NavList:

Dear Alan,

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:

SD_Sun = (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.

Warm regards,
Tony

60°N 30°E