Note: Since I published this text, I've done another, much easier sextant using a CD and its box. Click the banner to the righ to visit it.

But the octant described in this text is better ;-)

Scale Transfer - An alternative way to print the scale was suggested by Mr. Schmit, by email:
"When you print documents with laser printers or photocopiers it is done by fusing the toner on the paper. You can transfer the print on any solid heat resistant material by refusing the toner with a very hot 'cloth iron'. Place the laser printed document on the cleaned material (printed side facing the surface) then apply the iron on the paper back just long enough to melt the toner. I used that to make gratings and reticules on glass or metal sheets and it's working fine if your print is strong enough... but the print must be reversed! "

While this reversed print is not directly supported by the scale printer program, most printer drivers (e.g.: HP) offer this as an option. Open the "Printer Setup" dialog, click "Properties" and check the "reverse horizontally" checkbox.

Another way was suggested by Mr. Kunnar:

" I used inkjet iron transfer paper. It is a kind of plastic that can be transfered to clothes using a hot iron. Since the ink ends up trapped between the sextant frame and the plastic layer, this scale is water proof".

After this, you can use a marker pen to draw the parts (frame, arm and vernier).

The arm window (where the scale is read) must be sized and positioned so that you can see both the scale and the vernier touch point (that will define the arm radius). My octant arm is about 40 mm wide.

When designing the arm, measure the scale radius and add 10mm on either side (towards and away from the arm axis). My arm window is 30 mm wide.

The image to the left shows the 3 frame parts, marked and ready to be cut.

To drill the arm axis hole, first mark it with a hard point. Drill a 2mm lead hole and then a 7 mm hole. Use sharp drills. Do the same in the frame and make sure the hole center is precisely positioned.

I inserted a 7 mm hard plastic tube in the axis, so there is no contact between the arm and screw as the arm is moved (only between the tube and arm). Bond the tube to the frame.

The axis screw goes in the middle, with twin nuts. Make sure there is no slack in the arm axis setup

After cutting, drilling and sanding, you can assemble the arm and frame for the fist time. Then you can position and bond the vernier in the arm, using a couple small drops of loctite (Cyanoacrylate glue). Use small self tap screws to secure the vernier.

If you did these steps right, the vernier should be in smooth, close contact with the index, as you slide the arm along the arc.

Add a plastic device in the back of the arm to press it against the frame (the green part on the left image). It is important to have some friction between the arm and frame, so the instrument will hold the reading if left alone (i.e. the arm will not move by itself). This is also important for fine trimming.

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.

Make a point in all screw tips, to reduce the contact area between mirror and screw to a point. I also added thin metal plates to prevent the mirrors from moving sideways while trimming.

To ensure a perfect contact between the 3 screws and mirror, I used rubber bands (see right). These press the mirrors against the screws. Commercial sextants use metal springs for this function, but I could not find any suitable part in my junk collection.

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 the eye. This means your instrument must have a good Sun filter or shade.

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 supply stores. Use number #14 shade.
• Photography film. Use a dark negative, with metalic silver coating. This means black-and-white film. Color film does not contain silver and will not filter the UV rays. Hard to find in these days..
• Dark floppy disk media.
  

As shade for Sun sights, I used 35 mm dark negative photography film. The negatives were mounted in a slide frame. I used two layers of negatives for the Sun 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).

Sun shade

Trimming this octant is no different then any other sextant.

First trim the arm mirror adjustment screw. The arm mirror must be perpendicular to the arm/frame plane. This may be checked by looking at the index scale reflected in the arm mirror. The reflected index must be perfectly aligned with the index part you see directly (green arrow on the right).

Sun sight

As future improvements, I would change to a better shade positioning system, add a handle and a case (I'm currently using a cardboard box). The case is probably the most important of the three, for the sextant - like a violin - is not be left hanging around if it is to last long.

There are many materials, design ideas and garage junk devices that can used to build a Sextant. If you build such an instrument - using these ideas or not - I would like to hear about it. I would also be happy to show other design solutions here.

A sextant is a fun thing to build and getting a precise reading from your own instrument is great.

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