your own sextant
One of the obstacles to learn and practice celestial navigation
is the price and availability of sextants. Even the simplest plastic models
cost between US$ 50 and 150 and can only be found in a few specialized
stores. This scares interested people away from celestial navigation.
That's why a few months ago I published the X-tant
Project , a "do-it-yourself" octant design. But while
that octant is cheap to build, it requires electric tools, some hard to
find materials and considerable work.
So I went on to design an even simpler sextant, which I
called CD-Sextant. This small instrument is built using
a CD and box. As in the X-Tant Project, I used a few Lego blocks and glass
mirrors. No electric tools are necessary to build a CD-Sextant.
The CD-Sextant (assembled and parts)
The needed materials are:
- A CD with box. Use the traditional CD box.
- A4 Sticker paper (full page, without label cuts.
For printing the scale on inkjet or laser printer)
- 2 small glass mirrors ( 40 mm x 22 mm x 3 mm
- Lego bricks
- 1 2x4 brick
- 2 2x1 plates
- 1 2x2 brick
- 1 2x2 plate
- A couple more for the shade support (depends
on your design choices)
- Cyanoacrylate glue (aka Loctite Superbonder).
- Paper cutter (Olfa cutter)
The design takes advantage of the dimensional precision
of CD parts and Lego bricks. The sextant arm is the CD itself and the
sextant frame is the CD box. The angle is changed by turning the CD.
Of course the small CD radius gives limited precision to the instrument
when compared to larger sextants, but it is enough for celestial navigation
practice. Because the instrument is so small, care must be taken in all
steps (cutting, sticking etc) to achieve the best possible precision.
The CD-Sextant is not only useful for celestial navigation, but also
in coast navigation, using simple trigonometric relations. And it is surprisingly
strong (I have dropped mine a couple times, with pieces flying in all
directions, and it is still working).
Vernier scale sextants
Minutes of arc are very small and reading them is only possible with
a precise scale. There are two kinds of minute scales used in sextants.
Modern sextants use a drum to trim the instrument and read the
minutes. One full turn of this drum equals one degree ( or 60' ). This
device requires sofisticated machining to build.
The CD-Sextant uses a vernier scale, a simpler but usefull minute
scale type. Since the CD-Sextant degrees are small ( due to the small
CD radius ) I used a single 60' vernier (larger sextants usually divide
each degree in three ticks of 20' ).
For more details on vernier scales click
Printing the Scale
The scale is the most difficult sextant component to do using traditional
techniques. Fortunately, most of us have a precise printing equipment
right on our desktop: an inkjet or laser printer. These machines can print
300 dots per inch (1200 for laser), with enough precision to print a sextant
Sextant scale printer program
In order to achieve the best results, I wrote a small sextant scale
printing program. This will print the sextant scale using vector
rendering, for best resolution. This is the same program used in
the X-tant project.
XtantScalePrinter - version 1.1 - download
For Windows, 192.874 bytes
- This program can be freely used for personal, noncommercial
purposes, provided that the credit (name and URL) is not
removed from the printed scales.
1) To print the CD-Sextant scale, run the program
and check the CD-Sextant checkbox.
2) The CD-Sextant is a vernier sextant (see X-tant
Project for more details on Verniers). In the case
of the CD-Sextant, the small scale radius makes it impossible
to divide the scale degrees into smaller ticks. Set the div/degree
property to 1. This will give a scale with ticks only for full
degrees and a 60' vernier.
3) Select your favorite font. Use size 7.
4) Press the [Print scale] button.
5) After printing, cut around the scale precisely.
Don't cut the CD axis hole yet.
Sticking the scale
Sticking the scale is a critical operation. If the
scale is not perfectly centered when you stick, you will probably
have to remove it in pieces and print a new one. I did this:
|1) Lift an edge of the sticker paper backing,
in the scale part.
||2) Cut the paper backing edge, to expose a small
area of the sticker surface.
||3) Position the scale on the CD (use the data
side), making sure the scale is perfectly centered. Look against
a light source to check if scale is centered in the CD center
hole. Hold the CD and printed scale with both hands, making
sure the printed scale is not out of the CD in any side.
|4) Once the scale is centered, press the exposed
sticker surface against the CD to stick it. This will secure
the scale to the CD in the correct position.
5) Remove the rest of the sticker paper backing and carefully
stick the scale, working in one direction, to avoid bubbles
and ripples. In the end, the printed scale must be centered
and match CD surface perfectly.
6) Use the paper cutter to open the CD axis hole.
Save the printed Vernier (the small scale). It will
be the last thing to be stuck, after the mirrors are positioned.
I used 2 equally sized glass mirrors (40 mm x 22 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.
Go easy and 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 with a wet cloth or thin steel sponge
(the ones used to clean windows). In the end, the glass must be
clear and scratch free (fig. below).
Note: You may be tempted at this point
to use a thinner mirror and eliminate the transparent part altogether.
Don´t do that. This would introduce a refraction error.
The direct (horizon) light ray must pass thru the glass, as the
light ray from the star does.
This can only be done if you use a front mirrored
surface, such as a polished inox plate. However inox mirrors are
easily scratched. Glass mirrors are better.
I used Lego
bricks to hold the mirrors. They have good dimensional precision and
will guarantee a nearly correct 90° angle between the mirrors and
CD parts. They will also allow the sextant to be assembled, disassembled,
trimmed and parts to be replaced as needed.
