NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: accurate sextant
From: Fred Hebard
Date: 2008 Feb 22, 08:20 -0500
From: Fred Hebard
Date: 2008 Feb 22, 08:20 -0500
Alex' SnoT also has luminescent paint. The luminescence is obtained by exposing it to light before use, if I recall correctly. The use of the magnifier with no vernier then allows one to read the sextant at night without a light. Now how one writes down the numbers without a light is another story. The inverting scope is much lighter and slimmer than an equivalent binocular, if a bit longer. Fred On Feb 22, 2008, at 5:10 AM, George Huxtable wrote: > Another unusual feature is that there is a magnifier, to ease the > reading of > the micrometer drum, and also there's no Vernier with that drum > ( which, > otherwise, would be used to interpolate the minutes to tenths). I > suspect > these aspects are connected: if there was a Vernier, then the > magnifier > would have to be able to swing, to read it over its span of 10 > divisions or > so, around the drum's arc. So doing without the Vernier allows the > magnifier > to be fixed in place in a simple housing. The choice made by the > designer, > then, was either a simple magnifier or a Vernier, but not both. I > suggest > that any observer worth his salt should be quite able to estimate > to tenths > of a minute without a Vernier to aid him, especially with a > magnified image. > > One virtue in providing a magnifier could be in allowing the > diameter of the > read-off scale on the drum to be kept small. Otherwise, if a drum > is made > large enough to be read with ease directly, then the shaft joining > it to its > worm must be made long enough to avoid that large drum clanging on > the arc, > which increases the bulk of the complete read-out assembly. This is > a bit > speculative, as I haven't seen one of these sextants, just Bill's > pics. > > Finally, I ask those familiar with this instrument whether they see > any > advantage in the straight-through, inverting, 6x telescope, > compared with an > equivalent, non-inverting, prismatic ocular of similar power and > light-grasp? Presumably, the prismatic would be a bit heavier; are > there > other differences? > > George. > --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---