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    Re: Sextants with Polarizing filters
    From: Mike Hannibal
    Date: 2006 Jan 25, 13:27 +1100
    I use a sextant with a polarising filter on both the index and the horizon glass. They provide excellent filtering and I often use them alone - no other shade glasses. They are, as has been noted easily adjustable so it's easy to get specific brightnesses. This is particularly good when doing lunars - you can balance very easily the relative brightness of the other object with the moon.

    It is important to note however that even good polarisers change the index error when used. I know that the manuals say that this can happen with any shades but I have only ever measured it with polarisers - maybe it has to do with the two pieces of glass in each filter. In any event IE must be measured with the filters in place - orientation does not seem to matter.



    Peter Fogg <fthre@OPTUSNET.COM.AU> wrote:
    > From: George Huxtable
    > Some things I don't understand about Peter Fogg's recent message are
    > 1. How can you reduce the brightness of light by more than 100%? If you
    > have reduced it by 100%,
    > then you have already achieved complete darkness.

    An f/stop more is a doubling of light brightness, thus 100%, while an f/stop
    less is a halving, thus indeed 50%. A somewhat pedantic point hardly worth
    making, I would have thought.
    > 2. Yes, if you introduce one polarising filter, you will approximately
    > halve the light intensity,
    > for initially unpolarised light, because you are excluding the 50% of the
    > light that is polarised at
    > right angles to the direction of the filter that lets light through. Plus
    > a slight further
    > reduction, to allow for losses in the plastic material and in surface
    > reflections.
    , that
    > won't halve the light intensity again, ! because now all the light entering
    > it is polarised in the
    > preferred direction. This second filter will therefore cause a slight
    > reduction in intensity, as
    > before, because of further losses in that extra layer of plastic and
    > further reflections, but
    > nothing like the reduction that the first filter caused. That's a
    > deduction based on theoretical
    > reasoning, not on any practical tests by me, so it may possibly be
    > contradicted by experiment. I
    > would be interested to learn if it is.

    Take two polarising filters and try it out, George, as suggested by me
    earlier (I do hope it was included in the part you did understand). Once you
    see it working, more convoluted reasoning may be introduced to explain how
    it does, perhaps..

    > ... if you add a second filter, that's polarised in the same
    > direction as the first ...

    By turning the second filter the remaining light polarised in! the other
    direction is gradually choked off until no light passes.

    Getting back to sextant filters, the advantage is that the intensity of
    light passed by the two filters is infinitely adjustable, with, again; the
    proviso that the initial minimal light diminution is not.

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