Build your own sextant
From times to times I get tired of writing abstract computer programs. This time I decided to do a more concrete project: a sextant (actually, an octant). I'm not a very experienced craftsman and don't have a equipped shop. The design is simple and can be reproduced with hand tools. I used a small jigsaw and a belt sander. I have made no blue prints.
In tests, when compared to a Kern theodolite this octant did agree within 6' (see test results in the bottom of this page). Also tested a Davis plastic sextant.
Printing the Scale
The scale is probably the most difficult sextant component to do using traditional techniques. It must be very precise and allow reading degrees and minutes, with accuracy at least within 5' of arc.
Fortunately, most of us have a precise printing equipment right on our
desktop: a inkjet or laser printer. These machines can print 300 dots
per inch (1200 for laser), with enough precision to print a good sextant
Below are the three basic X-tant types. The octant can measure up to 90°, the sextant up to 120° and the quadrant up to 180° (see below). I choose to build an Octant, since this design fits better in a A4 printer page, giving the largest possible degree size in the scale, for best detail. And the 90° scale is enough for most observations.
I used A4 ink jet sticker paper to print the scale (the ones used for printing labels). Choose a paper with no cuts. After printing the scale, check how good is your printer, using a compass to see if the index is a perfect circle segment.
I used a 3 mm tick acrylic board for the instrument frame. This material is easy to machine and is relatively rigid. Acrylic boards are usually sold in large sizes, so you might want to search for someone who works with this material, in order to get the small piece you need with minimum expense.
The size of the sextant will depend on the printed scale size (that´s why there is no blue print). So, you will only "design" the instrument after you have stick the scale on the acrylic board.
The acrylic board comes with plastic layers in both sides, for protection. It's a good idea to keep this protection as long as possible, because the acrylic will be easily scratched. I carefully lifted the plastic protection (see below), stuck the scale and put the plastic back in place, so the sextant scale was also protected while machining the frame.
After printing, cut around the scale, leaving like 3 mm around the outside line.
Sticking the printed scale in the board is a critical operation. It must be perfectly stuck, free of air bubbles or ripples. Otherwise the scale will not be correct.
Remove from the sticker paper backing completely. Hold it with the two hands and gently place in the acrylic board, the arm axis circle first. Then use one hand to spread the scale, while holding the other side. Keep the paper slightly tensioned, but not so much as to distort it. If you make a mistake, you probably will have to print another scale and start again.
I cut the parts with an electric jigsaw, with thin teeth blade. Take care when cutting along the scale ticks. Never cut across the scale line. If you are careful, you can cut as close as 0.5 mm from the scale. Then you will have little trouble sanding out the rest, until you precisely reach the fine scale line. Use fine sanding paper for finishing the scale arc.
Also carefully cut and sand the vernier contact point, testing frequently against the scale arc. The vernier and scale contact must be as close as possible.
I used 2 equally sized glass mirrors (46 mm x 24 mm, 3 mm tick). Any glass shop will cut these for you. As you know, one of the mirrors must be half silvered. So you must remove half of the mirror silver backing. I used a paper cutter blade for this job (Olfa cutter).
First make a sharp longitudinal cut along the middle of the mirror. Then scratch half of the epoxy protective layer from the back of the mirror, with the blade inclined. The epoxy backing is a hard material, but will come out with patience. Don't use any abrasive material or the blade point, to avoid scratching the glass. Once the epoxy is gone, the silver is easy to remove, rubbing hard with a wet cloth. In the end, the glass must be clear and scratch free (fig. below).
I used a thicker acrylic for the mirror holders (4mm), as these parts are sometimes subjected to abuse. They will also have to be fixed with self tap screws so a thicker material is better.
Both mirrors must be supported by three contact points (from geometry, we know that 3 points are required to define a plane). I used 3 supporting screws to position each mirror. Some of the screws are adjustable, for mirror trimming, and some are fixed. For the adjustable screws, I used Allen screws, which have a large head, easy to turn by hand. The fixed points are regular inox nut screws.
The frame mirror holder is a T shaped part,
with 2 adjustment screws (below). Cut a window, so that the sight
thru the glass part of the frame mirror is clear.
After completing the two mirror holder setups (i.e. after drilling, cutting, sanding and securing the mirror screws), you can bond them to the frame and arm. Start with the arm mirror.
The arm mirror assembly must be positioned so that the center of the mirrored surface (the back surface of the mirror) is over the arm axis center. This way the center of the mirrored surface (i.e. the back of the mirror) remains fixed while the arm is moved.
Make sure you have space to introduce and remove the arm axis screw, or you wont be able to assemble and disassemble the arm. After bonding with Loctite, use small flat head self tap screws to secure the assembly to the arm. Make a housing for the screw head, to avoid interfering in the arm movement.
Make sure both mirror holders are firm, by bonding and securing with screws. Having reached this point, you already have a sextant to take twilight sights. But you still need shades do take Sun sights.
Observing the Sun can be dangerous. The UV radiations can cause cataracts. Excessive visible light can burn the retina. Extreme care must be taken while observing the Sun, to protect your eye. This means your instrument must have a good Sun filter.
For more on eye safety, read this page.
As shades for Sun and Moon sights, I used 35 mm dark negative photography film (there is one in the end of every film roll). The negatives were mounted in slide frames. I used two layers of negatives for the Sun shade and single for the Moon shade.
Both slide frames are removable and are attached to the instrument frame using Lego blocks (the yellow one in the pictures above).
The thing to watch here is the shade position. The filter surface must be perpendicular to the line connecting the mirror centers. This is to avoid introducing a refraction error. Don't make the same mistake I did, letting the shade support interfere with the arm at large angles. The arm must go at least up to 90° (for the octant).
No eyepiece ?
I was looking for a good 2x or 3x small telescope that I could use as an eyepiece for my sextant. I played with small toy telescopes, but results were poor. In the end, I decided to use no eyepiece. This actually gives a lot of freedom handling the sextant.
When taking a sight, remember to hold the instrument so that your eye is on a plane parallel to the instrument's and containing the fixed mirror silver-glass division. This is easy to find: turn the instrument up until you face the fixed mirror. In this position, you should see half of your eye in the silvered part of the mirror. Move it sideways until you see it. Then carefully turn the instrument back to observation position.
Trimming the mirrors
Then trim the frame mirror. This is a little trickier, because two screws have to be trimmed simultaneously. Set the arm to 0°00' and point the instrument to a far object (like a star or boat far away). Then trim the two screws until the object remains a single image while you swing and rock about the instrument axis.
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1. (jun/02) - Added text about inkjet printed
scale problem with water spray.
ęCopr 92-2012 Omar F. Reis - All rights reserved