A4 Sextant



Introduction: A4 Sextant

Sextant is an instrument to measure angles, used in celestial navigation. This DIY instrument is simple to build. No electric tools. just paper, foamboard, mirrors and Lego bricks ... and toothpicks.

This is my second navigation instrument project to be mounted on foamboards. The first was the Foamboard Octant ( https://www.instructables.com/id/The-Foamboard-Oc... ), a larger and better instrument, but more difficult to build. I recommend reading that Instructable, since this one borrows pieces from it.

Another problem with previous instrument was the paper size ( A3 ) used for octant parts, not handled by most printers ( size A4 is much more common ).

This time we go even simpler and smaller with the A4 sextant.

Sextant readings have two parts: degrees and minutes. Read the full degrees on the main scale. Minutes come from the secondary scale, the one with smaller numbers, also known as Vernier. The minute reading is the tick on the Vernier that aligns with the one on the main scale. --

Since this is a relatively small instrument - radius about 17 cm - the main scale degrees are small to be further divided. Main scale is divided in full degrees and a wide 60' Vernier.

If you take notice, the 60' Vernier scale is only 59' wide on the main scale. That extra tick is what makes the Vernier scale work.

There are two Verniers side-by-side. For large angles, one may go off the arc. Use either one.

This is another advantage of the 60' Vernier: minutes are read directly on the scale ( in the previous octant with 20' scale, you have to aa Vernier minutes + main arc minutes )

Check this maker video:


  • 1 foamboard A3 sized page ( or 2 pages of A4 )
  • 1 page of A4 premium presentation paper (density around 150 g/m2 or more)
  • 2 glass mirrors rectangular 50 mm x 20 mm x 3mm thick. Found in glass shops, cut from scrap.
  • 1 welder mask filter ( 5.5x11 cm shade #14 ). Shade for Sun observation. Found in construction shops

  • Lego bricks for mounting mirrors and sun shade (see below)
Lego brick list (using Pick-a-Brick names)

Arm mirror tower

  • 1 brick 2x2
  • 2 plate 2x4
  • 2 plate 2x2

Shade support

  • 1 plate 1x6
  • 1 plate 1x4
  • 1 brick 1x4
  • 2 plate 1x2
  • 2 hinge plate 1x4

Frame mirror tower

  • 1 brick 2x2 ( cut off bumps )
  • 1 cross axle 4M
  • rim wide 18/14 W. CROSS Ø4.8

other stuff:

  • Cyanoacrylate fast glue ( Super Bonder )
  • Spray Mount adhesive ( to bond the scale with minimum distortion )
  • White glue for paper
  • Epoxy 2 components 30 min cure time ( to bond shade to Lego brick )
  • Paper cutter ( Olfa type )
  • Steel wool sponge (window cleaner)
  • Scissors, square ruler, ruler, sand paper #200...

Step 1: Sextant Frame and Arm

Print scale
Print sextant scale on ink jet or laser printer.

Use heavy premium presentation paper ( 150 g/m2 or more). It will deform less than normal paper.

Page size A4.

Note: Instructable photos are low resolution. Don't use it to print you scale.

Instead Download and print high resolution image ( 600 DPI ) from:

Paste scale to foamboard

Paste the printed scale to one side of the large A3 board, covering half of it.

To minimize deformation while pasting the scale to the board, Spray Mount adhesive was used. ( spray mount is commonly used to paste pictures to frames, can be found on office supplies stores)

The scale must be bond flat without bubbles or wrinkles. Any visible deformation will ruin the instrument precision. Use minimum amount of adhesive spray (don't let the page get wet ). Do the job outside and spray the paper in regular movements, until all the page is covered. Allow the adhesive to dry for 15 seconds and paste the page to the foam board in one forward movement. A clean paste operation will result in a perfect flat scale. No removing and re-pasting allowed. If you mess it, print another scale .

Press the page with a heavy book on a flat surface and allow it to dry for some time.

Cut the A3 foamboard in two

With a sharp paper cutter and using a ruler as guide, cut the board in two equal A4 sized pieces: one with the scale on it and one blank.

Put the two parts together and make sure they have the same size, by cutting out any difference.

Make the instrument axis...with a toothpick

With the two boards together and aligned on a flat surface, make a guide hole on the arm center, going thru both boards. Use a pin for that. Remove pin and insert a toothpick in the hole, keeping it vertical.

Make another hole on the frame mirror center. Insert another toothpick.

Make a third hole near the printed eye and insert the last toothpick.

Now you have the two boards fixed together by the 3 toothpicks, forming a large triangle.

Using a scissor, cut the toothpick points on both sides, leaving about 5 mm each side.

Take the boards apart and place the scale alone on a flat surface. Put the blank board apart for now.

With the paper cutter, carefully cut along the arc of the sextant scale. Go easy and cut in progressive passes. Keep the cutter aligned with the arc at all times, so that the arc contact surface is vertical.

Once you finish cutting, you have two scale parts: the arm and the frame with the main scale.

