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    Re: Sextant certificates
    From: Ken Gebhart
    Date: 2006 Jan 27, 20:45 -0600
    Re: Sextant certificates On 1/26/06 8:59 PM, "Jim Hickey" <jimattac@AOL.COM> wrote:

    I did post this question at the end of an earlier post but should have posted it separately as it is an earnest request for some feedback.
     
    My Oaklet sextant (very similar to an Astra) that I have owned for about 25 years has a certificate that came with it that indicates an error of +10 seconds in the 20 to 30 degree range. The remainder of the ranges show 0 minutes and 0 seconds of correction.
     
    From the various specs I have seen for top end sextants, it looks like +/-10" is about as good as it gets which makes me think the Oaklet I own is reaching a little. That said, I have only had two other sextants I have worked closely with that also had great looking certificates. Not quite as good as for the Oaklet but < minute correction for only several angle ranges. Not enough to start looking at the certificate if a sight didn't work out well.
     
    Do others have certificates that look like this? When the claim is made that the spec is +/-10", does this suggest that in reality that most of the time the error is considerably less?
     
    I realize making corrections of 10 seconds of arc is getting pretty fine in practical terms but I am curious.
     
     


    Jim,

    Here is my take sextant certificates.  There are two formats: one which says “Free of errors for practical use”, and one that lists correction values for each 10 deg along the arc.  The former implies that there is no error greater than 20 sec.  Therefore the manufacturer is relieved of the requirement for listing the individual corrections.  If there is an error of greater than 20 sec. the manufacturer must list the corrections so the mariner can allow for them.  This system (I think) complies with German Hydrographic Office requirements. I am not aware of any other HO making ANY stipulations.

    Before turning to the second format, let me preface by saying that Celestaire has just announced its new Professional Model Astra sextant.  This is like the Tamaya line that has a bronze arc which is fused to an aluminum frame.  An accuracy of +/- 15 sec is stated.  In order to arrive at this end point we became intimately involved with the testing process, both in-house and with an outside party over the last 3 years.  Here are my comments from this experience.

    1.  In addition to the regular 10 deg correction values, correction values for each in between degree (which you are not shown)  can go quite far from expected values.  For example on one sextant, for 40, 50, and 60 degrees, we have corrections of +10, +9, and +2  secs respectively.  Not bad.  But at 47 and 54 degs we have a correction of 0!  So, you can see it is not a straight line variation between the 10 deg values.

    2.  Drum eccentricity (which has been discussed earlier on the list) can contribute up to 10 sec of error, either + or -  in between each deg value.

    3.  Variation in mirror flatness (nothing is ever perfect), and scope non parallelism (also discussed earlier) can both cause variations in results either in the field or during testing.  Bear in mind the scope you got with your sextant was probably not the same scope it was tested with.

    4.  Lastly, even the best testing machine may have +/- 3 sec. error, and operator (readability) another +/- 3 sec.

    So, if you combine all of these uncertainties, it makes the accuracy certificate something to be taken with a grain of salt.  I have a SNO-T sextant with a certificate that says straight zeros across the board.  Of course this is ridiculous.  Whenever I see a claim by a manufacturer of less than +/- 15 sec. I believe they are making a marketing statement rather than a true one.

    Ken
       
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