The first choice to make is between plastic or metal construction. Today's low cost metal sextants offer better accuracy and are easier to use. This will help you when first starting, and satisfy the professional's demands. Plastic models are perfect if your budget is restricted. They are also acceptable to some professionals who don't mind making frequent adjustments.
New or Used
Older sextants tend to have smaller mirrors and scopes which make them harder to use. Spare parts and maintenance are also more uncertain. Avoid discontinued models, and those out of date. Purchase only from someone you know and trust, or a reputable dealer. You will find that today's low cost metal sextants are very competitive with expensive used ones.
Accuracy
For all practical purposes, metal sextants are error free when compared to the many uncontrollable errors which may exist from such things as refraction, oblateness of the earth, and data tabulation. Generally, a minute of arc (one mile) is about the best anyone can hope to achieve. For these reasons, undue emphasis should not be placed on extreme accuracy guarantees. Plastic sextants can have errors in excess of 5 minutes, even when care is exercised. Although this is sufficient to make landfalls, precision navigation is difficult.
Mirror Size
The size of the mirrors on sextants generally vary directly with the quality of the instrument. Large index and horizon mirrors are desirable because larger mirrors allow more movement of the sextant while taking a sight, and lessen the possibility of losing the image as the body is brought down to the horizon.
Weight
Sextants are available with their major metal parts made of either aluminum, bronze or brass. The alloys of these metals are well suitable for use at sea. Some people feel that the heavier weight of a bronze sextant provides greater steadiness and more accurate readings, especially if it is windy. Others find that the lightweight models are less tiring to their wrist and arm and that the reduced fatigue gives better results. As the observer develops proficiency and speed in sight taking, fatigue becomes less of a factor. Lightweight plastic models can be difficult to use facing into a stiff wind because they tend to "flutter".
Scopes
A 3.5 x 40 scope is a good choice for stars. The large objective 40mm lens admits a great deal of light. The 3.5 power magnification helps you find and maintain stars in view in both calm or pitching seaways. A 7x35 monocular having greater magnification is well suited for sun sights, or the greater heights of eye associated with large ships.The increased magnification allows the sun'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 limb and the horizon. The increased magnification makes finding and holding sights more difficult on a moving deck. A Sight Tube of zero magnification affords a wider field of view for rough weather, horizontal angles, and finding stars. If your sextant is to have only one scope, a 3.5x would be the your best choice for yacht sized vessels.
Traditional or Whole Horizon Mirror
Many sextants have an option of either the traditional (half-silvered) horizon mirror or what is called a "whole horizon mirror". With the traditional mirror, the horizon glass is divided vertically into two halves producing a "split image." The half nearest the frame is a silvered mirror and the other half is clear glass. In some cases this clear glass is eliminated. A later development in sextant technology is the whole horizon mirror. Using specially coated optics, the whole horizon mirror superimposes both the horizon and the celestial body on the entire mirror with no split image. This greatly simplifies "bringing down" the celestial body and makes it easier to hold the body in view. A draw back to this system is a very slight reduction in light transmission and reflection which may affect marginally lighted observations. Some feel these two aspects are a "trade off" that is, one can more quickly take observations with the whole horizon mirror, and be finished before marginal conditions occur. In general, people on stable platforms such as large ships tend to favor the traditional horizon mirror while those on yachts tend to favor the whole horizon mirror.
Illumination
Sextant lighting is the least needed feature on a sextant, since a flashlight should normally be available in any event for recording observations.
What's In It For Me?
It's tempting to think those solemn warnings only applied in the days before we could carry a GPS receiver in every pocket, but even the least paranoid among us knows that's not true: the GPS system itself can be intentionally switched off or degraded very quickly. It can be physically damaged; it can possibly be sabotaged by hackers. None of those things is likely to happen. More likely are on-the-water problems: GPS receivers can be fried by lightning, dropped overboard, crushed and damaged. Batteries do run out or get soaked. Well, as any offshore sailor knows, you just carry a back-up handheld. Or two, or three. Plus batteries and waterproof bags.
It's tempting to think those solemn warnings only applied in the days before we could carry a GPS receiver in every pocket, but even the least paranoid among us knows that's not true: the GPS system itself can be intentionally switched off or degraded very quickly. It can be physically damaged; it can possibly be sabotaged by hackers. None of those things is likely to happen. More likely are on-the-water problems: GPS receivers can be fried by lightning, dropped overboard, crushed and damaged. Batteries do run out or get soaked. Well, as any offshore sailor knows, you just carry a back-up handheld. Or two, or three. Plus batteries and waterproof bags.
With all that, the chances of being far offshore without the ability to find a way home have diminished to a point where the demand for sextants has decreased markedly over the past 10 years. It's not what it was, but the demand for sextants has steadied. It's sailors rather than professional navigators who are interested in sextants now. There's a reasonable and steady demand. It's not so much that people need to have them, it's that they want to use them to navigate or just learn something new. Alot of today's interest in sextants stems from tradition.
There's more to it than that. Reading the sky not only tells us where we are, it connects us with all those who for centuries made their way over the world's waters with nothing more than simple instruments and tables, and the wisdom handed down to them on how to use them. Those skills, in turn, connect us in a practical way to the relationship of the earth to the sun, moon, and stars, a relationship that fewer and fewer sailors understand, to their detriment. The challenge of reading data on a GPS receiver pales next to the challenge of navigating by sextant, and for many people the resultant levels of satisfaction are proportional.
Selecting a Sextant
The clearer you are about your intended uses for the sextant, the better your chance of finding a place along the price range where your standards of value can be met. You can spend anywhere from $19.95 up to $3,000 for a serviceable sextant. The cheapest are made of cardboard (a German-made kit and plastic. While the cardboard kit is something of a novelty item with limited navigational potential (and no waterproofing) some plastic sextants are viable instruments for celestial navigation.
Still, most salts say plastic sextants aren't reliable enough for "real" navigation and that they should be limited to practice and/or lifeboat duty. Plastic sextants can yield results that are very close to those received from metal sextants: While most metal sextants can be shown to yield accuracy within a nautical mile the limitations built into plastic sextants give them a margin of error of five miles at best.
Instrument accuracy is really the least of your worries. Virtually all new sextants have negligible instrument or uncorrectable error. They also come with instructions for removing index (correctable) error. At that point you can bank on your new sextant as being virtually error free. Data reduction, refraction, and the oblateness of the earth are all more likely to be sources of inaccuracy than the sextant itself.
One of the biggest development in sextants over the past 20 years has been an across-the-board improvement of the optics involved. Mirrors have gotten bigger, coatings have been hardened, scopes made more versatile. Whether the sextant you buy comes from Germany, Japan, or China, you're likely to find that the optics are first-class.
Most navigators like a sextant with some "heft." Mass helps to steady the instrument. If, however, extending the sextant strains your arms and you're rushing your sights to be free of its weight, then you've got too much heft. Modern sextants range from 2 to just over 4 pounds. Finding one that's got substance enough to be steady but is light enough not to be taxing is a definite quest. Comfort in the grip of the handle is also important. Beware an overlycocked wrist and a sextant whose weight is not virtually centered around the handle.
Some top of the line sextants are made with brass armatures, but it's more common today to see framesmade of aluminum. This raises the topic of maintenance and the spectre of dissimilar metals. Even though you will obviously do your best to keep your sextant dry, smallboat use makes it likely that sooner or later it will get wet. Rinsing with fresh water and patting the sextant dry with a clean cloth is generally all that's required, but pay particular attention to those spots (like the stainless steel screws holding the mirror frame to the aluminum arm) where dissimilar metals are in contact.
