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IMHO, there are five major differences between plastic and metal
sextants:

1. Thermal stability:  Metal sextants are less susceptible to changing IE
due to thermal instability. This has already been discussed.

2. Precision:  Metal sextants can be machined more precisely. They tend to
have less "gear backlash". To see this, measure IE with your final
adjustment of the drum clockwise. Then do it again with your final drum
adjustment counterclockwise and compare. The difference is gear backlash.
(You can compensate for this by always making your final adjustment in the
same direction.)

3. Optics:  Plastic sextants tend to have plastic mirrors and lenses, and
these mirrors and lenses tend to be on the small side. Metal sextants tend
to have glass mirrors and lenses; these mirrors and lenses tend to be
larger in size and ground more precisely. To my eye, the difference in
the brightness and clarity of a star's image is like the difference
between a 10-watt light bulb and a 100-watt light bulb.

4. Weight:  Plastic sextants are lighter and less tiring to use.

5. Cost:  If money is an issue, plastic wins hands down. If you drop a
plastic sextant, you don't feel so bad.

Plastic sextants are certainly adequate for practical celestial
navigation. Many navigators have successfully used them for crossing
oceans and making landfalls. I don't mind using a plastic sextant, but a
metal sextant is a pleasure to use.

This is of course just one man's opinion; your mileage may vary. :-)

----------------
Chuck Taylor
SEASCAPE GB 32-2
Everett, WA, USA

   

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