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Well, the ultimate limit is the 1/8th wavelength of light "Raleigh
limit" since anything more perfect is not detectable due to the wave
character of light. This is the standard for telescope mirrors. But,
since you will not be trying see the moons of Saturn when doing celnav,
the artificial horizon doesn't need to be that perfect. Since the angle
of incidence equals the angle or reflection any error in the shape of
the mirror is doubled in the reflected ray. So, the answer to your
question is that it must be accurate to 1/2 the accuracy limit you are
trying to achieve. If you only want sight accurate to one minute of arc
then the mirror must be accurate to 1/2 of a minute. If working for one
tenth of a minute accuracy then the mirror must be accurate to one
twentieth of a minute.

gl

pls wrote:
> Does anyone know how accurate (i.e., level) the surface figure of a
> sheet of black glass must be to serve as an artificial horizon?  In
> particular I am trying to determine the point beyond which additional
> accuracy is irrelevant in terms of the result, given the other
> variables in a sighting with a hand-held sextant.
> best
> -p
>
> >
>
>


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