A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Tony Oz
Date: 2019 Oct 11, 05:38 -0700
The index error is the last to be addressed. You should work in certain order:
- make sure the index mirror is perpendicular to the sextant frame
- reduce the so-called side-error
- with the above adjustments done - correct the index error
The last step will possibly require to repeat the side-error adjustment.
The most accurate way to measure the index error by observing the Sun:
- hold the sextant so its' frame is horizontal
- put on the proper filters
- set the index arm at 0°
- aim the sextant at the Sun, you should see two overlapping images of the Sun
- turn the drum to decrease the values ("off scale") until the images separate a little, now turning the drum in the increasing direction bring the images to the touching position
- read the scale and drum. Values to the right on the "0" considered negative, write it down as 359° (or what you actually get), read the drum at the face value and write the minutes part of the measured angle
- now turn the drum all the way up to the point when the overlapping images just touch on their other sides
- the readings are now "on scale", write them as 360° + minutes off the drum
The index error is now calculated thus:
I.E. = 360° - (onscalevalue + offscalevalue) / 2
An easy check for the sanity of result - with the same values (the off-scale and the on-scale) do:
SDSun = (onscalevalue - offscalevalue) / 4
...and compare the result with the Sun's semidiameter in NA.
I'll be happy to help you more if needed.
The higher the scope magnification - the easier and more accurate are the measurements.