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    Re: Perpendicularity Error and the Davis Mk 3 Sextant
    From: Brad Morris
    Date: 2017 Nov 14, 06:51 -0500

    The degree of parallelism in float glass is such that it is not of any concern for celestial navigation.  If the surfaces were indeed out of parallel, inducing a prism effect, then that would affect our measurement.  However, the glass is more than sufficiently parallel.

    Other areas could be of concern.  Firstly, how crisp is the mirroring at the edge of the mirrored surface?  If the glass ends when the mirror does, then it will be difficult to maintain a sharp mechanical edge for the mirror.  Beveling the glass at the edge will also affect the mirrored surface and appear as a ghostly edge in the FOV.  

    It is far cheaper to install a completely mirrored glass in the inexpensive plastic sextant than it is to install a half mirrored glass.  


    On Nov 14, 2017 6:24 AM, "Robert VanderPol II" <NoReply_RobertVanderPolII@fer3.com> wrote:

    The reason I understand is that normally there is clear glass in the horizon path because there is a slight induced error in the optical path thru the horizon mirror (the 2 faces of the mirror glass are not perfectly parallel.  By having the same piece of glass in both optical paths that error cancels out.  You still have the error of the index mirror that isn't cancelled out, and I suspect the error is pretty small with currenty glass making technology, but if you want to do high precision work you take every error reducing step you can.

    For the Mark-III cost is prime consideration and accuracy around 1' is expected so the cost of this error correcting step is not worth the money.

    Re: Perpendicularity Error and the Davis Mk 3 Sextant
    From: Peter Monta
    Date: 2017 Nov 13, 13:19 -0800

    . . . 

    Incidentally, and this is off-topic to the question, but what is the reasoning behind a horizon mirror with glass in the horizon path?  The Mark 3 has just a mirror on the index-mirror path and empty air on the horizon path.  It seems perfectly usable this way.  Do "normal" sextants provide a split mirror to get the benefit of the 4% reflection from the plain glass?  Or they don't want to expose the edge of the mirror to the elements?  Or is there some other reason?

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