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    Re: Reliable Index Correction to a Tenth Minute of Arc
    From: Bill Morris
    Date: 2010 Feb 19, 23:08 -0800

    Dear Brad,

    I wonder whether I should congratulate my one of my instruments, a C Plath three circle vernier octant from about 1909. I used the x12 inverting scope with an eyepiece shade, to avoid discussions about prismacity, which I will investigate and report on at some other time.

    I did four estimates of the index error using the sun this morning before my neck began to protest. I tried to make them independent of each other, rather than using the preceding off the arc reading combined with the next on the arc reading. All four on the arc readings were 33'30" and all four off the arc readings were 30'00", giving an index error of 1'45" on the arc (and, for what it's worth, a standard deviation of zero).

    There is of course a problem with vernier sextants: one has to make a judgement about which lines coincide. If one is biased to getting the same reading each time, one may tend to choose a particular line. If however, one were to be biased towards "proving" that one is unlikely to get the same reading each time, one might choose another line. I freely admit my bias was towards finding variability.

    This evening I used a ladder frame Hughes and Son vernier sextant, new in 1920 , with a x10 'scope and eyepiece shade. For your delectation,the pairs were (on the arc first):
    34'00" 31'00" 1'30"
    33'20" 31'00" 1'10"
    34'10" 31'00" 1'35"
    33'50" 30'50" 1'30"

    These were not quite as consistent. Perhaps I was trying harder to be biased. More likely is that the tangent screw mechanism was a bit sticky. In both instruments, the tangent screw impels the index arm in one direction and a spring returns it in the other. If not working properly, slip-stick can be a very annoying problem. I recently rebuilt the Plath instrument (its story is on my web site), but it is some time since I serviced the Hughes. Within a micrometer instrument, there are several subtle sources of error that militate against consistency at the 10 second level and may account for some makers, in particular Freiberger Prazisionsmechanik and Tamaya, omitting a micrometer vernier.

    Kind regards

    Bill Morris
    New Zealand

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