NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Error of Perpendicularity
From: Bill B
Date: 2006 Apr 18, 17:39 -0500
From: Bill B
Date: 2006 Apr 18, 17:39 -0500
> Furthermore, I know of a few techniques for adjusting the index mirror for > perpendicularity, including the eyeball method and placing objects (such as a > pair of dominos) of exactly the same height at opposite ends of the sextant > limb but it seems to me that these methods are rather coarse and not nearly as > precise as those for reckoning index and side errors. > > Any comments on this most fundamental topic are welcome. Robert When I first obtained my Astra with front-silvered index mirror, I went through the process. (I had not yet obtained the matched brass cylinders.) All the dominos I could find were too big (double size). I took a micrometer to dice and other objects about the right size but they were not a close enough match. A little starter kit of Lego blocks (a few bucks at a toy store) were the solution. I mixed and matched them on a flat surface (a 1/2"-thick piece of float glass). You don't have to measure them, one can feel a difference of a couple thousands of an inch. I got really anal with the best match (with a few thousands difference) and slapped a piece of fine sandpaper face up on the float glass and lapped the "high" Lego down for an exact match (within a few ten thousandths). That being done I placed the sextant on a flat surface (3/8" thick float glass table top), placed the Legos on the arc, and measured from the table top to the top of the blocks. A shim of two index cards under one leg made both block tops equal height from the table top and level fore/aft and port/starboard. (A little circular bubble level on the frame could work too.) Then I attached an index card to a small scrap block of wood (a base to hold the card upright) and placed a pencil mark at the same height as the block tops. I placed this on the table top between my eye and the sextant. That gave me a reference mark to make certain my eye was aligned with the top of the bricks. NOTE: I needed a lot of light for this, to get enough depth of field in the old eyeball to have both the index card and Lego in focus. Seems to have worked. Bill