Optimize sun and moon sights; improve index correction
7x35 Sextant Telescope
7x35 monocular prism telescope, 6.5° field of view. Bright optics.
$199.95 ...item# 1840s
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.
The improvement in sight accuracy using such a scope is notable, but not dramatic. It makes it easier to get to sub 1' precision, but this is still possible with the conventional 4x40 general purpose scopes; it is just a bit harder.
Usually these scopes are not used with stars and planets because you do not benifit at all with magnification on stars (they will always be just a point of light) and you end up losing some light gathering since these are only 35 mm across compared to 40 mm of a conventional scope.
For routine celestial navigation this scope is a luxury; not a requirement. On the other hand, if you plan to extend your skills to doing lunar sights for finding longitude (called lunars), then this scope is more of a requirement, because the very best accuracy is required for those techniques.
The scope can also be used for general purpose monocular sighting (buoys, birds, etc), if you do not have access to higher-powered binoculars.
The only other sextant scope similar to this one is from Tamaya for over $500.
Note the location of the alignment notch below. On Tamaya sextants earlier than 1972 they placed this notch on the top side of the bracket, and therefore these scopes will not fit (unless you file off the ridge on the scope mount). In 1972, Tamaya switched their design to what is now the standard arrangement with the notch on the bottom.
Base of 45º triangle section is 5mm across; distance between center of gap and point of triangle is 5mm;
gap width is 5.6mm