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dashmanc@XXX.XXX writes:
 > Rick makes some good arguments and some points that I can and cannot agree with.  As
 > for the issue of quality variations:  I only have experience with my Davis Mark 25 and
 > 3 and I have not seen the problems Rick describes, only the ones I have described.  As
 > for the telescope:  unless you are engaged in shooting marginally visible objects
 > (granted that can happen) the optics on my Davis don't lose much.  It is true that the
 > larger the objective, the more light you should be able to collect--but only if the
 > magnification is kept constant.  Check with binoculars--a 7x50 will collect far more
 > light than a 10x50 (and of course more than a 7x40). Again, for tough visibility you
 > need a higher quality instrument.
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.
 > I agree that the whole horizon vs half is a matter of preference--again I have no
 > experience with other Davis sextants, but my whole horizon gives as large a field of
 > view as my Tamiya, albeit a half horizon.  Perhaps the half version in a Davis is too
 > small--I don't know.
 > As for shooting the sun on a clear day--magnification is nice, but unnecessary. Would
 > it help a beginner? Well, my opinion is maybe and for that I can't justify the added
 > cost of even an Astro (though maybe a Mark 15).
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.
 > 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.
A Davis Mk15, 20, or 25, an Astra III, or a Plath all have about the
same features.  In fact, my Astra has one more horizon filter than my
Plath.  But they all have, essentially, the same features.  None of
them connect to computers (whatever became of those projects?), have
gyro stabilizers (why doesn't Canon work on that?), or other "chrome".
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.
 > 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]
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.
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.
Rick
S/V One With The Wind, Baba 35
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