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    Re: Calibrating a sextant scale
    From: Geoffrey Kolbe
    Date: 2007 Nov 25, 20:25 +0000

    Well Alex, this is what the 3T2KP looks like...
    http://www.airshow.ru/expo/304/prod_25.htm
    
    But it is difficult to tell from external appearances what a theodolite is
    capable of. One capable of reading to 1" that a surveyor would use, is very
    similar to one that reads to 20" like the standard "building site"
    theodolite, or even my 2T30P which only reads to 5 minutes - yes, that is
    minutes.
    
    As you say, my 2T30P would be no use at all as the primary reference in
    calibrating the arc of sextant. (I have another theodolite for that :-)) I
    think they were the Soviet version of standard "building site" theodolites.
    The fact that the scale is 15 times less precise than the equivalent
    theodolite here in the West may be a comment on the standard of Soviet
    brick layers... ;-) That said, I find it a very useful theodolite which is
    easy to adjust, easy to set up, easy to use and easy to interpolate angles
    to 1 MOA which, in my experience, is usually as good as you can reliably
    get without going to a whole different level of sophistication ranging from
    the tables used to the temperature control of the instrument.
    
    It is worth commenting here that just because a theodolite can be read to a
    precision of 1" does not mean that the measured angle is that accurate.
    Thermal effects and even flexing of the telescope tube can lead to errors
    amounting to a good fraction of a minute. An interesting paper to read is
    "Modern Methods of Finding the Latitude with a Theodolite" by John Ball,
    The Journal of the Geographical Society, Vol. 49, Part 6, Pages 440-458,
    1917, The paper is now 90 years old, but Ball was measuring latitudes to 1"
    when surveying and mapping the Egyptian Western desert in the first decades
    of the 20th century and I think his experience is still valid today.
    
    Geoffrey Kolbe
    
    
    
    At 17:54 25/11/2007, you wrote:
    >
    >
    >
    >Gear Geoffrey,
    >
    > > The type of theodolite is the 3T2KP.
    >
    >That's what I wanted to know, thanks.
    >Now I know at least one brand of Russian 1"
    >theodolite. Is this the one that looks like
    >Wild T2 ot like Wild T3?
    >
    > > I too have one of their theodolites,
    > > a type 2T30P,
    > > It only reads to five minutes,
    >
    >Do you really mean "minutes" (not "seconds"?)
    >This is the accuracy of a good wooden cross-stuff:-)
    >Pretty useless for sextant calibrating business.
    >
    >Alex.
    
    
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