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: Problem shooting index error
    From: Tony Oz
    Date: 2019 Oct 11, 05:38 -0700

    Dear Alan,

    The index error is the last to be addressed. You should work in certain order:

    • make sure the index mirror is perpendicular to the sextant frame
    • reduce the so-called side-error
    • with the above adjustments done - correct the index error

    The last step will possibly require to repeat the side-error adjustment.

    The most accurate way to measure the index error by observing the Sun:

    • hold the sextant so its' frame is horizontal
    • put on the proper filters
    • set the index arm at 0°
    • aim the sextant at the Sun, you should see two overlapping images of the Sun
    • turn the drum to decrease the values ("off scale") until the images separate a little, now turning the drum in the increasing direction bring the images to the touching position
    • read the scale and drum. Values to the right on the "0" considered negative, write it down as 359° (or what you actually get), read the drum at the face value and write the minutes part of the measured angle
    • now turn the drum all the way up to the point when the overlapping images just touch on their other sides
    • the readings are now "on scale", write them as 360° + minutes off the drum

    The index error is now calculated thus:

    I.E. = 360° - (onscalevalue + offscalevalue) / 2

    An easy check for the sanity of result - with the same values (the off-scale and the on-scale) do:

    SDSun = (onscalevalue - offscalevalue) / 4

    ...and compare the result with the Sun's semidiameter in NA.

    I'll be happy to help you more if needed.

    The higher the scope magnification - the easier and more accurate are the measurements.

    Warm regards,

    60°N 30°E

    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