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    Chasing tenths of an arc minute
    From: Bill Morris
    Date: 2010 Mar 11, 20:42 -0800

    Lately there has been much written about accurate determination of index error, some of it relating to the instruments used and some relating to the observers’ physiology. Recently, Richard Pisko mentioned the subject of backlash, in the context of surveying instruments and I see there was quite extensive (though not always well-informed) discussion of this on the list in 2005. Not for the time being having any new old sextants to pull apart, I have devoted a little time to considering aspects of accuracy of a few micrometer sextants of various ages.

    All are second hand but in good condition and have been completely overhauled. I first made a start on the subject in my blog post of 6 July 09 at www.sextantbook.com. under the head of "Interesting Overhaul Problems" and I have followed it up with another account, posted today, under the head of "Chasing tenths of an arcminute".

    The account is rather lengthy and is illustrated, so I have posted it on my web site rather than here,for those interested in the details, but briefly my findings are as follows.

    In sextants that follow C Plath’s original micrometer design, backlash is negligible, under 3 seconds. This design, which uses a leaf spring to oppose axial movment of the worm shaft by pressing a collar on the shaft against a thrust surface on the bearing, was followed by Tamaya, Hughes and Son after 1939, USNavy MkII, Astra, SNO-M, Cassens and Plath and, of course, C Plath.

    In sextants that depended on close manufacturing or hand fitting of the worm shaft bearing to reduce backlash, such as the Freiberger Trommelsextant, SNO-T and pre-1939 Hughes and Son, backlash was a respectable 6 seconds, but by further attention to detail could be reduced to close to that of the first group. If known to be in good condition, it probably doesn't matter from which direction you approach a reading with this type and the Plath type.

    The third group, that mounted the worm shaft and swing arm between adjustable coned centres as bearings, includes Heath and Co and later Kelvin Hughes sextants. In my example, I could not by any means adjust out the backlash of about 75 seconds, but it was at least fairly consistently of this amount. I remain puzzled as to its source. My findings might caution you always to approach a reading from the same direction with this type of micrometer design.

    The bearing between the swing arm and index arm, and the index arm bearing itself are in principle other sources of backlash. If well made in the first place and properly adjusted, wear here should be negligible, and backlash from this source is unlikely to be important.

    As I have mentioned in a previous post, calibration certificates tell us only about instrumental accuracy at various whole degrees and nothing about how accurately the micrometer divides these degrees. In my blog post for 6 July 2009 (“A Worm Turns”) I gave some results and in the latest post I give some more. Many people, I am sure, will be surprised at the size of some of the errors. Bear in mind that the worst are those that deviate most from a horizontal straight line as, over a full rotation, there are always exactly 360 degrees in a circle and the errors have to sum to zero (in practice there is nearly always a deficit of plus or minus a handful of seconds).

    On this basis, the SNO-T, a 1938 Husun with a new worm, a 1938 Husun with the original worm, an Ajax Engineering US Navy MkII and a Freiberger all perform well, but before deciding that the Tamaya, Heath and SNO-M don’t cut the mustard, consider that they were second-hand instruments and that 10 seconds at the periphery of a radius of 150 mm represents a movement of about 4 thousandths of a millimetre. Invisible particles of dust and fibres and minute nicks can thus easily lead to variations of this amount.

    Bill Morris
    Pukenui
    New Zealand
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