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    Re: Graphs of Lunar Distances.
    From: Frank Reed
    Date: 2010 Oct 24, 20:59 -0700

    Hewitt, you wrote:
    "But after five days practice ashore and five days at sea, using our one and only David Mk 15, they were consistently getting sun LOP intercepts of 1', and often even better."

    Wow. That's pretty good! Did you have any special procedure? For example, some people suggest dealing with the variability of plastic sextants by taking IC observations immediately before and after each sight and then averaging.

    Let's compare two instruments with and without telescopes. Suppose I have a Davis plastic sextant and a good metal sextant, let's make it a new Astra IIIB just to be definite. I use both of them without a telescope for some kind of standard observation that I know I can trust, like Moon-Jupiter lunars. I could expect errors (at the 1 s.d. level) of about 1 minute of arc for the Astra sextant, and, if we use your Davis instrument (or whatever procedure you use to make it work as well as you do) I could expect errors of about 1 to probably 1.4 minutes of arc for the Davis sextant. Now, the Astra sextant is really only limited at this point by the resolution of human vision. So if I now attach a 7x telescope and repeat the observations, I can expect errors (again at the 1 s.d. level) of about 0.14 minutes of arc, and improvement of a factor of 7 in direct proportion to the magnification. This assumes I've also re-done the IC test with the telescope to get the same level of accuracy there. But by contrast, with the Davis sextant, magnifying probably won't help much at all. It's very likely already close to its limit. We might reduce the error from 1.4 to 1.0 (at 1 s.d.) but we can't take out that residual error that arises from the instability of the instrument due to its plastic construction.


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