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    Re: Getting to know a sextant.
    From: Frank Reed
    Date: 2017 Feb 14, 16:23 -0800

    Brad Morris, you wrote:
    "I was less than clear in describing the telescope alignment."

    I hate to say it, but your clarification post is digging a deeper hole. :/

    You wrote:
    "Certainly, holding your binocular as a monocular in sequence with your sextant will put that collimation entirely into your wrist!"

    A simple sighting ring, as described by Greg (if I remember correctly) will deal with this, and in any case, it is a relatively minor issue. Best solution is obviously to acquire a scope designed for the instrument. 

    Next you wrote:
    "The yaw of the long axis of the sextant (rotation perpendicular to the arc) is built into the telescope mount, with the residual error accounted for in your index error.  That is, you adjust the yaw by rotating the index mirror."

    Now that just doesn't make sense. There is no additional impact on index error arising from the telescope in any way --no matter how I interpret this (no, your follow-up, which I have just read, does not fix this). Index error exists if the horizon glass is not parallel to the index mirror when the arm is set at zero. It is a simple zero-ing error, like setting an old-fashioned spring postal scale to to zero. The quantity of index error is inconsequential (in other words, you can live with it) except to the extent that it requires an additional step to correct it, and every step in the clearing process yields a potential for error. But it is not augmented by the orientation of the telescope. That's just wrong.

    While we're here, if at all possible I recommend zero-ing index error as standard practice today. 

    You added:
    "Side error can also be present, where the horizon mirror is not perpendicular to the arc of the sextant."

    Side error is no error at all. It is merely an inconvenience, and even then, it is usually not much of an inconvenience. Many people find some benefit in a few minutes of arc of the lateral offset erroneously known as 'side error'.

    You concluded:
    "I highly recommend Commander Bauer's excellent Sextant Handbook for determining and eliminating these errors. 
    As, I am sure, will others."

    Twenty-five years ago, when there were few other options, Bruce Bauer's book was the best thing available. It is no longer that by any stretch, and I would not recommend it to anyone --unless my goal was to waste that person's time and money. You can find anything you need to know online. Bauer's book has a number of errors in content and several points of questionable advice, and the second edition (the common one) is a mess of editing errors. In addition, the book claims to be a history of the sextant, but it is only a rough, un-historical sketch of the instrument's history. Stay away!

    Frank Reed

       
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