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Philippe Posth you wrote: Hello, Can you tell me exactly what kind of plastic the Davis Mark 15 and 25 sextants are made of. This question because I need to buy some glue to repair one of this sextant, and I want to be sure that I am buying the correct glue. If you know the name of the best glue, that would be great.

There are apparently many thousands of recipes for plastics.  The best thing to do would be to ask Davis at info@davisnet.com, but you would need to specify the year and which part, because the chances are that more than one type of plastic is used in the sextant, and the types used might have changed over the years because of operating experience.  The frame, which includes the arc, and the arm need to be rigid, stable, and have a low coefficient of thermal expansion, without being brittle.  For other parts such as the scope tube, the manufacturer might prefer to forego the exacting requirements above for a material which was easier to mould.  DaveP