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2 Mirror Making 3 Telescope Design 4 Telescope Construction 5 Telescope making books and other published information 6 External link |
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 that 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 abrasives. Silicon Carbide is typically used to about 500 grit, after which Aluminum oxide is used. Fine grinding to a 3 micron 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 impotant 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 completed, a polishing or "Pitch" lap is made from the tool. A Pitch 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. The polishing operation is very similar to the grinding process, with the exception that the resistace between the mirror and the lap is much higher, and will require that cleats be fastened to the work surface to keep the lap/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.
Although it is possible to test the unfinished mirror by putting it in a telescope assembly and making a star test, most mirror makers construct a simple device known as a Foucault tester, which greatly aids the process of figuring the mirror surface.The Foucault tester consists of a pinhole light source and a vertical knife edge which is mounted so as to allow X-Y axis adjustment by use of micrometers or home made screw adjusters.
After the mirror is polished out, the mirror is placed vertically in a stand. The foucault tester is set up at a distance close to the mirror focal point The tester is adjusted so that the returning beam from the pinhole light source is interrupted by the knife edge. Viewing the mirror from behind the knife edge will show a pattern on the mirror surface. If the mirror surface is a perfect sphere, the mirror will appear evenly lit across the entire surface, when viewed through the tester. If the surface is parabolic, the mirror will look like a donut or lozenge.
It is possible to calculate how closely the mirror surface resembles a perfect paraboloid by placing a special mask over the mirror and taking a series of measurement with the tester. This data is then reduced and graphed against an ideal parabolic curve.
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.
Although the finished mirror will work in the telescope with out a reflective coating, the image will be very dim. For this reason a very thin coating of a highly reflective material is added to the front surface of the mirror. Historically this coating was silver, which was put on the mirror by allowing a slurry to evaporate, leaving the silver behind. This was then polished. Silvering was typically done by the mirror maker.
With the advancement and invention of modern coating techniques, the mirror maker now almost always have the coating applied by a firm specializing in the work. Modern coatings usually contain Aluminum and other compounds.
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 ancillary 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.Introduction
Mirror Making
Mirror Testing
Figuring the mirror
Aluminizing or "Silvering" the mirror
The mirror is aluminized by placing it in a vacuum chamber with electrically-heated nichrome coils that can evaporate aluminum. The mirror blank is electrically charged which causes the evaporated aluminum to be attracted to the mirror and deposited in a layer only a few atoms thick.Telescope Design
Telescope Construction
Tube
Circular disks called altitude bearings are attached to the side of the tube at its center of gravity so it can tilt on its mount. Usually a small finder telescope is attached to the tube too to aid pointing.
Mount