NavList:

Herman Dekker, you wrote:
"IE correction.
If I understand the AntiSpoof app correct it has to correct the real calculated H value on its display with the conversily value of IC, so you can directly compare what you read on the sextant scale with the AntiSpoof display."

Yes. And I would add "of course" except that anyone can get befuddled by signs. It's easiest to keep things straight if we think about a positive index error. Let's make it two minutes of arc exactly for the sake of discussion. The error is 2.0' "on the arc" (always has been counted as positive index error). The index "correction" in that case is -2.0'. In the app, you enter the index correction. And then the app displays what your sextant would read which is 2.0' higher than otherwise.

As I emphasize in my classes, index error is nothing complicated, and it is completely analogous to the zero-ing or "taring" a common postal scale. As I have saied before, I recommend making every effort to zero out index error. There's no reason to fuss over it in the modern world. The idea of the "tortured sextant" is an antiquated bogeyman --a story told to navigators in the mid-twentieth century designed to scare them from messing around with their instruments (navigators who adjust their instruments too much will probably grow hair on their palms, too). This is an unnecessary prohibition. So set your index error to zero once a season.  

Having said that, there can be potential benefits to setting an exotic index error on a sextant. Suppose your height of eye is always 45 feet implying dip is 7'. Suppose you're only shooting the Sun when the refraction is close to 1', and of course you only shoot the Sun's lower limb. Subtracting those two corrections from the Sun's SD gives a net correction of 8'. You need to add 8' to every sight. You can accomplish that by adjusting your sextant so that it reads 8' when it should be reading zero (again comparable to taring a scale). If you force this +8' index error, you are letting the sextant do one small bit of the math for you. It automatically adds those constant corrections.

With some old vernier sextants, you may find that the arc is unreadable in the vicinity of zero. Suppose the finer divisions on the scale have been worn off by polishing. You can still potentially take excellent sights with an instrument like that by setting it to zero a couple of degrees up the arc (this may not be possible without shimming a mirror). Set it to be perfectly adjusted for exactly 2° on the arc. Then your index error is +2°. You subtract 2° from every sight you take.

But don't get carried away by these last two paragraphs! To reiterate, in the modern world, I highly recommend zero-ing out index error. Set it and forget it. Or in the case of a plastic sextant, zero it before every sight.

Frank Reed
Clockwork Mapping / ReedNavigation.com
Conanicut Island USA