When looking to buy a sextant your biggest decision is quality and purchase price. The two basic sextants to look at are plastic and metal, with the price ranges varying. Their is quite a bit of difference in the price range between the two, choose whats best suited for you and how much it will be used.
The plastic sextant's advantage is price, disadvantage is 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 you take a set of sights. The navigator will 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 corresponds to one nautical mile on the plot.
Secondly, plastic sextants weigh less than a pound and some have considerable wind resistance, making it more difficult to hold the sextant vertical when sighting in windy conditions.
Thirdly, the quality of the components is less, the filters, the mirrors, the zero to three power viewing scopes. And last, the life of a plastic sextant is shorter, depending on the amount of use. Filters break off, the plastic gearing wears out, 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. As a primary sextant when a celestial fix is important, I would recommend investing in the better sextant. 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 better accuracy and the life of the instrument is long if care is used in usage and storage.
The metal frame may be made of either brass or an aluminum alloy, lightening the weight from roughly four to three pounds. The size of the frame varies too, also changing the weight. 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 is 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 drum. The battery case, wires and bulb socket are all subject to corrosion at sea and batteries tend to wear down when when needed. Cases usually come with the sextant and are included in the price.
Second hand metal sextants are a rare and not much of a price bargain. Some sextants are sold as antiques and application of this prices them beyond their 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 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 you, how comfortable the instrument is to use, and how experienced the navigator is in handling the sextant.
