Of all the optional equipment for a sextant, this is the only one we can recommend as actually improving the sight data. It is used for sun and moon sights to increase the magnification from the typical 4 power to 7 power. It is also valuable for improving index corrections. Its sole purpose is to make the horizon and limbs of the moon or sun larger and sharper.
Why a higher powered scope? The increased magnification allows the sun's diameter to appear larger, and also better defines a more distant horizon as may be seen from a ship. This helps the navigator determine the point of tangency of the Sun's limb and the horizon. As a side issue, taking Lunar distances (lunars) has become a popular addition to celestial navigation activities. It is a way to tell what time it is from the Moon's position with respect to other stars, planets and the Sun (see our explanation in the book section). A higher powered scope is very helpful in meeting the high accuracies required by taking these measurements.
What makes a good scope? Although this scope has higher quality optics than any being currently made for sextant use, the most important feature is the fork (sometimes called a rising piece) that should be exactly perpendicular to the optical axis. Each degree of error in this will add a one minute error in high altitude sextant sights. The fork on this scope is cast as part of the aluminum body, and has no error of perpendicularity. All other currently available high powered scopes are merely one-half of a binocular set, with a fork screwed onto the side. We have seen errors as high as 3 degrees on some of them.
The 7x35 monocular prism telescope features, 6.5° field of view, Bright optics, and will fit on most all sextant models: Astra IIIB, Cassens & Plath, C. Plath, Tamaya, and others.