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Thanks for the prompt reply. It is my first message
to this discussion group, and I never suspected
I would get such extensive and complete answers.

I know the hard part to build a sextant is the scale.
But I found that inkjet and laser printers can do a
good job (at least some of them can).

I'm a programmer and have written a scale printer
program, were one can specify the scale parameters
(scale radius, ticks/degree, font size etc). The program
works with vector math. The scale of course is
of the vernier kind.

In the following address you can find an output of this
program. It is a 300 DPI image of a 514mm octant
scale, with 4 ticks/degree (vernier of 15').

www.tecepe.com.br/nav/xtant/SextantBMP.jpg

I have already built a 230mm octant, which I printed
on a HP 810C inkjet printer. In tests, I found that
the scale error can be as high as 10' for large angles,
but if a correction table is used, the instrument can
be precise, for the error is predictable and stable.

I now want to build a larger instrument, to see how
much precision I can get out of this construction method.
I found the Bowditch octant to be elegant
(the image is from a museum site called the Bowditch
Initiative ( www.nathanielbowditch.org/ )

Omar

   

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