Amateur telescope making

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(Redirected from Telescope making)

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, also known as a dosonian telescope. Amateur telescope makers typically make the most critical parts of a Newtonian-reflector, which are the primary mirror and mount.

Making a telescope should be fun, but is technically challenging. For a modest cost, a first class instrument can be constructed. 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.

Contents

Mirror making

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 carbide 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 radius).

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 carbide 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 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 or cerium(IV) 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.

Mirror testing


Foucault test

The basic trick to making a good mirror is to measure it, and then fix its problems by altering the lap to polish the problems away. The foucault test is the traditional mirror test for amateur mirror makers. It is also the least subjective of tests available to amateurs.

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.

Foucault test setup to measure a mirror
Enlarge
Foucault test setup to measure a mirror
Foucault test appearance depending on knife-edge position
Enlarge
Foucault test appearance depending on knife-edge position

The Foucault tester consists of a pinhole light source and a vertical knife edge which is mounted on micrometers or home-made screw adjusters. The knife-edge is mounted at the focus. The light illuminates the mirror, and the user looks at the mirror past the knife-edge.

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. If the surface is parabolic, the mirror will look like a donut or lozenge. Imperfections as small as a half-wavelength high (a quarter of a micrometer) show as flat spots or bumps.

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.


Matching Ronchi test

Ronchi testing (http://www.atm-workshop.com/ronchi-test.html) is similar to the foucault test in set-up. A light source is emitted through a diffraction grating, reflected by the mirror, then passes through the refraction grating again and observed by the person doing the test.

The result is a pattern of interference that reveals the shape of the mirror. The interference is compared to a computer generated diagram of what the mirror should look like. Inputs to the program are line frequency of the diffraction grating, focal length and diameter of the mirror.

The ronchi test is much faster to set up than the foucault test but more subjective. It offers a quick glimpse at the mirrors shape and condition, and can quickly identify a 'turned edge' (rolled down outer diameter of the mirror).

Star test

Star testing (http://www.astunit.com/tutorials/startest.htm) tests the entire optical system of the telescope. It is a very subjective test that requires a high degree of skill to interpret, but is considered to be the best test available to amateurs.

A star is brought into field under high magnification and then observed inside and outside of focus. Diffraction rings appear around the star. The shape and brightness of the diffraction rings are analyzed to determine the quality of the mirror.

For primary mirror testing, all optical components are assumed to be perfect and the results are applied to the primary mirror only. In a cassigrainian telescope, for example, the secondary may be complete and the primary can be tested and adjusted to correct for errors in both the primary and the secondary.



Figuring the mirror

The true "art" of mirror making is in shaping the final curve or "figure" of the mirror. The figuring process begins after the mirror is fine ground and polished. The polishing lap continues to be used in this process.

The first goal in figuring a mirror is to obtain a perfectly spherical section. If the mirror is spherical, the surface of the mirror will appear to be evenly lit when inspected with the Foucault tester. The rotating "W" stroke described above will tend to bring the surface to a spherical shape.

It is important to allow the mirror to thermally stabilize or "cool down" after each figuring session. The friction of the lap against the mirror will cause the glass to expand slightly and will affect the figure.

The parabolic curve is obtained by slightly deepening the curve in the center of the spherical mirror. This is done by changing the "W" stroke so that more time is spent with the lap against the center of the mirror. It can take less than a minute of work to turn the spherical surface to a paraboloid, however the process is not exact and may need to be repeated numerous times. The shape of the curve can also be manipulated by flipping the mirror/lap over, or by using a small sub-diameter lap.

The Foucault tester and testing mask are used to obtain data used to calculate how closely the mirror matches a parabolic curve. Visually, the mirror surface will appear to have a slight circularly-symmetric donut appearance when viewed with the Foucault tester.

Aluminizing or "Silvering" the mirror

Although the finished mirror will work in the telescope without a reflective coating, the image will be very dim. So, a very thin coating of a highly reflective material is added to the front surface of the mirror.

Historically this coating was silver. Silvering was put on the mirror chemically. This was then polished. Silvering was typically done by the mirror maker.

Since the 1950s most mirror makers have the coating applied by a firm specializing in the work. Modern coatings usually contain Aluminum and other compounds.

The mirror is aluminized by placing it in a vacuum chamber with electrically-heated nichrome coils that can sublime aluminum. In a vacuum, the hot aluminum atoms travel in straight lines. When they hit the surface of the mirror, they cool and stick. Some mirror makers evaporate a layer of quartz on the mirror, others expose it to pure oxygen or air in an oven so that it will form a tough, clear layer of aluminum oxide.

Telescope design

The most common telescope design for the amateur telescope maker is the Dobsonian Telescope. The Dobsonian is very simple and easy to make. A Dobsonian telescope can be optimized for both planetary and deep sky observing. It is not suitable for astrophotography because it does not have the ability to automatically track the sky.

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 (100 mm) mirror is a moderately easy science fair project. An 8 inch (200 mm) mirror is a good compromise between ease and constructing an instrument that would be expensive to purchase. A 12 inch (300 mm) mirror is difficult, and a telescope over 24 inch (600 mm) usually must be ground and lapped with mechanical assistance. Amateurs have constructed telescopes as large as 1 metre across (39 inches), but this is foolhardy for anyone other than the best-funded, experienced clubs.

Telescope Construction

Tube

Once the mirror is done, it is mounted in a mechanical tube. The idea is to maintain the optical alignment between the mirrors 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). The eyepiece is aimed at the secondary mirror on the spider, from outside of the tube.

Alignment is achieved by adjustment. The secondary mirror's position and tilt must be adjusted. The primary mirror must be tilted to focus through the secondary on the eyepiece.

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 to aid pointing.

Mount

In a Dobsonian mount, the altitude bearings rest on pads of teflon. This provides very little friction so that the telescope can be moved very small angles without jerking. These pads sit on the rocker box, which itself rotates in the azimuth on pads of teflon.

A dobsonian mount can be adjusted by computerized motors to track the stars, but the star field rotates, making the field of view useless for photography. It's theoretically possible to correct this with a rotating erection prism, but most people opt for a standard equatorial mount.

For astrophotography, an amateur equatorial mount is usually a "T" shape made of pipe, with roller bearings around the stem and top of the "T." The telescope tube is attached to one side of the T, with a counterweight on the other side to balance the weight of the telescope. The fixed axis (the stem of the "T," the one closest to the ground) is aimed at the pole-star, parallel to the axis of the Earth. In this way, moving the telescope to counter the rotation of the Earth requires movement only on one axis, the bearing wrapped around the stem of the "T."

Another form of equatorial mount is a two-tined fork. There are three bearings. One is wrapped around the "handle" of the fork. The handle of the fork is parallel with the axis of the Earth. The other two bearings are on the tines, and support the telescope tube from two sides. This mount is very popular for small professional telescopes because it weighs less (no counterweight) and since it gives better support, the tube and optics distort less. It's less popular with amateurs because it has three bearings, of two sizes, and it can be difficult to align the two secondary bearings.

Some amateurs construct setting circles on their mounts, or use motors that can move by very precise amounts. These let the amateur "dial in" astronomical objects by coordinate. A few amateurs have even constructed precision setting circles, and performed astrometry, measuring angles to nearby stars!

Telescope making books and other published information

The classic reference is:

  • Albert G. Ingalls (ed.), Amateur Telescope Making (Vols. 1-3). Orig. edition: Scientific American; new rearranged edition: Willmann-Bell Inc.


See also

External links

de:Amateur Telescope Making

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