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    [Fwd: [Fwd: [Fwd: Davis Sextants]]]
    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
    It's been fun guys, but I must get to work.

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