Thursday, May 1, 2008

Choosing a Sextant

In choosing a sextant, the biggest decision is quality versus purchase price. There are two basic "grades" to investigate, plastic and metal, with the corresponding retail price ranges varying.

The gap here is considerable the buyer should be better armed to choose the side of the gap best suited to him or her.

The plastic sextant's one advantage is price. This should be weighed against several disadvantages. First, since plastic will expand and contract with varying temperatures, the index correction (instrument error) is constantly changing. This can be partially compensated for by obtaining an index correction each time a set of sights is taken. However, the navigator may find that even between the first and last sight during a twilight series, the change can be considerable. Remember, a minute of error in sextant altitude directly corre­sponds to one nautical mile on the plot.

Secondly, plastic sextants weigh less than a pound and some varieties offer considerable wind resistance, all making it more difficult to hold the sextant vertical when sighting in breezy conditions.

Thirdly, the quality of the components is less, the filters, the mirrors, the zero to three power viewing scopes. And lastly, the life of a plastic sextant is shorter, depending on the amount of use. Filters break off, the plastic gearing wears down, the micrometer drum develops slop.

Any of the plastic sextants make excellent teaching aids where principle, not accuracy, is important. Also, as a back-up sextant, the micrometer plastic sextants can be very valuable. However, as a primary sextant when the celestial fix is important, I would strongly recommend investing in the better grade.

The advantages of a metal sextant are obvious after reading the disad­vantages involved in using a plastic sextant. Index correction is always the same unless the sextant is dropped or mirror adjustments are made. The weight (2 to 4 pounds) and open work frame reduce windage problems, the better optics and filters give guaranteed accuracy and the life of the instrument is indefi­nite as long as care is exercised in usage and storage.

The metal frame may be made of either brass or an aluminwn alloy, lightening the weight from roughly four to three pounds. The size of the frame varies too, also changing the weight. I find that the lighter sextants are easier to hold for a length of time.

The telescope power varies from three to eight power, the advantage of the higher power being mainly its ability to pick up the light of a star earlier in the twilight when the naked eye still cannot see it. The disadvantage of greater power is reduced field of view, and this becomes critical when the navigator is trying to keep the celestial body in the field while bouncing around on a small vessel. A four power scope is a good compromise:

Lighting is another option. Of course this isn't needed during the day but near the end of twilight, it is convenient to press a button or turn a switch to illuminate the arc and micrometer drwn. The battery case, wires and bulb socket arel all subject to corrosion at sea and batteries tend to wear down when most needed so often this "luxury" is questionable. The extra cost for lighting will finance an inexhaustible supply of penlight flash­lights which will clip on to clothing or store in the sextant case. Cases usually come with the sextant and are included in the price.

Second hand metal sextants are a rarity and often not much of a price bargain. Some sextants are sold as "antiques" and application of this title prices them beyond their useful value. The true antiques, the vernier sextants or octants, are nice as display items but the difficulty of reading a vernier versus a micrometer drum is a big disadvantage. Sometimes Navy surplus sextants can be found at reasonable prices. In any of these situations, instrument cleaning or mirror resilvering may be necessary but this cost will be minimal compared to the price of the instrument.

The final choice of instrument to buy comes down to how much you can afford, how essential celestial navigation is to your voyage, how comfortable the instrument is to use, and how experienced the navigator is in handling the sex­tant. Guaranteed accuracy ratings become less important when rough sea conditions and navigator inexperience prevail.