Smooth both sides of the arc surface with sand paper. Remove any bumps on the arc.

Paste the main scale to the frame base

Using white paper glue, paste the main scale (the lower part) to the blank base board. Use the toothpicks as guides to position the scale as before, while the glue dries.

Mount the arm on the frame

Using the toothpick as axis, mount the arm to the frame, using previous axis hole.

Check arm rotation. Should be smooth across the arc.

Rotate the arm a few times to break in.

Printed scale precision check

The scale image in this design was generated using formulas and is - in theory- geometrically correct. What your printer will print is a different matter. But I found modern printers can do a good job.

After pasting the scale to the foamboard, do a couple checks see how precise it is:

  • Measure the radius R of the arc at different points ( 0, 45, 60, 90, 120° ). They must match.
  • Measure the distance D from tick 0 on the arc to the center line ( 60° radius ). This must be:

D = R * Sin(30°) = R * 0.5
For instance: If R=17.10 cm, D must be 8.55 cm

  • The distance from 120° to the center line must be the same.
  • With a navigation divider, measure a fixed arc at different points on the scale.
  • This will give you and idea of the precision that can be obtained with your sextant.

Step 2: Mirrors

This sextant uses two equal sized rectangular mirrors. Those can be obtained and cut at glass shops. These little mirrors can be cut from scrap, should not cost much.

Ask for two rectangles of size 20x50mm - 3 mm thick glass mirror ( make it 3.. )

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)

One of the mirrors in the sextant must be half-silvered. This is the one fixed to the frame. It combines the light ray coming from the other mirror to the horizon image.

Check this video about making the half-silvered mirror:

The half silvered mirror is obtained by splitting it in two and removing the epoxy and silver layers on one side. Half of the mirror becomes regular transparent glass. The other half remains a reflective.

The protective epoxy in the back of the mirror 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 wet steel wool sponge (window cleaner) to remove the silver. The glass is very hard to scratch, but work with care. 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 mirror was bonded to the Lego brick with cyanoacrylate glue ( Super Bonder ). It is important to bond the mirror at 90° angle to the horizontal surface of the brick. Use the square ruler face-to-face to the mirror on the bond operation, working on a flat surface.

The mirror towers are made with some shallow Lego bricks.

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.

Bond the mirror base to the arm with Super Bonder, in position as indicated in the printed image.

Frame mirror assembly

The frame mirror is mounted with a Lego plastic axis (see images). This kind of axis allows rotation if forced (to calibrate) but will remain in place in normal instrument usage.

Paste the base of the frame mirror to the frame, so that the mirror is positioned as indicated.

Step 3: Finishing Touches

  • Paste a couple Lego pieces to the arm, to work as rotation knobs (see images)
  • Paste a couple plastic guides, bond to the frame arc, to keep both sides of the scale together and even as you rotate the arm. Cut from plastic card.

Sun shade

Last but not least, for Sun observations, we need a Sun shade. This is because the Sun light is very strong and dangerous to the eye. Sun UV rays can cause cataracts and other problems. We used welder mask filter for that. Welder mask protects the welder from similarly hazardous UV rays produced by the intense weld heat.

Welder mask filter can be found on construction shops, for a couple dollars. Ask for shade number #14. The one I found is rectangular, 5.5 x 11 cm. I had it cut in the glass shop, dimensions 5.5 x 4.0 cm, so it is a little larger than the mirrors, to cover them completely.

The filter must be placed between the mirrors, and should cover the whole area of the mirror, so that no part of the arm mirror can be seen from the frame mirror outside the shade.

Again I used Lego parts to fix the shade to the frame.

There is no mark on the scale for the shade position, so you have to improvise.

Position the shade assembly ( the shade and attached Lego bricks ) so that the shade surface is perpendicular to a line connecting the two mirror centers. One mirror should not see the other outside the shade.

In this position, bond the base 1x2 Lego brick to the frame (w/ Super Bonder). Make sure the shade does not interfere with arm rotation (see photos)

I used Epoxy glue ( 2 components 30 min) to bond the shade to the Lego brick ( see photo ). The filter is made of dark glass and is difficult to bond.

This basic shade is installed and removed by coupling it to the Lego brick on the frame (in and out). This proved to be boring. I found a nice hinge brick that allowed to fold the shade instead of removing it (see photos).

Yep... This is my final version... ;o)


Further eye-safety: https://eclipse2017.nasa.gov/safety

This page reviews materials for Sun filters and its safety, including welder glass #14:


Initial calibration

Loosen the frame mirror by pulling it out.

Set the arm position to 0.0 (i.e. align the A tick on the Vernier to the 0 tick on the main scale)

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. Holding the instrument vertically, aim to the chosen point looking through the frame mirror. You will see the selected point directly and the reflected image on the arm mirror. Rotate the frame mirror firmly and gently so that the reflected and the direct images are near and horizontally aligned. Once the mirrors are parallel, press the mirror on its axis to fix it in that position. Tricky to get right. Use the index error to correct for mirror mis-alignment.

Related Links

  • A4 Sextant maker video:

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