new mirror blank is “sanded” down to create a precise parabolic curve in a process called grinding. To do that, the blank is paired with a device called a grinding tool, usually made of glass, plaster, or ceramic. Unlike typical sandpaper strips, the so-called sandpaper used in grinding is actually a mixture of grit and water. A layer of grit is sprinkled on the blank. Then water is sprayed onto the grit and the machine is turned on to spin the grinding tool. More grit and water is added as needed. The coarser the grit, the quicker the grinding — but also the rougher the surface. That’s why it’s important to frequently measure the depth of the curve throughout the entire process.
After grinding comes polishing. At this point, each mirror is tested to check the initial concave curve. A spherical concave curve is usually acceptable for high-focal-ratio mirrors (usually f/10 and above). But the steep angles in lower-focal-ratio mirrors, especially in today’s fast sub-f/4 Newtonian systems, require ultrasmooth, precisely crafted parabolic curves to correctly focus incoming light. Therefore, the mirror’s surface must first undergo a process called figuring.
Figuring is done by passing a second tool, referred to as a pitch lap, over the mirror by hand while the mirror rotates on the grinding machine. The grit used for this stage of production, technically called polishing compound, allows for very fine adjustments to the curve. This is where the artistry really comes in. Painstaking testing and inspection, followed by slowly correcting hills, valleys, and other irregular zones in the parabolic curve — first with machines and then by hand — are all critical for exceptional results.
To test for any optical imperfections at the final stage, the mirror undergoes several optical assessments using interferometers in controlled environmental conditions. Interferometry testing compares a reference mirror with a known accuracy to the newly manufactured mirror. A laser beam is shot between the two mirrors, creating a series of lines, called fringes, on the surface of the mirror. A computer then reads those lines and averages the results from a series of tests to determine the mirror’s accuracy. For manufacturers like Terry Ostahowski of Ostahowski Optics, this computer-generated report is customarily given to the purchaser to objectively prove the mirror’s quality.
Once a mirror is fabricated, all that is left is for it to be coated. Some premium mirror makers send their products to reputable coating companies, while others do that step in-house. Regardless, most premium mirrors feature enhanced aluminum coating to reflect 96 percent of the light striking its surface. By comparison, standard aluminizing reflects around 90 percent. The reflective coat is deposited in a vacuum chamber, where the aluminum is evaporated to create a vapor, which evenly coats the mirror. To create an enhanced coating, several layers of dielectric film are added on top of the aluminum. Finally, to prevent scratching, an overcoating of silicon dioxide (SiO2), which is both hard and transparent, is deposited to protect the delicate surface.