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    Re: Bris Sextant
    From: Alexandre Eremenko
    Date: 2005 Nov 7, 22:37 -0500

    Dear George and Frank,
    I thought a bit  more about "rocking" the Bris device,
    and it seems that it needs two kinds of rocking
    to obtain the best precision.
    1. The one we discussed: rotation abut vertical axis
    to remove the collimation error, and
    2. "Ordinary rocking" that is rotation about the axis
    between the eye of the observer and the horizon. To ensure
    that you really measure the angle in a vertical plane.
    It was a mistake when I said that rocking of the first kind replaces
    the ordinary rocking. One needs both kinds.
    Remark. When I had to practice with ordinary sextant but without
    telescope last summer, I noticed that some rocking about a vertical axis
    is also needed to avoid the collimation error which is almost
    always present if you don't use a scope (or some other visor)
    which ensures that your line of sight is parallel to the frame.
    (That is perpendicular to the axis of the index mirror).
    If one attaches a Bris to a telescope, the first kind of rocking will
    be probably unnecessary.
    But the attachment cannot be as simple as I thought first.
    a) the Bris device (or at least its filter part has to cover ALL
    the objective lens (for safety considerations) and
    b) it should be quickly adjustable for the "number of the sun".
    I imagine some arrangment which has 8 positions and adjusts by clicks.
    c) collimation adjustment can be probably fixed once and forever.
    (No modern sextant has it, as I understand, except the SNO's inverting
    On Mon, 7 Nov 2005, Frank Reed wrote:
    > George wrote:
    > "Now, yaw the instrument about,  so
    > that first its right-hand edge, then its left, is closest to the eye.  Only
    > at one position, when it's exactly at right angles, so the edges  are
    > equidistant from the eye, will the viewline be in exactly the  right
    > direction to avoid collimation error. What I ask is this; in use, is  that
    > optimum orientation obvious to the observer?"
    > Alex wrote way back  on October 11th:
    > "Rocking. When you slightly rotate the devise about VERTICAL  axis,
    > you will see that the reflected "Suns" move up and down slightly.
    > You want to measure the altitudes when the reflected Sun is in the LOWEST
    > position. This happens when the horizontal lines in the planes of your
    > glasses are perpendicular to the line of your sight.
    > This rotation plays  the role of rocking the usual sextant. "
    > He's clearly describing the same  thing here. This rotation about the
    > vertical axis is designed to eliminate  collimation error --as much as possible. This
    > is *not* the same thing as  'rocking the usual sextant'. So here's the tricky
    > part: can you rock the Bris  "sextant" about its vertical axis to eliminate
    > collimation error and then  independently rock it about the line of sight axis
    > to ensure that the  measurement is perpendicular to the horizon? I think it
    > would be difficult to do  these two motions correctly and indepenently, and I
    > think this is the biggest  problem with the accuracy of the Bris "sextant".
    > A note on semantics: is  it a sextant? Technically, no. But then again, I
    > have a David White Co. sextant  (US Navy, 1944) in front of me that can measure
    > angles up to 142 degrees. So  it's not a sextant --it's a quintant. Of course,
    > if I call it that, it just  generates confusion. In the present era, if the
    > word sextant means anything at  all (*), it means angle-measuring instrument.
    > But there's a distinction that I  think is worth making anyway. The biggest
    > difference between the Bris "sextant"  and a standard sextant is that a standard
    > sextant can measure ANY angle within  the limits of the arc. So maybe we could
    > call it a "Bris fixed-angle  sextant".
    > I've been entertained by the discussions people have had about  improving the
    > Bris sextant by attaching a telescope, different shades, and so  on. Myself,
    > I've been thinking about mounting the glass pieces on a frame.  Perhaps one of
    > the mirrors should be allowed to rotate. And as long as I've got  a frame, I
    > could permanently attach a small telescope. Hmmm... I'll need some  sort of
    > handle to rotate the movable mirror... And maybe I could record the  angles on a
    > table affixed directly to the frame indicated by the position of the
    > rotating mirror handle! I'll call it a "Variable Angle Bris device" You  can see
    > where I'm going with this. This all leads me to wonder whether the early
    > inventors of the sextant came upon the idea by playing with homemade devices  very
    > similar to the Bris sextant and went on to invent the complete instrument  by
    > considering the very issues that have been discussed on the list in the past
    > couple of weeks.
    > *(continuing a thought from above) DOES the word  "sextant" mean anything
    > today? Twenty years ago, I could say "sextant" and most  people with some
    > education were aware that the word refered to some instrument  for looking at the sky
    > from the deck of a ship. Today, I find that people of  similar education
    > stare blankly when I use this word, even giggling a bit  because it sounds like
    > "sex tent".
    > -FER
    > 42.0N 87.7W, or 41.4N  72.1W.
    > www.HistoricalAtlas.com/lunars

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