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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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