NavList:

---

Nial

Both sextants have a rack radius of about 160 mm (pitch of rack is 1.4 mm)

The Navistar Professional feels unbalanced near the lower end of the scale, and as the horizon mirror is "fully silvered", there is very little overlap of direct and reflected images in the central field of view. It can be set down (face down) only in its case, as it has no proper legs, just pegs to limit the range of the index arm, and even if it had legs, there is no part of the frame that can be grasped to set it down face upwards. The drum is only 28 mm in diameter and the figures moulded into the plastic are small and hard to read, while the plastic itself tends to disintegrate with age.

The Observator Mark IV does have legs, but again there is nothing of the frame to grasp in order to set it down face up. The horizon mirror is again fully silvered, but there is slightly more image overlap in the central field. The telescope has internal focussing and the shades are inside the telescope. However, they are made of slips of photographic film and the index film tends to cook with any prolonged observation of the sun, because the films are close to the focal plane of the objective. The drum is only 30 mm in diameter, but the divisions and figures are properly engraved so it is much easier to read than the Navistar.

I don't think either sextant is an improvement on traditional micrometer sextant design. The SNO-T, however, _is_ an improvement and was probably the best sextant ever made. The horizon shades in every example I have seen have been mounted upside down, but that apart, it is hard to fault its performance.

Bill Morris
Pukenui
New Zealand
----------------------------------------------------------------
NavList message boards and member settings: www.fer3.com/NavList
Members may optionally receive posts by email.
To cancel email delivery, send a message to NoMail[at]fer3.com
----------------------------------------------------------------

: http://fer3.com/arc/m2.aspx?i=125685