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    Re: Calibrating a sextant: with a sextant and a spotting scope
    From: Bill Morris
    Date: 2011 Oct 7, 19:54 -0700

    Frank

    With a quick lashup - a piece of old plywood mounted on a theodolite tripod for a table - and two SNO-T sextants set at thirty degrees each without index or side error, the method tells me that there is an error of 1.4 minutes, when by other means, one has an error of +4 seconds and the other -3 seconds. I used the side of a barn 6 km away for an object and the x6 inverting scope supplied with the observing instrument, and made sure to observe in the centre of the field. The 15 mm plywood is a bit bowed. Could that have something to do with it? I struggle with two dimensional geometry, never mind three.

    I'll try again with a really flat table and set both frames in the horizontal plane with a level.

    I wonder, from the comfort of my armchair, whether your powerful spotting scope does "make the process really accurate". It may make the observations more precise, but doesn't the accuracy depend on the known errors of whichever instrument you are assuming to be the standard?

    I'll report again after I have done more work and, for the avoidance of doubt, will also try the method with two C Plath sextants with known errors.

    Bill Morris
    Pukenui
    New Zealand
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