Welcome to the NavList Message Boards.


A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Add Images & Files
    Re: Compact Lightweight Metal Sextants - Freiberger Yacht vs. Astra IIIB
    From: James N Wilson
    Date: 2012 Aug 10, 20:25 -0700
    I agree with many of your sextant comments, except that I have no use for verniers on the micrometer drum. Their ostensible increase in accuracy is questionable, and they introduce the possibility of reading from the wrong end of the vernier, introducing an eleven minute error.
    I deplore the idea of eliminating the clear glass on the horizon mirror. I've found it handy with Sun and Moon shots. I also dislike "whole horizon" mirrors, finding the horizon difficult to discern at twilight. Has anyone produced a horizon mirror with a gradation from full on the right to zero on the left? That sounds handy. Should I patent the idea?
    Jim Wilson
    On 10 Aug 2012 19:23:36 -0700 "John Karl" <jhkarl@att.net> writes:

    Yes, Bill Morris is right (as usual). The Freiberger Yacht sextant has the clear-glass left-side of the horizon mirror missing. And this has the effect of not showing the images from the index mirror on that left-side field of the view (FOV). What may surprise some readers is that in sextants with Galilean scopes and half-silvered horizon mirrors, you really never use the silvered half of the horizon mirror when shooting sun or bright moon sights. The horizon is not visible on the right side of the VOF, but as Bill says, the unsilvered left half of the mirror reflects plenty of sun light to make both the horizon and the sun visible there. So, perhaps without thinking about it, you make the sight on the left half of the VOF (even though you paid for the whole FOV). But with the Yacht sextant, with its left half missing, you get a region about ½ d wide to make the sight. That’s about one eighth of the normal 4d FOV.

    In small vessels this FOV is really important in rough seas, and in general, important for comfortable and pleasant viewing. In general, the optics in most sextants is pretty miserable. They have severely limited FOV, extraneous light in the scope, and shades reducing the FOV in many sextants. Just compare the sextant’s view to that of even a modest cost binocular. The only sextant that does the optics right is the Navy Mark II, it has a very wide, high quality, FOV. It suffers only slightly from lack of intensity in difficult star shots because of its smaller aperture. You can read more about this topic (and the effects of prism scopes) in my book, and in the 2008 issue of Ocean Voyager (the summer issue of Ocean Navigator).

    The smaller size of the Yacht sextant is really nice, specially in small boats, and in air-travel luggage to various passage legs. But my main objection is that its micrometer drum is too small and lacks a vernier scale. Even worse, its shinny surface reflects light, making it very difficult to read in many circumstances. The sextant also has no lighting. (In addition, the arc error has no bragging rights. Mine ranges up to 48’.)

    Happy force four winds,

    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

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.

    Email Settings

    Posting Code:

    Custom Index

    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site