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Picture of The Foamboard Octant

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 with precision within a couple minutes.

Well...We'll see about that !

Step 1: Materials

  • 2 Foamboard 5 mm thick, large enough to mount a A3 size page
  • 2 pages of heavy A3 premium presentation paper (around 150 g/m2)
  • 2 mirrors rectangular 50 mm x 20 mm x 3mm thick
  • 1 welder mask filter ( 5.5x11 cm shade #14 )
  • Lego plastic shaft and two nuts (see below)
  • 2 shallow Lego bricks with axis holes ( to be embedded on the foam as axis)
  • more Lego bricks for mounting mirrors and filter (see below)
  • Epoxy glue two components ( 30 minute cure )
  • Cyanoacrylate glue ( Super Bonder )
  • Spray Mount adhesive ( to bond the printouts with minimum distortion )
  • White glue for paper ( for bonding the Vernier scale )
  • Paper cutter ( Olfa type )
  • Steel Wool ( window cleaner )
  • Scissors, square ruler, ruler, paper clips, adhesive tape, sand paper #200...

The instrument frame and arm are made of foamboard. Foamboard is basically a paper sandwich with foam in between. It is commonly used for the mounting of prints and photographs and as backing in picture frames. It's about 5 mm thick and that gives it a relatively solid structure and flatness.

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 resistant 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.

JesperE28 months ago
Wow, that's probably the simplest way I've seen of making a working navigational instrument. Brilliant! I like the lego, especially as a holder for shades.

About the shades though: not all welding masks protects properly from the type of UV-radiation that's the most damaging one from the sun. Older masks and goggles are more likely to be sufficient, since they are more broadband dark, but newer ones are more selectively dark so that they can be more protective in the wavelengths that a welder is more likely to encounter. Please be careful to check wich wavelengths your shades protect against and to which degree, and perhaps ask a supplier of hobby astronomy stuff or something if it's good enough for the purpose. (sorry if I'm incoherent, English isn't my first language)
omarreis (author)  JesperE28 months ago
Thank you for the message. There is concern over the safety of using of welder mask glass as a filter for Sun observation. Most of it is directed to the attaching those filters to eyepieces or binoculars, that concentrate the light even further. With those optical devices, even a temporary mechanical failure of the filter may cause instant harm to the eye.

Not the case with this project, which uses no eyepiece. But of course the concern is valid and the danger is real. This Nasa site suggests that you should use at least welder shade #12 ( #14 is even better ). The text focuses on eclipse observations, which are particularly dangerous because people stare at the Sun for a long period in those events

To be on the safe side, I edited the project to use shade #14 instead of previous #12. There is also this discussion about the subject:

This one discusses welder glass #14 safety in detail:

Alternatively, there are Mylar and aluminized polymer filters for Sun observation like this:

Other Sun safety tips:
* Install the Sun shade before the observation starts
* Previously set the expected instrument altitude before the observation, so you don't waste so much time looking for the Sun.
* Perform the observation as fast as you can. With preparation, it should take only a few seconds to adjust the octant arm
* If it takes too long or if you feel uncomfortable, stop the observation and rest

rkrishnan78 months ago
Great Instructable! I have been wanting to build one for a long time.
A tip when working with foam board, to minimize the effects of paper delamination due to moisture: You can use Minwax Polycrylic protective finish. This is the water based version, but has been tested successfully in RC model plane scratch building. Alternatively, you can use any polyester resin - apply and spread uniformly with a rag, then let dry 24 hours. This coating also adds considerable strength.

Lorddrake8 months ago
I have heard of sextants before, but never heard of Octants. This is a very cool project.
seamster8 months ago
This is excellent!

For what it's worth - I think your 4th photo in the introduction would make a fantastic cover image and might help draw more readers. Just a thought! : )
omarreis (author)  seamster8 months ago
Done that.. Thanks