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    Re: Simplified Bris Sextant (calibration)
    From: Bill B
    Date: 2012 May 15, 14:41 -0400

    Are two crossed polarizing filters (with on other filters) dark enough
    to view the sun with the naked eye?
    I tested a Tiffen crossed with a Nikon, and they measured max 6.5 f
    reduction.  The sun pack on my Astra is 11.5 f stops--32x reduction over
    the polarizers.
    Even with a 2-stop ND filter included, the sun image was still to bright
    to view with the naked eye IMHO.
    I am also concerned with pointing a camera at the sun for more than a
    second or two with 6.5 f-stop reduction. At some point in the lens
    barrel the sun's image is reduced to a point, that point being near the
    aperture-control leaves if memory serves.  Like setting a leaf on fire
    with a magnifying glass, severe damage is possible.
    Bill B.
    On 5/15/2012 12:55 AM, Greg Rudzinski wrote:
    > "How did you attach a dark filter to it?"
    > Two linear polarizers are held with fingers for shading. The degree of
    > shading can be adjusted for best contrast.
    > "Can you look through polarizing filters
    > at the Sun?"
    > Yes. My Cassens & Plath sextant is equipped with polarizer shades. See
    > attached image showing how to hold the filters and reflector.

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