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Introduction
There is a strong tradition of amateur telescope making.within the amateur astronomy community.
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 parabolic primary mirror and mount.
Other than the "geek" factor of making one's own telescope, the foremost 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 parabolic mirror, 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 shape of the the mirror surface is periodically checked with a Foucault tester.
Mirrors are usually ground from low-expansion borosilicate glass (Pyrex (TM) is the brand name). Alternatively, a special ceramic called Cer-vit is used. This material produces superior mirrors, but costs more.
The mirror blank is ground against another piece of glass made from either a piece of thick window glass such as a porthole or another mirror blank. This second piece of glass is called the tool. An abrasive such as silicon carbide mixed with water is used between the mirror and tool.
The mirror maker strokes the mirror blank back and forth across the tool, which is usually placed on a barrel in order to provide access from all sides. The mirror maker takes a step around the barrel every 10 or 30 seconds or so. This will ensure thet the initial shape of the mirror is a perfect concave spherical surface. Periodically stepping around the barrel will average out surface errors. Fresh abrasives are added as required.
During grinding The piece of glass on top will become concave as the piece of glass on the bottom becomes correspondingly convex. The mirror is rough ground using course abrasive until the curve begins to approach the desired depth or radius. The depth of the curve will define the focal length of the mirror and hence the f-stop of the telescope.
The same basic step is repeated, using successively finer abasives. Silicon Carbide is typicaly used to about 500 grit, after which Aluminum oxide is used. Fine grinding to a 3 micron size abasive will greatly speed up the polishing step.
Clean the system carefully when reducing grit sizes to prevent scratching from the previous size abrasive.
After that, one constructs a "lap" from tar or a mixture of rosin and beeswax. The lap has channels cut in it to let water and abrasives run off.
Then, using the lap, one begins to polish the mirror using rouge. The scratches of the rouge are smaller than a wavelength of light, and the mirror thus becomes a specular (mirror-like) reflector.
At some point, the mirror is polished, but it is still a section of a sphere. At this point, a light and knife-edge are set up at the focus of the mirror. Using this arrangement, viewing the mirror past the edge will show shadows showing the "figure" of the mirror.
The lap is cut away, or a different stroke is used to polish more in the center. When the above inspection method shows a slight circularly-symmetric dough-nut appearance, then this indicates the sought-for parabolic mirror shape.
The mirror is then aluminized, by placing it in a vacuum tank with electrically-heated nichrome coils that can evaporate aluminum.
Both mirrors are front reflecting, meaning the light bounces off only the front surface of the glass.
First, the amateur decides what size to construct. The difficulty of construction grows roughly as the square of the diameter of the mirror. A 4-inch mirror is a moderately easy science-fair project. An 8 inch mirror is a good compromise between ease and constructing an instrument that would be expensive to purchase. A 12 inch mirror is difficult, and a telescope over 24 inches usually must be ground and lapped with mechanical assistance. Amateurs have constructed telescopes as large as a meter across (39 inches), but this is foolhardy for anyone other than the best-funded, experienced clubs.
Once the mirror has been crafted, it is mounted in a mechanical tube. The idea is to maintain optical alignment relative to ancilliary optical components, the secondary mirror and eyepiece.
In a Newtonian telescope, the primary mirror is located at the bottom of the tube, the small secondary mirror is suspended in the middle of the tube at the top using a low-profile mount (called a spider), and the eyepiece is adjacent to this on the outside of the tube.
Alignment is achieved by:
The altitude bearings rest on pads of teflon. This provides very little Static friction so that the telescope can be moved very small angles.
These pads sit on the rocker box, which itself rotates in the azimuth on pads of teflon.Mirror Making
Telescope Design
Telescope Construction
Tube
Circular disks or bearings are attached to the side of the tube at its centre of gravity so it can tilt on its mount. Usually a small finder telescope is attached to the tube too to aid pointing.Mount
