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    Re: Lunar distance sights
    From: Frank Reed
    Date: 2013 Apr 22, 12:20 -0700

    You should expect to get the scatter down to about a quarter of a minutes of arc in individual observations but only if the following conditions are met:
    1) 5x or better magnification
    2) a properly adjusted, good-quality, metal sextant
    3) carefully measured index error
    4) no significant arc error

    If the first condition is not met, then you can expect your accuracy to be reduced in proportion to magnification. The second condition is a bit broad. What constitutes a "good-quality" sextant? If you're using a Davis plastic sextant, I would expect a scatter (in the "standard deviation" sense) of about 1-2 minutes of arc. A "properly adjusted" sextant has had its mirror perpendicularity checked, its telescope collimation adjusted, and any "looseness" adjusted out where possible. Careful estimation of index error is critical to this sort of observation since, of course, any error in IC will translate directly into an error in the lunar observation. This is true in common altitudes, too, but the scatter in altitudes usually swamps small residual errors in index correction. Finally "arc error" is generally out of our control. It amounts to an index correction that varies along the sextant scale (equivalent to the calibration certificates formerly included in sextant cases). Shooting lunars is, in fact, one of the best ways to assess arc error.

    The short answer is, yes, you can expect better. Tell us more about your observational conditions, and let's see if we can improve things.


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