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Quoting "Foley, Jim" :

> Has anyone tried to use a mirror and bubble level as Richard Hubbard
> described in his book "Boater's Bowditch"?  He describes using a thick
> glass
> mirror with a plywood back and thumb screws for leveling.  What is the
> down
> side of this and what kind of errors would you expect?  Is the error all
> in
> the level or are there others?  Thanks.
>
>
> Jim Foley
>
Jim
There are three problems with using a glass mirror,  which can be overcome.
First the mirror needs to be front silvered so that there is no reflection from
the non-silvered glass surface
Second the glass needs to be thick enough so that there is no bending of the
mirror by the supporting frame and
Thirdly the spirit level needs to be sensitive enough to  detect an out of level
of better than 1 minute of arc this means using a precise engineering level and
these are rather expensive.
I have used a 6 mm thick glass mirror which I epoxied to a mounting disk. The
epoxy shrank during setting and this was enough to produce a blurred image of
the sun on reflection. I recommend using at least 15 mm glass. Ordinary window
glass is not flat but Float glass is nearly as good as ground and polished plate
By the way, as the glass is to be front silvered it need not be transparant.

Best of luck.
Clive.S.

   

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