NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: sextant index error measurement
From: Paul Hirose
Date: 2006 Nov 04, 00:02 -0800
From: Paul Hirose
Date: 2006 Nov 04, 00:02 -0800
I wrote: > To check parallax, make another measurement at half the original > distance. It would be best to construct another target so the sight > picture looks the same at the shorter distance. The results from this > second test will reveal any residual parallax, since IE is constant with > distance while parallax varies in inverse proportion. This residual parallax could be used to correct the measurement of the sight-line offset. In fact, the precise measurement method George Huxtable described never occurred to me. I imagined that an approximate measurment would be gotten by eye and ruler, then refined with the parallax test. In turn, with a precise offset value you can prepare a 3-line target custom-tailored to a particular sextant. If the interval between the white space (separating the paired lines) and the single line is just right, the index error test becomes independent of distance. In practice, if you cut the observing distance in half, the widths of the white space and the single line must also be cut in half to maintain sensitivity. Dimensional accuracy of the test pattern becomes twice as critical too. I think it would be easiest to create and print the pattern with a computer. The self-test patterns my inexpensive ink jet printer generates are amazingly crisp; I think it could do a fine job if I knew how to prepare a suitable drawing. The bisection method occurred to me because some of the theodolites I've handled use that principle in the optical micrometers that read the graduations on the circles. For example, on a Kern 1-second DKM2 instrument you rotate the micrometer knob to center a single line in the narrow space between paired lines. This is more precise than simply putting one line atop another. The globe sight on a target rifle also relies on the eye's ability to judge centering with great accuracy. -- I block messages that contain attachments or HTML. --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---