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    Re: Getting to know a sextant.
    From: Frank Reed
    Date: 2017 Feb 16, 16:45 -0800

    Tony Oz, you wrote:

    Is my understanding correct:- the optical axis of a telescope should be:
    • parallel to the sextant frame (i.e. - no collimation error)
    • level with the horizon's mirror transparent/reflective border
    How important is it to have the proper frame-to-axis distance (the point #2 above)?

    The scope collimation is moderately important. The height of the scope above the frame is not especially important and is adjusted for convenience. This latter adjustment is something that you would do at the time of taking a sight, and its function is comparable to selecting the correct shades for a sight. It's not a permanent adjustment of the instrument.

    But you should understand that you do not need a scope. The resolution of the human eye is about one minute of arc. The limiting accuracy of celestial navigation regardless of magnification is also about one minute of arc. So you only get a modest improvement in the normal observations of celestial navigation (excluding lunars, which are not "normal") by adding a scope to your sextant. The scope removes some "noise" in the observations. It's nice to have a sighting ring though, as Greg Rudzinski described in his post here: http://fer3.com/arc/m2.aspx/g38309. You could sail the oceans your whole life without a scope on your sextant, and I know a number of real ocean navigators who prefer that. If you decide you want to get into special sights, like lunars, then you'll want a telescope with a magnification of 5x to 10x (about 7x yields the best results in my experience).

    Getting your sextant ready for use consists of four normal steps and one optional:

    • set the index mirror perpendicular
    • reduce the index correction as near to zero as possible
    • reduce so-called "side error" to a level that is not inconvenient
    • measure and record any remaining index error
    • (optional) adjust the scope collimation

    If you need to know how to do any of these, just ask. 

    Frank Reed
    Conanicut Island

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