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    Re: Sextant Arc Calibration : FIRST LIGHT
    From: Brad Morris
    Date: 2014 Mar 31, 16:03 -0700

    http://www.lightglassoptics.com/Davidson-Optronics-D-633-12-Optical-Polygon-12-Sided_p_520.html

    I present this link for those who wish to understand a bit more about metrology instruments.  For the benefit of a reader some years on, let me summarize what can be seen at the link.  All of these optical instruments are considered to be in the same class.

    First is shown the 12 sided optical polygon.  Each face is constrained to be within a certain tolerance of 30° spacing (12*30°=360°).  That tolerance is specified as no worse than 5" of arc error, but could be 1", since he does not have the calibration certificate.  The optical polygon is listed at $4100.

    On the same page, related items are shown.  Two of them are an  Ultradex.   The $6600 dollar Ultradex is stated to be accurate to 1/2 second and have increments of 1°.  The other Ultradex is a mere $4800, and has the same accuracy spec.

    An Ultradex is the equivalent of a 360 sided polygon.   I have seen a 72 sided polygon (5° per facet), for which they wanted $29000.  The Ultradex is a superior instrument and well known in metrology.

    I do hope this helps to clarify the instrument and in what class of instruments it resides.  It is used to verify the angular precision of another device.  In our circumstance, that would be the sextant.

    There is one other instrument on that link.  Its an autocollimator.  Surprise!

    Brad
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