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    Re: Visit to Freiberg
    From: Courtney Thomas
    Date: 2005 May 23, 11:02 -0500

    Thank you Alex.
    Courtney
    
    
    On Mon, 2005-05-23 at 09:36, Alexandre Eremenko wrote:
    > On Mon, 23 May 2005, Courtney Thomas wrote:
    >
    > > What is backlash ?
    >
    > Backlash is the free movement of the worm screw with respect
    > to the arc. You can
    > rotate it back
    > and forth a little without moving the sextant arm.
    > The master who checked my sextant actually just tried to shake
    > the arm back and forth with his hand, apparently feeling whether
    > it moves or not. In other words, whether you can shake the arm
    > when the wormscrew is engaged, and without rotating the wormscrew.
    >
    > The usual procedure of measuring backlash is touching the
    > direct and reflected image of the sun twice, on the same
    > side (say Lover Limb), like you do when measuring
    > the index correction, but by rotating the
    > screw in two
    > different directions, first clockwise then anticlockwise.
    > The difference between your measurements is the backlash.
    >
    > But in the factory they do it with a special measuring device
    > which has apparently 1" precision. The same device measures the
    > arc error.
    >
    > The adjusting skrew is on the bottom side of the drum,
    > it is perpendicular to the plane of the arc.
    > (if the sextant lays on your table horizontally in mormal position,
    > the screw is on the bottom side of the drum).
    > He adjusted it with some special wrench (not with the wrench supplied
    > with the sextant. The strange wrench coming with the sextant is
    > for disassembling the drum. Both things are discouraged by the manual:
    > they say this should be done in a workshop.
    >
    > > Without going to Germany, how do you know if it's a problem with your
    > > sextant ?
    >
    > They said they would do everything by mail,
    > except that they do not accept credit cards, but only bank transfers.
    >
    > > Can you [yourself] approximately 'measure' it's significance with your
    > > sextant and correlate turning said 'screw', recheck and approximate a
    > > satisfactory adjustment ?
    >
    > I never knew that my sextant had this problem. I blamed the arc
    > excentricity, and brought it to Freiberger factory to measure this
    > excentricity precisely.
    >
    > Alex.
    
    
    

       
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