Welcome to the NavList Message Boards.


A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Add Images & Files
    Re: sextant precision.
    From: Bill B
    Date: 2005 Jun 23, 16:14 -0500

    >> I am coming around to your way of thinking on sextant mirrors.
    > How gratifying it is to find a sinner who sees the light by thinking about
    > it and convincing himself...
    Careful George. "If you make people think they are thinking, they will love
    you; but if you make people really think, they will hate you."   I do not
    know the source of the quote, but have always liked it and valued people who
    do not resent being made to think.
    > As a general rule the mid-point of the reflecting plane of the index mirror
    > is indeed aligned with the pivot axis of the index am. But it's in no way
    > crucial that the axis should be placed exactly there, in the reflecting
    > plane or across the middle of the mirror's area.. If it wasn't, then as you
    > shifted the index arm, that mirror would be displaced sideways a bit, and
    > in the end you might lose a bit of light because one edge would move in and
    > shrink the width of the light-path to the horizon mirror. But it wouldn't
    > alter the sextant readings, not a jot.
    Understood, and a nice breakthrough in my thinking.  Somehow I felt it had
    to be that way because traditional sextants were designed so the axis was in
    the center of the mirror vertically AND in line with the rear-silvered
    surface.  So I was always suspect of front-silvered mirrors in the same
    mount as rear-silvered, but took it on faith until now.
    > Alex Eremenko has commented that in his Russian SNO sextant that is indeed
    > the case. The pivot line lies behind the front-silvered face of his index
    > mirror, not on it, as I recall. And the only disadvantage that has resulted
    > is that with such a geometry, a favourite trick for checking
    > perpendicularity of the index mirror doesn't work. That trick is to look at
    > at the continuity of the direct and reflected arc as seen in the index
    > mirror. It only applies if the effective reflecting plane is exactly
    > aligned with the pivot. Because, in the past, all sextants were made that
    > way, that restriction on the use of the method was never made clear. Alex
    > has to use a different method to check perpendicularity.
    My Astra IIB Deluxe, like Alex's SNO-T, has a front-silvered index mirror.
    Like Alex I have to use gauge blocks to align the index mirror.
    Thanks again

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.

    Email Settings

    Posting Code:

    Custom Index

    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site