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    Testing sextants in 1885
    From: Frank Reed
    Date: 2009 Jul 15, 19:06 -0700

    A couple of months ago we were talking about the required quality of sextant 
    mirrors. I found a nice description of a good practical test today in "The 
    Sextant" published in 1885 by H. Wilberforce Clarke of the Royal Engineers. 
    This guy has a way of making simple things complicated, but he is also 
    extremely thorough in his writings (he has another book on "lunar distances" 
    which is the epitome of "difficult" lunarian texts).
    
    His advice for testing the mirrors:
    "To test the reflectors, look with a small telescope obliquely and separately 
    into each reflector, and observe the image of a distant object.
    In every part of the reflector the image should be clear, distinct (not 
    streaky), single, and well defined about the edges."
    
    This idea of looking at a reflection at a very shallow angle is a good basic 
    test. I tried it today with a sextant that I know has a slightly concave 
    index mirror (it's on a Davis plastic sextant so not unexpected), and the 
    imperfection of the reflections was very obvious.
    
    He also offers a suggestion for measuring arc error:
    "Imperfect centering may be detected by comparing the distance between 
    two(N.A.) stars measured by the sextant under trial with the same distance 
    observed with a standard sextant, or with a circle; or with the same distance 
    deduced from computation. The error being found for certain places on the 
    arc, the correction for any angle may by proportion be obtained."
    
    ...with a footnote showing how to calculate the correction for refraction 
    correctly --albeit by a long method. The idea of comparing the observed angle 
    with two sextants, one as a standard, is an obvious way to avoid the math 
    work (so obvious, even I didn't think of it!), and it also strikes me as a 
    good "sanity test".
    
    There are some other good ideas in the book. They're primarily intended for 
    explorers and surveyors working on land. Clarke shot many, many lunars and 
    other sextant sights on the Upper Nile in the 1870s/80s, but most of the 
    advice also applies to accurate work at sea.
    
    "The Sextant" is a quick read, only 45 pages in length. It's available here: 
    http://books.google.com/books?id=epAoAAAAYAAJ
    
    -FER
    
    
    I was browsing on google books today (I needed a statistic: there are 2,160 
    books from the 19th century indexed with the word "sextant" in their text).
    
    
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