NavList:

Hi Frank

Recently, you wrote:

“I actually have recently found a method that does give the index correction to the tenth of a minute of arc, again and again, reliably, and with near perfect repeatability. But it's a land-based method.”

Please…do tell!  I would love to try this!

My method is one of super-imposition, using the sun’s image on top of its reflected image, whilst paying attention to the edges or margins of the sun.  The index mirror is perpendicularity is first set using Cmdr Bauer’s domino method.  But then, I set the vernier’s nonius directly to zero, using as much care as possible, not changing it until the process is complete.  Next, I tweak the horizon mirror for perpendicularity and index while simultaneously looking at the sun’s image.  The sun is a bright crisp image (using the appropriate filters, of course) and therefore, it is quite easy to see when they don’t precisely overlap or overlay.  Then I put the sextant aside until the evening and pick out a bright star.  Without dark adapting my eyes, I check to see if the two star images super-impose.  If they don’t, I tweak the horizon mirror again while simultaneously looking at the star images.  Again, put it aside.  The next day, actually use the index arm to bring the sun’s two images in contact for both the upper and lower limbs.  Create the difference to find the standard IE correction.  This readily gets me well below ½ arc minute, and typically in the 10 seconds to zero IE.  This method can be performed solely on a star image, but I have not found the same degree of success.

This CAN be done at sea, no reason why not. 

As to the recalled difficulty with different telescopes, I would suggest that this implies a lack of parallelism between the plane of the arc of the instrument and the long axis of the telescope. Some sextants do have an adjustment for parallelism.  Pick out two bright objects about 90 degrees apart.  Assuming stars for a moment, superimpose them.  With a vibratory motion (to quote 1856 Bowditch!) see that they maintain contact on both sides (toward and away from the plane of the arc) of the telescope.  If your telescope has the sighting wires, use them!  Adjust the telescope’s parallelism until the images remain in contact on both sides. 

Best Regards

Brad