A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
[Fwd: [Fwd: [Fwd: Davis Sextants]]]
From: Carl D
Date: 1999 Jan 26, 5:16 AM
From: Carl D
Date: 1999 Jan 26, 5:16 AM
> > > It's possible to go overboard on magnification, either in binoculars > or a sextant scope or monocular. Past a certain point, the platform > you're shooting from becomes a signifcant issue. As the boat pitches, > rolls, and yaws, that motion is also magnified. 7X seems to be a > reasonable compromise on this issue for binoculars, for example. With > a sextant, where the field of view is limited to begin with and > putting two images into a specific relationship is crucial, lower > powers are needed. ABSOLUTELY! I always tell people at football games--don't go for power, go for objective size and apparent field of view--you'll enjoy them more. 6x is FAR more useful than 10x from the stands. > The question is one of accuracy; not ultimate, "good enough only for > NASA" accuracy but reasonable, "I'm pretty sure we're here" accuracy. > If a round of sights is erratic, the confidence in the final result is > reduced. With magnification (subject to the constraints above), it's > possible to get the limb of the Sun or Moon smack on the horizon with > greater confidence *and do so repeatedly*. This produces (with > practice) consistant accurate results which, in turn, leads to > justifiable confidence in the sights. > > Talking about optics, I want to clarify a point about magnification. > 6X30 monoculars (the "30" is the diameter of the front lens in > millimeters - i.e., 30mm) are intended for Sun (and Moon) sights while > the lower power 2.5X or 3X scopes are much better for star and planet > shots. As the magnifcation of a system goes up, assuming the lens > diameters remain constant, the scope's light gathering ability > decreases. Clearly, neither the Sun or Moon are dim objects so the > loss of light gathering in a 6X30 monocular is not an issue. With the > dimmer nav stars or planets (Mars or Saturn), reducing the > magnification helps. Since all of these targets are effectively point > light sources, there's little information lost (save, possibly some > resolution of the horizon) with the reduction of magnification. I mostly agree, but as all astronomy buffs know, light gathering ability is far more important than magnification. You can see far more with a 10" scope at 75x than a 3.5" scope at 150x, especially when dealing with point light sources. > > > > As for the lathe analogy: here I believe Rick has missed my point. The items he omits > > are necessary to the operation of a lathe (specifically, a metal lathe, not a wood > > one, as my dad was a machinist) and to overcome them would require even a skilled > > machinist to go to extraordinary lengths are are akin to omitting the sight tube, sun > > filters and vernier marks on the index arm. Yes, it's that extreme. It's nice to > > have auto-feed, auto-cleaning, and computer control on a lathe--it certainly speeds > > both setup and production and are pay for themselves, but they are not needed. > None of > them connect to computers (whatever became of those projects?), have > gyro stabilizers (why doesn't Canon work on that?), or other "chrome". GPS killed those projects. You can buy a GPS w/ PC connection for the cost of a Mark 25. If electricity is not a concern, why bother with sextant to PC? (of course I don't want to get into the GPS/ sight reduction debate--we all know the issues). I LIKE the idea of a Canon stabilizer! How 'bout it guys? > Where they differ most is in the precision of their construction and > the materials they're made from. Lathes, it seems to me, are far more > varied and complex. Naw, not old ones. My dad has one in his cellar that must be70 years old and still working. > > > > Rick has clearly been better at finding good, used and new sextants at bargain prices > > than I have and I will defer to him on that. > > Gee, I wish my wife was as kind on that point! [grin] If she figures every buck spent on a sextant is wasted, you are SOL. [double grin] > > > I don't argue that a potential student should rush out and spend top > dollar on a sextant any more than, in my ski instructor days, I'd > argue for buying top of the line skis to learn to snowplow (sorry, > PSIA, make that "perform a gliding wedge"). I do argue, however, that > below some level, the student's equipment makes the learning process > avoidably more difficult. Now you know why I don't ski... > > > I saw this happen with a friend who was trying to make a Freiberger > Yachtsman work. He was having little success in getting a good round > of sights so I asked to have a look. The filters were hazy and the > mirrors were more mirrors in name than in deed. Exchanging the > Freiberger for an Astra, he made a great deal of progress in minutes. > The condition of this particular sextant is, I hope, atypical but it > does point out that less than adequate gear is a significant hindrance > to learning. I was shocked, on the WindSurf, to learn they are stocked with Friebergers, not Plaths or Tamiyas. It's been fun guys, but I must get to work. ATB, Carl.