OVERVIEW
The first choice to make is betweenplastic or metal construction. Today's low cost metal sextants offer highaccuracy and ease of use. These reward the beginner's efforts, and satisfythe professional's demands. Plastic models are perfect for lifeboat provisioning,and for restricted budgets. They are also acceptable to some experts whodon't mind making frequent adjustments. The following characteristics ofsextants should be considered.
NEWER USED?
Older sextants tend to have smallermirrors and scopes which make them harder to use. Spare parts and maintenanceare also more uncertain. Avoid discontinued models (ie. those not shownin this catalog), and those greatly out of date. Purchase only from someoneyou know and trust, or a reputable dealer. You will find that today's lowcost metal sextants are very competitive with expensive used ones.
ACCURACY
For all practical purposes, metalsextants are error free when compared to the many uncontrollable errorswhich may exist from such things as refraction, oblateness of the earth,and data tabulation. Generally, a minute of arc (one mile) is about thebest anyone can hope to achieve. For these reasons, undue emphasis shouldnot be placed on extreme accuracy guarantees. Plastic sextants commonlyexhibit errors in excess of 5 minutes, even when great care is exercised.Although this is sufficient to make landfalls; precision navigation isdifficult, and student progress may be retarded.
MIRRORSIZE
The size of the mirrors on a sextantgenerally vary directly with the quality of the instrument. Large indexand horizon mirrors are desirable because larger mirrors admit more light,making it easier to obtain sights in marginal conditions. Larger mirrorsalso lessen the possibility of losing the image as the body is broughtdown to the horizon.
WEIGHT
Sextants are available with theirmajor metal parts made of either aluminum or brass. The alloys of eachmetal are both suitable for use at sea. Some people feel that the heavierweight of a brass sextant provides greater steadiness and hence more accuratereadings, especially if it is windy. Others find that the lightweight modelsare less tiring to their wrist and arm and that the reduced fatigue givesbetter results. As the observer develops proficiency and speed in sighttaking, fatigue becomes less of a factor. Lightweight plastic models canbe difficult to use facing into a stiff wind because they tend to "flutter".
SCOPES
A 3.5 (or 4) x40 scope is a goodchoice for stars. The large objective 40mm lens admits a great deal oflight. The 3.5 to 4 power magnification helps you find and maintain starsin view in both calm or pitching seaways. A 6x30 or 7x35 monocular of greatermagnification is well suited for sun sights, or the greater heights ofeye associated with large ships.The increased magnification allows thesun's diameter to appear larger, and better defines a more distant horizon.This helps the navigator determine the point of tangency of the sun's limband the horizon. The increased magnification however makes finding andholding sights more difficult on a moving deck. A Sight Tube of zero magnificationaffords a wider field of view for rough weather, horizontal angles, andfinding stars. If your sextant is to have only one scope, a 3.5x or 4xwould be the logical choice for yacht sized vessels.
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| TRADITIONAL | WHOLE HORIZON |
HORIZON MIRROR
Many sextants have an option of eitherthe traditional (half-silvered) horizon mirror or what is called a "wholehorizon mirror". With the traditional mirror, the horizon glass is dividedvertically into two halves producing a "split image." The half nearestthe frame is a silvered mirror and the other half is clear glass. In somecases this clear glass is eliminated. A later development in sextant technologyis the whole horizon mirror. Using specially coated optics, the whole horizonmirror superimposes both the horizon and the celestial body on the entiremirror with no split image. This greatly simplifies "bringing down" thecelestial body and makes it easier to hold the body in view. A drawbackto this system is a very slight reduction in light transmission and reflectionwhich may affect marginally lighted observations. Some feel these two aspectsare a "trade off; that is, one can more quickly take observations withthe whole horizon mirror, and be finished before marginal conditions occur.In general, professional navigators tend to favor the traditional horizonmirror while beginners tend to favor the whole horizon mirror.
ILLUMINATION
Sextant lighting is the least neededfeature on a sextant, since a flashlight should normally be available inany event for recording observations.
VALUE
Contrary to the adage that you getwhat you pay for; global exchange rates, tariffs, and labor costs havecombined to produce variations in value. In this monetary respect only,we would rate the ASTRA IIIB sextant highest, and the Tamaya sextants lowestin value for the metal sextants. The Davis Mark 15 is the best in valuefor the plastic models.
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olav hansen aps. 86 99 15 10 |
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