The sextant’s compromising views

The instrument
First, we need to know the workings of sextants, see illustration below. The marine sextant measures the angle between a celestial body and the horizon (or between any two objects) by using a rotatable index mirror attached to the index arm whose position can be accurately read (to at least 1.0’ of arc) from a calibrated arc and micrometer drum. The index mirror reflects rays from the body onto a second mirror, the horizon mirror. This specialized mirror simultaneously reflects both the image from the index mirror and transmits the direct view of the horizon, forming a composite image of the two in the observer’s eye.

In addition there are separate shades for adjusting the brightness of the celestial body and the horizon. Beyond the obvious protection that shades provide for bright objects (the sun, moon and bright horizons), they are very useful for achieving optimum viewing. To make a measurement while looking through a telescope (or a simple sight tube), the observer brings the two images seen in the horizon mirror into visual coincidence by adjusting the index arm. The angle is then read from the arc and micrometer drum.

When it comes to selecting a sextant, you might think that the most important requirements are accuracy, ruggedness, durability and stability. But not so. The reason is, happily, that all of the common makes and models meet all these requirements for practical use. Except for the plastic models, there are only slight differences in these characteristics. The hard choices come not from their mechanics, but from their optics — the compromising views through their optional telescopes and horizon mirrors.
These optical components determine the view used to bring the celestial body and the horizon into visual coincidence in taking the sight. The easy sights have high contrast: the sun, the moon at night and bright stars in mid twilight. The difficult ones have low contrast: dim stars in a bright sky, stars with a faint horizon and a pale moon or Venus in bright daylight. Therefore, the view’s contrast is an important factor for making difficult sights — and that depends on the sextant’s telescope and horizon mirror.

The sextant’s views
The telescope. Usually sextants are supplied with telescopes designated 3.5×40 or 4×40, where the first number is the magnification and the second is the objective lens diameter in millimeters. Many manufacturers offer optional 6×30, 7×35, or 8×30 scopes. The higher magnification helps to bring the celestial body more exactly into coincidence with the horizon. However, we’ve all probably experienced the difficulty of picking up a distant buoy with 7×50 binoculars on a small boat bouncing around in rough seas. Thus these higher magnification scopes can be a disadvantage on small boats.

Larger objectives lenses increase the brightness of point sources, such as stars. But this effect only becomes noticeable when the lens size doubles, so that the common 35 or 40 mm scopes show no difference, while stars in a 35 or 40 mm scope are noticeably brighter than in a small 20 mm scope.

But for extended sources, magnification decreases their surface brightness because this higher magnification spreads out the light across a larger image area. This means that for a given objective lens diameter, the lower magnifications have superior surface brightness useful for viewing dim horizons, such as encountered in star shots. But the higher magnifications are useful in sun and moon shots where surface brightness is not a concern. However, I’ve found that this surface effect is extremely subtle, verified only in the most difficult dim conditions. Nonetheless, some navigators call the 3.5x and 4x star scopes, and the higher magnification 6x, 7x and 8x sun scopes.
The Horizon Mirror. There are two types of horizon mirrors that produce a composite view of the horizon and the celestial body. The traditional mirror is silvered (or aluminized) on the right half (toward the frame), and clear on the left half. So you see a split image, the body on the right side of the horizon mirror and the horizon on the left. As shown in the illustration (a) on page 21, typically the telescope produces about one-half degree central overlap region of these two images.

Moving the sextant left and right will usually show the sun’s image all across the view, as shown in illustraton (b), page 21. The sun’s image on the right is reflected from the silvered half of the horizon mirror. The sun’s image on the left is a reflection from clear half of the horizon mirror; furthermore, darker horizon shades increase the sun’s brightness because of the increased contrast with its background. Covering the silvered half of the horizon mirror with a piece of black paper reveals no difference in the sun’s image on the left — proving its reflection comes from the left-half clear glass.

While this reflection from the clear half of the horizon mirror works okay for sun shots, dim stars appear even dimmer on the left because there’s no increased background contrast from horizon shades. This leaves the star’s brightest image on the right and in the small overlap region in the center. Again, this is verified by blocking off the silvered half of the horizon mirror. Thus, the best image of the star occurs in the right half the sextant’s field of view — right where there is no horizon image. The same holds for pale moon shots in bright daylight.

The newer type of horizon mirror isn’t divided into halves. Rather, its whole surface is uniformly treated with a thin film (dielectric or metal oxide), giving partial reflectivity from the index mirror, while also transmitting the view of the horizon. Because the horizon and the body are treated the same across the horizon mirror, there’s no split image; the two are simply superimposed, as shown in the illustration on page 24. Since there’s no split in the image, these mirrors are called whole-horizon (WH) mirrors, or various other names such as all-view or full-view mirrors.

