Index error (I.E.) can be determined more precisely during the day by using a custom fitted solar mylar film filter (the type used to observe a solar eclipse) over the objective lens on the sextant scope. The result will be a pair of sharp uniform orange Sun images. Overlap the Sun images for direct index error readings or take half of the difference between the two limb to limb readings.
note:
-When off the arc the reading is 60' minus the micrometer value.
-Shading the sextant scope directly allows the horizon mirror shades to be pulled aside which opens up better access for the adjustment tool.
Link: http://www.thousandoaksoptical.com
Thousand Oaks Optical (928)692-8903
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