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    Re: Bubble sextants on e-bay
    From: Jean-Philippe Planas
    Date: 2009 Jun 7, 02:11 -0700
    Yes, you are right, there is no fine screw to adjust the IE of the instrument. If the error is large (degrees), you have to adjust the instrument by untighting the large bristol screw squeezing the index mirror on it's shaft.
    The finer adjustment can only be done via adjusting the screws on the vernier scale. the ajustment range is about 6 arcminutes.
    At a known geographical position, you have to compute the altitude to be measured at LAN including refraction (as well as the exact time LAN occurs), and quickly make the adjustment of the vernier within two minutes of that time.
    Begin by setting the vernier in the middle of its range, then you note the error and adjust accordingly. In the LAN time frame of 2 minutes you have time to confirm or refine the adjustment via a couple more sun shots and readings.
    When you are within less than 2 to 5 arcminutes index error, you have to be content.
    Also note that the index error is different at 0°, 45°, 80°, so make the adjustment in the middle of the scale range (LAN shoots are convenient now for UK latitudes) and identify the index error for other altitudes.



    --- On Sat, 6/6/09, douglas.denny{at}btopenworld.com <douglas.denny{at}btopenworld.com> wrote:

    From: douglas.denny{at}btopenworld.com <douglas.denny{at}btopenworld.com>
    Subject: [NavList 8550] Re: Bubble sextants on e-bay
    To: NavList@fer3.com
    Date: Saturday, June 6, 2009, 5:42 PM


    I would be interested to know the 'official' method of adjusting the index error for this instument.

    Not having had to take it apart completely (it is in very good condition) I cannot see any obvious index adjustment other than removing the two screws holding the Veeder indicator, and rotating the shaft to obtain the nearest reading - then noting the index error with suitable checks for accuracy.  There does not seem to be any continuous fine screw adjust on the instument.
    Any comments appreciated.

    Douglas Denny.

    Chichester. England.



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