In bright light conditions the WH mirror, with its two images completely superimposed across the field of view, is convenient for locating celestial bodies and is particularly nice for keeping them in view in rough weather on small boats. However, under the difficult conditions mentioned earlier — such as dim stars at twilight exacerbated by a low-contrast horizon — it produces a washed-out image. This is because superimposing two low-contrast images further reduces the resulting contrast of the combined image. This loss of contrast with the WH mirror is easily demonstrated by viewing the horizon with the index mirror aimed at a clear daytime sky; then swinging down the darkest sun shade to eliminate the sky’s interference shows a dramatic increase in the horizon’s contrast.

The WH mirror makes high-contrast sights more convenient, but it makes low-contrast sights more difficult — sometimes impossible. The traditional mirror does well with low contrasts, but has an inconvenient view. So with high-contrast sights, the WH mirror’s convenience outweighs the wash out problem, but with poor viewing conditions, the traditional mirror can make possible an otherwise impossible shot. Or to put it another way, the WH mirror makes easy sights easier and hard sights harder.

Views of traditional horizon mirrors. Our above discussion of traditional horizon mirrors assumed that we were looking through the most common scope provided on sextants — the Galilean scope. The higher power scopes, such as 6×30 or 7×35, use prisms, as commonly seen in binoculars. The Galilean scopes use a diverging lens for the eyepiece, making them the simplest, lightest and least expensive design, having only two lenses. These two scopes produce very different views of the traditional half-silvered horizon mirror.

Prism scopes have an internal focal plane where the image from each half of the traditional horizon mirror is focused all across the entire image plane, superimposing both images. This superposed image is similar to the image from the thin-film WH mirror. However the amount of superimposition depends on the relative contrast between the images from each side of the horizon mirror. Since we’re superimposing the light from the two images, a brighter image can wash out a dimmer one. But viewing two non-competing images will produce a complete overlap all across the field of view, just as shown in the illustration on page 24. For example, viewed through shades, the image of the sky with a bright sun is perfectly dark except for the sun’s disk. In this case we see a complete superposition of the sun and horizon all across the field of view, exactly as we do with the thin-film horizon mirror.

However, sights that do not have this extreme contrast will produce a washed-out view, similar to the thin-film WH mirror. But rather than a uniform wash out across the entire view, the horizon fades out toward the right edge of the view, and a body’s image fades out toward the left edge of the view. Thus the WH-effect grades gradually across the view. This effect is easily demonstrated: Looking at a horizon split into two images with a prism scope and then swinging down the darkest sun shade eliminates the wash out from the index mirror image.

It’s possible to control the relative effect of this wash-out by simply sighting to the left or right of center, or, in some cases, by adjusting the scope’s mount laterally to favor one side’s intensity over the other. (Unfortunately, some scopes have a recessed seat for the scope’s mounting screw that locks the scope in the center position. This can be modified with a little machining.) This control over the composite-image grading can make possible an otherwise impossible shot.

Field of view. The field of view (FOV) of a sextant is important for easily locating celestial bodies, particularly in star, wide-angle and coastal navigation sights — concerns that are exacerbated in rough seas. Typically the FOV of a Galilean scope is around 4°. But since traditional horizon mirrors with these scopes show only the horizon on the left, these combinations have an effective working FOV of only about 2°, as shown in the illustration on page 21. In some sextants, shade and mirror frames further obstruct the FOV. Since the shades aren’t used on star shots, and frequently dim stars aren’t sufficiently bright to show in the left half, then only about the middle 1/2° overlap remains to take the sight. On the other hand, prism scopes typically have a much wider FOV, around 7°, effectively, several times that of the lower power scopes.

Compromising views
Since accuracy and durability are very comparable among many sextants, my personal view is to select a sextant for overall quality, light weight, nice feel, at a reasonable cost. Then come the four compromising views:

The traditional mirror with a prism scope gives the best of both worlds: typically a full 7° WH view for sun and bright moon shots, with negligible compromise in low-contrast capability. If purchased new, the prism scope is an extra expense (but then a Galilean scope is also included). The higher magnification of the optional 6x prism scope is useful in smooth rides, while low enough to be tolerable in small-boat rough rides. With its wide FOV access to all sights, this combination accommodates the serious navigator, provides the most practice opportunities and the most fun. That’s the best compromise — in my view.

The WH mirror with the Galilean scope gives the wide-view advantage for sun and night-moon shots under most conditions, and even bright star shots in the middle of the twilight window. Its disadvantage is in low-contrast situations: poor horizons, dim stars at twilight limits and daytime pale moon and Venus shots. While omitting these sights can be a sensible compromise for the part-time navigator, or for standby navigation, they are important for the serious navigator interested in additional LOPs and fixes.

The traditional mirror with the Galilean scope has an advantage in poor viewing conditions. But the excruciatingly narrow FOV takes the fun and usefulness out of too many sights — a poor compromise in my view.

The WH mirror with the prism scope gives a wide FOV and high magnification. But since it suffers in poor contrast conditions, with no compensating advantage, it only makes sense for high-contrast shots on large ships.