- A reflector telescope has two mirrors. The primary mirror has a parabolic shape enabling it to gather and focus light. The secondary mirror is flat and reflects the light to a focal plane at the top of the telescope tube.
- Early telescope mirrors were made of fused quartz. Most telescope mirrors today are made of Pyrex sheet glass or low-expansion crystallized glass-ceramic. The glass ("mirror blank") is shaped and ground before the reflective coating is applied.
- The mirror blank begins as a roughly flat square of a size and thickness specified by the person building the telescope. The corners are cut off several times to make a "rough round," a disc shape having between 16 and 30 sides. The glass is not perfectly flat and must be ground, either by hand or by machine, to an optical-quality smooth surface.
- Two types of tools can be used for grinding: a "solid" tool is a disc usually made of glass, and a "segmented" tool is usually a round ceramic tile with a plaster face. In either case, the tools can be no smaller than 75 percent of the diameter of the mirror blank.
- The mirror blank is placed on a working surface, generally a heavily weighted drum with a flat, carpet-covered top. The mirror blank is then wetted and a No. 60 abrasive is applied to the glass. The tool is moved with short W strokes over the mirror blank, rotating the blank under the tool. Wetting and grinding repeats over and over until the mirror blank is sufficiently flat. Fine grinding is then done with No.120 and No. 220 abrasives before going to No. 320 micron grit. This process takes a minimum of 20 hours per side.
- The mirror blank is coated with pitch, a viscous tarlike substance that serves as a carrier for the cerium-oxide-based polishing agent. The polishing agent is mixed with water and squirted onto the mirror. A "pitch lap" is set on the center of the mirror and about 40 pounds of weights are placed on the lap for 15 minutes (a "wet press"). The weights are then removed and polishing begins by moving the lap around to stir up the polish on the mirror. This takes considerable strength because the lap is very difficult to move and a great deal of pressure must be applied while moving it around. This continues until the mirror is free of pits and is of optical quality. It is then ready to have the reflective coating applied.
- The reflective coating can be applied using various methods. Many coating companies have developed their own processes, such as Spectrum Coating Inc.'s Ion Assisted Depositions (IAD), and Mead Instruments Corporation's Ultra-High Transmission Coatings (UHTC). The mirror blank is placed in a vacuum chamber and the air is pumped out. The mirror is spun while the coating is applied, ensuring even distribution. Manufacturers have their own "secret recipe" of ingredients and patented formulas for their reflective coatings.