Of course other materials can be used if Lego bricks are
not available. Try to use dimensionally precise objects, to build the
mirror supports with right angles.
Bonding the mirrors
CD Center mirror:
||1) Working on a flat surface, bond the
center mirror back to the large side of a 2x4 Lego brick. Make sure
the mirror is perpendicular to the flat surface.
||2) Assemble the 2x4 brick over two 2x1
Lego plates (gray ones). The space between the plates will be over
the CD axis hole.
3) Bond the mirror assembly to the CD. A few things
to watch here:
- Align the the large Lego side to the 180° scale line.
This way, the mirror silvered surface (i.e. the back surface
of the glass mirror) will be over the CD center.
- Take care to center the Lego plates well. They will be very
close to the CD center hole. Don't let them interfere with
CD Box mirror (half silvered):
high definition 300 DPI image
I used a 2x2 Lego brick mounted on a 2x2 Lego
plate, to hold the CD box mirror. Cut the 4 brick bumps out,
because they will be visible thru the transparent part of the
half silvered mirror.
1) Working on a flat surface, bond the half silvered
mirror to the 2x2 Lego brick. Make sure the mirror is perpendicular
to the flat surface.
2) Assemble the box mirror brick to the plate.
3) Position and bond the box mirror assembly in
the CD box corner. Make sure that:
- The CD is positioned pointing more or less as shown in
the layout to the left, so you will have space to place
the Vernier in the other box corner afterwards.
- Place the half silvered mirror assembly parallel to the
center mirror. Position it visually.
- Bond the assembly to the CD box.
Sticking the Vernier
At this point, your CD-Sextant is almost done. You must
now stick the Vernier in 0° position:
1) Cut the vernier in a triangular form, to fit the CD
box corner. I did stick the vernier on a blank sticker paper piece,
in order to make the vernier paper ticker. This is important because
the vernier edge will be unsupported.
1) Trim the mirrors (see trimming the mirrors below)
2) Turn the CD until the mirrors are parallel.
3) Hold the instrument in observation position, looking
thru the half silvered mirror and focus on a far away object. Turn the
CD slowly until the reflected image and the direct image coincide. This
must be the instrument zero, so....
4) ...Carefully place and stick the vernier in the CD
box, reading 0°00'. This means that the vernier tick A most coincide
with the 0° scale line. On the other side of the vernier, the 60'
tick must coincide with the 59° tick in the scale.
Make sure the vernier and CD scale are very close together.
The vernier probably will be a little higher than the CD, and you might
want to bend it down a bit.
the Sun can be dangerous. The UV radiations can cause
cataracts. Excessive visible light can burn the retina.
must be taken while observing the Sun, to protect your
eye. This means your instrument must have a good
- Make sure the shade is in place before observing the
- Avoid observing the Sun for more than a few seconds.
- Never stare the Sun directly.
- If you feel uncomfortable, stop the observation immediately.
- In this case, consider a stronger filter.
Materials that can be used to make the filter:
- Aluminized Mylar film - this is a material specifically
developed for solar observation. Can be found in science
suply stores. Probably the best material available.
- Welder's glass - strong filter, used to protect the
welder eye. Difficult to cut, can be found in construction
- Photography film. Use a dark negative with silver
coating. This means black-and-white film. Color film
does not contain silver and will not filter the UV rays
(they are dark for visible light, but not for UV light).
- Dark floppy disk media.
For more on eye safety, read this
As shades for Sun and Moon sights, I used 35 mm dark negative
film (there is one in the end of every film roll). The negatives
were mounted in slide frames. I used two layers of dark film for
the Sun frame and single for the Moon. Both slide frames are removable
and are attached to the instrument frame using Lego pieces. I
did trim the lower edge of the slide, to make it thinner. The
slide window must match the imaginary "tube" formed
by the mirror edges.
In the image to the left we have 3 different shade support
The shade must be positioned between the two mirrors and
the filter surface must be orthogonal to the line connecting
both mirror centers. This is to avoid introducing a refraction
Try to position the slide center in the line connecting
the two mirror centers. The Sun observation is made by looking
thru the half silvered mirror, below the shade.
Trimming the mirrors
For simplicity, the CD-Sextant is not equipped with screws to
trim the mirrors. But the mirrors can be trimmed by inserting
small sticker paper pieces between the Lego brick and plates (or
by sanding the brick) at suitable positions.
First check the angle of the CD mirror (center mirror). As you
look to this mirror, the reflected CD edge must be perfectly aligned
with the edge you see outside the mirror (green arrows in the
image). This must hold for all directions.
The half silvered mirror can be trimmed by setting the instrument
to 0°00', aiming to a far away object and making sure the
direct and reflected images coincide.
After this initial trimming, the Lego bricks will hold the trimming
surprisingly, even after disassembled. Of course you must read
the index error after each set of observations, as with all sextants.
CD center mirror trim
Sun sight with the CD-Sextant
Bond a round 1x1 Lego piece to the CD surface, to use as a turning
knob (the blue piece).
The CD may be hard to turn. In this case, rub the CD hole with
a pencil. The graphite is a good lubricant, and will make turning
the CD easier. This is important for fine adjustments.
I made 8 such sextants. I will be publishing test results here
soon. Stay tuned.
|>> "The American Practical
Navigator " by Nathaniel Bowditch
ISBN 0781220211 - 1200 pages