NavList:

I haven't posted about this in a while. After some years of experimentation, trying various different techniques, I have found that the best way to get an index correction is to take off the sextant's standard telescope and place a higher power spotting scope in line with the instrument on a table. Then you look at some suitably distant target, some miles away, and do the usual alignment. With a 30x scope, you should find with a metal sextant that you get the same IC over and over again to the limiting precision of reading of the micrometer (in other words, you'll get +0.4 ten times in a row without fail). A plastic sextant will continue to show a lot of variability, but by using a spotting scope, you take the observer error due to the limited resolution of the human eye out of the equation. Then you can assess the real limitations of the plastic sextant and possibly find circumstances where the IC is stable.

For those interested in sextant calibration beyond IC (for arc error at angles other than zero), you can extend this by placing two sextants mirror to mirror on a table using one as a standard for the other. As with the index error check, you remove the telescopes and place a spotting scope in line with the usual position of the telescope on the instrument being tested. Then if you set both sextants to some angle, e.g. thirty degrees, the arc calibration is reduced to a simple index correction check. The "calibration standard" sextant delivers light to the sextant being tested at exactly thirty degrees angular deviation (assuming the calibration standard is good, of course) and then the sextant being tested straightens it back out so the observer should see two identical images superimposed. Any difference is the arc error for that angle.

-FER

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