NavList:

Hi Steve, you wrote:

"As I checked the instrument for index error, I notice that the when I attempt to superimpose the image of a star upon itself, there appears to be some type of misalignment in that the image of the star can only be brought alongside the star as I obtain the measure the index error. My history with regard to index error has been 0.5’ on the arc. Now, the first several attempts at determining index error gave 2.5’ on the arc as I tried to imagine when the star and its image were in alignment with one another.

I don’t know what else may have been affected in the accident. The instrument came with a little square headed wrench that fits the mirrors. I am reluctant to try to make any adjustments by myself."

Nothing to worry about, and you can definitely do this yourself. First of all, that "side offset" is usually called "side error" though it is not necessarily a source of error and is really mostly a nuisance. Obviously when aligning star images beside themselves, you discover its largest nuisance value. Try instead a distant straight line. For example, look for a radio tower or a wind turbine a few miles away. Hold the sextant on its side for this, and align the direct and reflected images. Then read off the index error as usual. You can leave it at that. The instrument works fine, and you have the new index correction. If it happens to be 2.5' of arc, so be it. That's fine.

You can also use that little tool and adjust out most of the "side error" (a little left over is quite harmless), and you can even adjust the index correction itself down to a negligible level (I.C.=0.0' is certainly convenient!). Look for the adjustment points on the back of the horizon mirror. There are two with those little square heads that match the tool. Turn those and they tilt the mirror frame in two directions. This can be a little annoying since you have to put your hand in the line of sight to perform the adjustment while simultaneously looking through instrument. You can make things easier by setting the sextant on its side on a table, again sighting at some distant vertical structure. Just "fiddle" with those two adjustments, and you will find in short order that you can re-align things quite nicely. In older sextants, the two motions are not independent, you really just have to experiment to get it right.

Oh... but before you mess with the horizon mirror, you should really check the index mirror for perpendicularity. Do you know how to do that by sighting the arc from above the index mirror? It's easy, too, but you really should do this first when you buy a new/used sextant or after a sextant has been dropped or bounced around too much.

Bottom line: unless something like a mirror is physically broken, or the impact was hard enough to damage the hub where the index arm rotates, the sextant is designed for adjustment in the field, and you can definitely do it yourself.

-FER

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