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    Frank: welcome back (and questions)
    From: Alexandre Eremenko
    Date: 2012 Mar 27, 20:26 -0400

    Welcome back. While you were absent several questions to you accumulated.
    1. About Nav List archive. All messages for 2012 are listed under January.
    (I already addressed this 2 times, but you were away, and probably missed
    these messages).
    2. Question about your Lunar sight and Almanac.
    How far back it is reliable?
    I know it is OK for XVIII century. Is it OK for BC dates?
    Actually I don't even know how can I enter a BC date:-)
    3. You have "predicted lunars" for many more stars than your
    "clear a lunar online" allows. Many years ago you said that you
    consider adding stars to the calculator. Why you decided not to do this?
    On Tue, 27 Mar 2012, Frank Reed wrote:
    > I haven't posted about this in a while. After some years of experimentation, 
    trying various different techniques, I have found that the best way to get an 
    index correction is to take off the sextant's standard telescope and place a 
    higher power spotting scope in line with the instrument on a table. Then you 
    look at some suitably distant target, some miles away, and do the usual 
    alignment. With a 30x scope, you should find with a metal sextant that you 
    get the same IC over and over again to the limiting precision of reading of 
    the micrometer (in other words, you'll get +0.4 ten times in a row without 
    fail). A plastic sextant will continue to show a lot of variability, but by 
    using a spotting scope, you take the observer error due to the limited 
    resolution of the human eye out of the equation. Then you can assess the real 
    limitations of the plastic sextant and possibly find circumstances where the 
    IC is stable.
    > For those interested in sextant calibration beyond IC (for arc error at 
    angles other than zero), you can extend this by placing two sextants mirror 
    to mirror on a table using one as a standard for the other. As with the index 
    error check, you remove the telescopes and place a spotting scope in line 
    with the usual position of the telescope on the instrument being tested. Then 
    if you set both sextants to some angle, e.g. thirty degrees, the arc 
    calibration is reduced to a simple index correction check. The "calibration 
    standard" sextant delivers light to the sextant being tested at exactly 
    thirty degrees angular deviation (assuming the calibration standard is good, 
    of course) and then the sextant being tested straightens it back out so the 
    observer should see two identical images superimposed. Any difference is the 
    arc error for that angle.
    > -FER
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