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    Re: Testing sextants in 1885
    From: Henry Halboth
    Date: 2009 Jul 15, 20:38 -0700

    Your quotation from Bro. Clarke's 1885 work, which is quite familiar to me, includes the statement, with reference to centering error (arc) error .....

    "The error being found for certain places on the arc, the correction for any angle may by proportion be obtained."

    In my opinion, for what it may be worth, this statement is not technically correct. Arc error, which necessarily includes centering error, graduation errors, deformation error, and the residuals of any other uncorrected errors of mirror parallelism and perpendicularity, as well as telescope collimation and mirror prismatic errors does not necessarily change proportionately over the full extent of the arc and should be individually determined at any measured angle - or at least at far lesser intervals than is normally done. This error may also change over the years and old certificates certainly should not be taken as Gospel.

    Clarke's little book is great, but there are others out there, including "The Sextant and its Applications", Simms 1858, which go into the subject far more exhaustively, and may well appeal to the more technically minded.



    --- On Wed, 7/15/09, frankreed@HistoricalAtlas.com <frankreed@HistoricalAtlas.com> wrote:

    From: frankreed@HistoricalAtlas.com <frankreed@HistoricalAtlas.com>
    Subject: [NavList 9099] Testing sextants in 1885
    To: NavList@fer3.com
    Date: Wednesday, July 15, 2009, 10:06 PM

    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


    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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