The Foamboard Octant project
Sextants and Octants are similar instruments. Both measure angles and are used in celestial navigation. The sextant has this name because its scale is one sixth of the full circle ( 60 degrees ). The Octant scale is thinner: 1/8 of the circle or 45 degrees. The instrument optics are the same with a scale of two arc degrees per degree ( so the octant, with its 45 degree scale, can measure up to 90 degree angles)
Modern celestial navigation instruments are smaller than old ones. This shrinking was possible by precision optics and mechanics. In the old times, when instruments were made of wood, precision was obtained by increased sizes. For those large instruments, the octant design was more convenient than the sextant, because it is slender.
This DIY project is like those old octants. It is large in size, making it possible to read angles within a couple minutes.
The scale was originally designed to be printed in
A3 paper (297 x 420 mm), but it can also be printed
I actually used A3+ format paper, the widest my printer can handle. This is a little larger than A3.
The base design consists of two large resolution PNG images, to be printed with 600 dpi resolution on ink jet or laser printers. A3 format is twice the size of regular office paper (A4).
The instrument frame and arm are made of foamboard.
Foamboard is basically a paper sandwich
Note that this material will not stand either rough handling or moisture. There are other kinds of plastic and wood boards that could be used to build a more resistent instrument. You are encouraged to do that. I went with foamboard because I had it around and it is so easy to cut. Very light too. But fragile.
The instrument frame and arm images were printed on premium presentation paper ( density 145 g/m2 ) which is heavier and deforms less than regular paper. The design uses a minimum of printing ink, to reduce paper deformation while printing.
Also to minimize deformation while bonding the paper to the board, a spray mount adhesive was used. The scale must be bond completely flat to the board, without bubbles or wrinkles. Any visible deformation will ruin the instrument precision.
With a sharp paper cutter, cut the frame and arm pieces . Cut the main scale arc very carefully. Smooth the arc foam with sand paper. Note that the main scale and arm scale (Vernier) must touch each other and the arm rotation must be smooth across the whole arc.
Also cut off the arm window.
The arm axis
The arm axis setup must be sturdy and allow arm rotation, without slack. The foamboard itself is not capable of that (too soft), so I choose to use Lego pieces with axis holes, which were embedded to the foamboard.
There are many kinds of Lego pieces that can be used to build a simple rotation axis. I choose two
shallow bricks with axis holes,
a small-headed short shaft and two nuts.
The shallow bricks used are conveniently about the same thickness of the foamboard ( about 4 mm). The axis bricks were bonded to the foamboard with epoxy two component glue (30 minutes cure time). As it cures, the epoxy hardens and joints the brick and foamboard firmly. Any force the axis receives is distributed to the board evenly, to avoid axis displacement.
In preparation to the bonding operation, position the Lego brick over the frame so that the axis hole center coincides with the printed center. Trace the brick outline with a pencil.
Cut out the top layer of paper of the foamboard ( the rectangle to fit the Lego piece in ). Scratch the foam out, leaving only the bottom paper layer untouched.
In the Lego bricks, protect the axis hole to avoid epoxy contamination. I used adhesive tape rolls for that, fitted to the hole diameter ( 5 mm )
Mix a small quantity of the epoxy glue (equal parts of the two components), and drop it on the rectangular pool on the foamboard. Spread the glue evenly. Insert the brick, so that the piece and foamboard surfaces are about even. Clean up any epoxy that overflows.
Allow some 15 minutes for the epoxy to half-cure. Now hold the brick and gently remove the axis hole protection rolls, before they get too tightly attached to the epoxy. Make sure not to contaminate the axis hole with epoxy, or displace the brick while removing the roll.
Press the brick on a flat surface and allow the glue to cure for a couple hours.
Embed the other brick similarly to the front of the frame.
This octant has two 2 rectangular mirrors. This can be obtained and cut in any glass shop.
Both have the same size: 2 cm x 5 cm ( 3mm thick regular mirror )
Mirror is glass coated with a thin layer of reflective silver on one of the surfaces. Since the silver rusts in contact with air and humidity, it is covered with a protective epoxy layer (the back of the mirror)
The protective epoxy is hard. I used the paper cutter blade for the removal job. With the blade point, make a longitudinal cut along the middle of the mirror. Then, with the blade inclined, gently scratch the epoxy out. It will come out as a fine powder. The glass itself is very hard and will not be easily spoiled. But avoid using the blade point on it, except for the center line. Use the blade edge inclined instead. After a couple minutes of scratching, you begin to see the silver under the vanishing epoxy.
Once the epoxy is gone, use a steel wool ( window cleaner) to remove the silver.
After a while you have your new half silvered mirror.To mount the mirrors to the frame and arm, they were bonded to regular sized Lego bricks (10mm thick). I used 2x2 bricks. For the semi-transparent mirror I had to cut out the 4 brick bumps ( because I could see their tips through the transparent part, which is also 10 mm )
The mirrors were bonded to the bricks with cyanoacrylate glue ( Super Bonder ). Make sure the mirror and brick form a square angle. Use a square ruler face-to-face to the mirror, working on a flat surface, on the bond operation.
The mirror positions are marked on the printed scale and arm. The half-silvered mirror tower base is bonded to the frame, as seen in the image below ( to the right. On the left is the Sun filter assembly )
Use 1 extra shallow brick on the frame mirror tower, so that both mirrors are at the same height in relation to instrument plane. The arm mirror tower, attached to the arm, is lower.
The arm mirror must be positioned so that its mirrored surface (i.e. the back of the mirror ) is over the center of the axis, and it pivots on that axis. In order to avoid interference between the instrument axis and the arm mirror, I chose a shaft with a small head.
Bond the arm mirror tower base to the arm, using Super Bonder.
Position the small arm scale, also known as Vernier, in the arm window, underneath the arm piece. Do not bond it yet. This will be done last, after the mirrors are mounted, on the initial calibration. Fix it temporarily with paper clips.
Mount the arm and the frame using the shaft. Lock it with the two nuts. Make sure the arm can swing smoothly over the 90 degree range. Use fine sand paper to remove any bumps in the main scale arc.
Set the arm position to zero ( align the A tick in the Vernier with 0 tick in the main scale )
Now take a sight of a point located far away, at least a few hundred meters.
In this zero position, the two mirrors should be parallel. But there is probably a small error in this parallelism. Holding the instrument vertically and with the horizontal axis aiming to the chosen point, look through the frame mirror.
You will see the selected point directly and its reflected image on the arm mirror. Swing the arm gently so that the reflected and the direct
images are both at the same vertical altitude. Press the arm and frame together to freeze the arm in that position. Now bond the Vernier scale, so that the point A and scale 0 ticks match perfectly. I used white paper glue for that. Super Bonder is too fast for a precise positioning.
The Vernier is a kind of scale used before micrometer screws found in modern instruments. In the case of this octant, each degree of the main scale is divided in 3 parts, each 20 minutes of arc wide. The A tick in the Vernier points the measured angle.
Reading Vernier scale
When Vernier tick A is between two main scale ticks, the fraction of the 20 minutes must be estimated. The Vernier scale is 20 ticks wide. Find which of the ticks in the Vernier aligns with one on the main scale. This is a number between 0 and 19, and must be added to the main scale reading. For instance:
foamboard octant optics
- published dec-18 by Omar Reis
- jan19: Om: changed welder glass spec from #12 to #14. See eye security notes
ęCopr 92-2012 Omar F. Reis - All rights reserved