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    Pistor & Martin's Prismatic Sextant
    From: Brad Morris
    Date: 2016 Dec 21, 19:35 -0500
    A rare Pistor and Martin's Prismatic Sextant is on eBay.  This sextant can read very large angles compared to a nominal sextant.  The scale reads from -10° to +255°.  When set to 180°, and held in just the right way, you can observe your left eye with your right!  This one also has the right angle eyepiece, so as to avoid this problem.

    This sextant is quite similar to my Circle of Reflection, except the sextant only has one vernier, whilst the CoR has two. The two vernier setup permits any centering error in the index arm to be minimized. 

    The sextant is a high quality device, and should be considered by any serious celestial navigation enthusiast



    On Dec 21, 2016 7:14 PM, "Brad Morris" <NoReply_Morris@fer3.com> wrote:

    At the risk of sounding pedantic, it appears as if Paul has suggested that the altitude correction should come from the planets and stars table, because (and this is the important part) he expects the direct path and index mirror path images to be superimposed when observed.  In doing so, the SD is ignored, as superimposition of the images transforms the observation into a point source observation, like a star observation.  

    You indicated that you used a lower limb observation, which implies you performed a limb to limb contact observation.  In that case, the SD does apply.

    I believe you gentlemen are just having a small communications glitch.


    On Dec 21, 2016 6:15 PM, "David C" <NoReply_DavidC@fer3.com> wrote:

    You have used the sun altitude correction table.

    With AH-sights you have to use the star and planet altitude correction table instead.

    Which means that the observed altitudes are smaller and the intercepts bigger than in your calculations.

    The difference between an AH sight and a horizon sight is that the former does not require a dip correction. As I am using an AH I am not correcting for dip.

    Consider an altitude of 37 degrees.

    From the Air Almanac 2016 R = 1' and SD = 16.3'. I was using the lower limb so the correction is +15.3'. Note that refraction is given to a precision of 1'.

    From the NA 1973 Sun's Correction Oct-March (scan attached) the correction is +15.0. This is to a precision of 0.1'.

    I believe that I was using the correct value but maybe I am missing something?

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