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The classic amateur telescope is the Newtonian reflector with a dobsonian type mount. Amateur telescope makers typically make the most critical and expensive parts of a Newtonian-reflector, which are the primary mirror and mount.
It's fun to make one's own telescope, and also, some people who would like to have a large telescope can't afford one. Another reason to grind and figure the primary mirror of a telescope is that it is possible to produce a hand made mirror that is far superior to commercially made mirrors. It is well within the range of any reasonably competent person to produce a primary telescope mirror that is diffraction-limited.
The Newtonian reflector has two reflecting surfaces: the primary mirror (usually parabolic), and a small flat secondary mirror. The primary mirror reflects and focuses incoming parallel light rays back through the tube of the telescope until they are intercepted by a flat secondary mirror set at a 45 degree angle. This flat secondary mirror reflects the light sideways to an eyepiece mounted on the side of the telescope, where it converges at the focal plane .
The mirror is usually ground and polished to a shallow spherical section, and then carefully "figured" to a paraboloid using a special polishing lap and a rotating W shaped stroking motion. (The depth of the mirror's curve will define the focal length of the mirror and hence the f-stop of the telescope. If the focal length of the mirror is long enough, such as f/12, a spherical curve's performance will be nominally equivalent to a parabloid, and the more difficult task of achieving a parabolic shape becomes unnecessary. Also, the longer the focal length, the greater the resulting magnifying power of the primary mirror when used with a given eyepiece, although the field of view will be smaller.)
The shape of the mirror surface is periodically checked with a Foucault tester, this will be described in the mirror testing section.
Mirrors are usually ground from a "mirror-blank" of low-expansion borosilicate glass (Pyrex (TM) is the brand name). Alternatively, a special ceramic called Cer-vit is used. Cer-vit costs more, but produces mirrors that deform less as the temperature changes.
The mirror-blank is ground against a "tool," which is another piece of glass, either a thick piece of window glass or another mirror blank. An abrasive such as silicon carbideSilicon carbide (SiC) or moissanite is a ceramic compound of silicon and carbon. Most SiC is man-made for use as an abrasive (when it is often known by the trade name carborundum), or more recently as a semiconductor and moissanite gemstones. The simplest mixed with water is used between the mirror and tool. The tool is usually placed in a frame on a barrel in order to provide access from all sides.
To grind the hollow in the mirror, the mirror maker strokes the mirror-blank back and forth across the tool. Each stroke grinds the abrasive against the blank and tool. The mirror maker takes a step around the barrel every 10 or 30 seconds or so to average out surface errors. This ensures that the shape of the mirror grinds to a perfect concave spherical surface. Fresh abrasives and water are added as required.
During grinding, the piece of glass on top becomes concave as the piece of glass on the bottom becomes convex. The mirror is rough-ground using coarse abrasive until the curve begins to approach the desired depth (or radiusThe word radius ( Latin for "wheel spoke"; plural radii pronounced ray dee-eye has several meanings in English: In classical geometry, a radius of a circle or sphere is any line segment with one endpoint on the circle (i. the circular boundary) and the ot).
The depth of focus is checked by wetting the mirror's surface, and seeing where a light's image focuses against a cardboard card
The same basic step is repeated, using successively finer abrasives. Silicon carbideSilicon carbide (SiC) or moissanite is a ceramic compound of silicon and carbon. Most SiC is man-made for use as an abrasive (when it is often known by the trade name carborundum), or more recently as a semiconductor and moissanite gemstones. The simplest is typically used from 60 down to about 500 grit, after which Aluminum oxide is used. Fine grinding to a 3 micrometre size abrasive will greatly speed up the polishing step.
It is important to clean the system carefully when reducing grit sizes to prevent scratching from the previous size abrasive. It is also important to periodically check the focal length of the mirror during the grinding process. The curve of the mirror will continue to deepen as long as the mirror is on top. If the curve becomes too deep, the system is flipped over, and grinding continues with the tool on top. This will cause the curve to become shallower.
After fine grinding is done, a polishing or "pitch" lap is made from the tool. A pitchPitch is the name for any of a number of highly viscous liquids which appears solid. Pitch can be made from petroleum products or plants. Petroleum-derived pitch is also called bitumen. Pitch produced from plants is also known as Resin. Products made from compound is heated in a double boiler until it becomes liquid. This compound is poured over the mirror, and the tool is pressed on top of the tool and pitch so that the lap will take on the exact shape of the mirror. It is important to coat the mirror with rouge before this step to prevent the lap from sticking to the mirror. After the lap and mirror are separated, the lap has channels cut in it to let water and abrasives run off. Alternatively a rubber mold can be used when the lap is poured, to make the channels.
Then, using the lap, one begins to polish the mirror using rouge or cerium oxide . Polishing is very similar to grinding, except that the resistace between the mirror and the lap is much higher. Cleats must be fastened to the work surface to keep the lap and mirror from sliding. The scratches of the rouge are smaller than a wavelength of light, and the mirror thus becomes a specular (mirror-like) reflector.