NavList:

Randall,

It is best to not fool with the screws once you are within a minute or two of zero. Paint a dab of clear nail polish on the index and horizon screw adjusters to lock in place. There will remain some slop in the readings which is normal for plastic sextants. Be sure to use a light grip and light fingers on the micrometer drum for improved consistency.

Greg Rudzinski

[NavList] Index correction sun sights
From: Randall Morrow
Date: 20 Feb 2012 07:51

I spent some time yesterday doing repeated index correction sun sights with my Davis Mk 15, using the techniques described in a Starpath article. In more than 20 observations I got 20 different values, both on and off the arc before I finally gave up. Previously I had posted that sun and moon sights using the appearance of tangency always give me trouble due to the distortion I observe as the edges approach. But in addition to this, when the images do not fall on a vertical line I adjust the horizon mirror as the article suggests, but when I repeat the sight for verification the images are again off line. It has been suggested that I do not turn these screws on the mirror often but they do not stay in line from one sight to the next so re-adjusting seems necessary. When turning the horizon mirror finger screws the image seems to be affected by movement of the plastic frame of the mirror by the pressure from my fingers as I turn the screw. Is lack of rigidity in this component a factor? Before making the sights I removed the telescope and looking at the reflection of the arc, brought the direct and reflected images in line for perpendicularity. Am I missing something? Please advise.
Randy

Randall F Morrow PT
Kern County - Bakersfield

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