Once the sextant is obtained, adjustments to the mirrors may be necessary to reduce the index correction to a minimal amount. One or two adjusting screws are located on each mirror for this purpose. Each mirror should be perpendicular to the sextant frame and when the sextant is set at zero the two mirrors should be parallel to each other. Three tests are involved. The first test is for perpendicularity of the index mirror. Hold the sextant on its side (with handle down) and with the index arm set to 35°.
Place your eye close to the sextant near the index mirror so that you can see the sextant arc in the mirror (reflected) and also just to the right of the mirror (direct). If these two images are not in a straight or continuous line, the mirror is not perpendicular to the frame. Adjusting the screws will bring the images in line.
The second test is for perpendicularity of the horizon glass. Actually, the "glass" is only half glass with the right half of the frame filled with a mirror. The horizon is viewed through the glass, the reflected image of the celestial object viewed in the mirror. If this horizon glass is not perpendicular to the frame, the error is referred to as side error. If a star is viewed both in the glass and in the mirror with the sextant set near zero, by adjusting the altitude, the star should pass over itself, become superimposed. If instead the reflected image of the star passes to the right of the direct image, side error exists and can be minimized by adjusting the two screws at the base of the horizon glass. Other celestial bodies may be used for this test as well as reasonably distant terrestrial objects.
The third test is for parallelism of the index mirror and horizon glass when the index arm is set exactly at zero. If at this setting the horizon or a celestial body appear higher or lower in the mirror than in the glass, the mirrors are not parallel and should be adjusted until they are. This error is called index error.
This is an error in the sextant itself and can be found by setting the sextant to read exactly zero and observing the sea horizon, a distant mountain top (a reasonably flat one), or a celestial object. At zero reading, the objects observed should appear the same height in the horizon glass and mirror.
If this is not the case, in other words, if the horizon or object in one side is above or below that in the other side,adjust the micrometer drum or the tangent screw until the objects are level with each other. Note the sextant reading. This is the amount of index correction. If the arrow is to the left of the zero or "on the arc", the I.C. is negative. If the arrow is to the right of the zero or "off the arc", the I.C. is positive. An easy way to remember this,though perhaps at first confusing,is to memorize. If it's on,it's off. If it's off, it's on. With a plastic sextant, the index correction should be ascertained for each set of sights since plastic will expand and contract with varying temperatures and will have different instrument errors. With a brass or aluminum framed instrument, the index correction should always be the same barring tampering with the mirrors or dropping the instrument.
