Next message: Alexandre Eremenko: "Re: Visit to Freiberg"
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.
