Star Finder

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OUR dials do all the brainwork when you use this table-top star finder—no need for paper-and-pencil figuring. Given four facts, you can quickly seek out and identify heavenly bodies that otherwise you might never see in the vastness of the sky. These facts are: • The date you're using the finder. • Your local standard time. • The right ascension of the star in degrees or hours. • Its declination. Right ascension and declination are coordinates that astronomers use to locate stars. The first measures along the celestial equator, the second to the north and south of it. Just as you can locate a tiny island at the intersection of its longitude and latitude, you can pinpoint a sky target by its right ascension and declination. But as the earth turns on its axis and through its orbit, the sky that you see keeps changing. That's why you need the hour and date, too. The star finder is also a seven-power telescope. Low magnification provides a generous field of view—about six degrees in diameter—making it easier to locate the sky objects you're seeking. It also makes the stars some 40 times brighter than when viewed with the naked eye. Where you can normally see only 5,000 stars, this telescope brings more than
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100,000 into view. And it emphasizes the color of the brighter ones. It also lets you see certain star clusters, nebulae, and some of the nearby galaxies—and it makes a fine satellite tracker. It's easy to operate, simple and inexpensive to build. Buy only what you can't make yourself, and the total cost will be under $5. The eyepiece. If you buy a regular 7x50 binocular eyepiece, that cost is nearly doubled. But you can make your own from instructions that come with the lenses you'll need. Our eyepiece contains two identical plano-convex lenses having diameters of 21 mm. and focal lengths of 41 mm. Assembled as shown in the crosssection sketch on the facing page, they produce a Ramsden eyepiece with a 1" focal length. The outside diameter of the housing determines the size of the hole to

be bored in the eyepiece holder. The inside of the tube is painted dull black. The telescope itself is a 1/8"-thick cardboard mailing tube with a 2" inner diameter. A ring is sliced from one end to form the cell for the objective lens. By stripping off one or two inside layers of paper it's possible to wedge in the lens; but be sure it's seated at right angles to the optical axis. At the other end of the tube, make a 2-1/4" cut along the side, then slice down at right angles so you can peel half the tube open. Glue in the center light stop, then the rear light stop and eyepiece block. After the glue sets, drill a hole through the tube, centered on the light stop and perpendicular to the optical axis. A dowel can be run through this hole as a guide for gluing on the declination disks and brackets. Glue the mirror to its mount, silvered side up.

116 POPULAR SCIENCE DECEMBER 1961

CONCENTRIC DIALS do the figuring for you. Glue time dial to mount with 12-hour mark offset from centerline as shown. Date ring rests

on bottom flange of fork unit, but turns independently. Declination dials are glued to the disks that flank the telescope tube (below).

THE RIGHT-ASCENSION RING turns with the fork, but has a dial on each face and must remain reversible. To switch dials, swing tube down into fork, remove eyepiece, slide ring up over fork, flip over, and replace.

ORIENT THE STAR FINDER by placing it outdoors in the position shown. Set the declination dial. Shift mount until North Star is centered in field of view. If a more precise orientation is desired, offset Polaris one degree (one third of space from center to edge of field) in the direction of the constellation Cassiopeia.

Make a trial assembly to check alignment of the optical elements. When viewed from above, the diagonal mirror should show the open front of the tube as a circle of light centered in the hole for the eyepiece. You may have to shift or trim the mount to obtain this result. Insert the eyepiece to make sure the telescope can be easily focused. Once satisfied, remove the unglued parts and paint the interior of the tube (and all parts that go in it) dull black. The cell with the objective lens is attached with masking tape. This permits removal for cleaning. The fork consists of two uprights anchored to a disk and spaced far enough apart to allow the telescope tube to fit snugly between. In assembling, run a dowel through the holes in the sides to align the declination axis at right angles to the polar axis. The unpainted slip disks are glued on last. A slight taper on the edge of the fork base and filler pieces assures a tight fit for the removable rightascension ring. The mount is of the equatorial type—

that is, when the telescope is oriented, its polar axis parallels the axis of the earth, making the time-dial plane parallel to the plane of the equator. Before starting construction, consult an atlas to determine your latitude to the nearest degree. Subtract this from 90 degrees and you have the angle at which the time-dial plate must tilt from horizontal. Attach the forward leg to the time-dial plate with glue and nails; then drill the hole for the pivot screw, making sure it's perpendicular to the plate. Glue on the crescent-shaped time-dial extension, and paint the mount a bright color. When fastening the fork to the mount and the tube to the fork, tighten the pivot screws just enough to hold the telescope on target yet allow a smooth shift to a new target. Your telescope can't function as a star finder, however, until it's equipped with the all-important dials that point the way to celestial objects. Since these dials must be accurately graduated, POPULAR SCIENCE has arranged to have full-size reproductions of all six included in a kit containing the optical elements you'll need to make your Star Finder (see note at bottom of column at left). Adding the dials. The time dial does THE SIX READY-TO-CUT-OUT DIALS are available in the PS Star Finder Kit, No. 60,- not turn. It is glued to the mount. But 223—$3.95 postpaid from Edmund Scien- it must be properly related to the centertific Co., 101 E. Gloucester Pike, Harrington, N.J. This price also covers an eyepiece set, line of the mount. Go back to the map to first-surface mirror, and 7 x 50 objective lens. determine your longitude to the nearest
118 POPULAR SCIENCE DECEMBER 1961

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