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    Re: Calibrating a sextant scale
    From: Fred Hebard
    Date: 2007 Nov 23, 11:20 -0500

    Yes, Alex and I forgot this means of calibrating.
    For those without access to a theodolite, the church steeple method
    may be the quickest.  An advantage of the star-star or lunars methods
    are that you improve your technique while trying.  Of course then you
    have to decide when your technique has improved enough to start
    collecting useful calibration data.
    Some important parameter of technique are looking through the center
    of the telescope in line with the object, always approaching the
    point from one direction, calibrating the micrometer beforehand, and
    accounting appropriately for real barometric pressure (to correct
    refraction).  The last effect is not of significance below elevations
    of 250' above sea level or so, or, for sextant observations of
    celestial objects, periods of abnormally high barometric pressure.
    On Nov 22, 2007, at 3:22 AM, engineer wrote:
    > The interested amateur can calibrate a sextant(which should of course
    > be in adjustment) without too much expensive gear, now that so much is
    > available on e-bay. He will need two collimators. A collimator is
    > essentially a lens with illuminated crosswires or other graticule at
    > its focal point, so that the image of the wires appears to be at
    > infinity. He will also need to own or have access to a modern
    > universal theodolite, reading directly to single seconds of an arc,
    > e.g Wild T2 or later. It will be helpful to know how to operate the
    > theodolite, but they are robust and not impossibly complicated.
    > In principle,the procedure is as follows. Focus the theodolite on a
    > distant object, level it and then point at one of the collimators, set
    > up in the same horizontal plane a few metres away. Sight on the
    > vertical wire. It should be in focus, If it is not, make it so by
    > adjusting the collimator cross wires, not the thedolite. Note the
    > reading(s) on the horizontal circle of the theodolite. Rotate the
    > theodolite through, say, fifteen degrees, set up the second collimator
    > and adjust it until its vertical wire coincides with that of the
    > theodolite(it helps to have an assistant and /or a rotary table to do
    > this). You now have, as it were, two images at infinity, fifteen
    > degrees apart. With your sextant supported horizontally, see how it
    > compares. Repeat for as many angles as you like.
    > In practice there are many refinements to get the angles as exact as
    > possible, but the amateur can improvise a collimator from any
    > telescope that has cross wires by the simple expedient of substituting
    > a green LED for the eyepiece; and Edmunds Optical once had a short
    > book "Collimators and Collimation", which describes how to build your
    > own.
    > National testing laboratories like the National Physical Laboratory in
    > Britain would have a battery of up to fourteen collimators set up
    > permanently on masonry columns.
    > There is another method using two instruments known as
    > autocollimators, a large surface plate and a plane mirror, that
    > depends on there being always exactly 180 degrees in a straight line.
    > It is described(in French)in :Canadian Journal of Physics 1976. Vol
    > 54, Number 9, pp 917 - 927. The principle is again simple, but the
    > execution I suspect is not. I will be trying it out over the next few
    > weeks and if there is any interest will give an account of how a
    > Pioneer-Bendix Mk II, a Mark IX bubble sextant and an SNO-T sextant
    > perform.
    > Bill Morris
    > On Nov 21, 12:33 am, Isonomia  wrote:
    >> Whilst it is easy to calibrate a sextant's zero reading, is there any
    >> way to calibrate any other readings?
    >> My thought was to lean over a cliff and view my own reflection, and
    >> then align the mirror with the horizon, this should be somewhere
    >> close
    >> to 90deg (give or take the variation of the horizon from horizontal).
    >> Having established the true 90degree, the 45degrees could be
    >> calibrated by finding two objects precisely 90degrees and finding an
    >> object exactly halfway between.
    >> Then it occurred to me that I could use a bucket of water at the
    >> seaside, to give two readings (direct and false horizon) one being
    >> exactly** twice the other.  (** allowing for known errors)
    >> at which I've just realised that star charts are far easier .....
    >> but it might amuse others to think about leaning over a cliff!
    > >
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