Adjustment Of Horizon And Index Mirror - CASSENS & PLATH POLARIS Handhabung

Instructions
Page 13 ______________________________________________________________

Adjustment of Horizon and Index Mirror

There are two different kinds of sextant errors: the so-called small errors (mirror position errors)
and the large errors (caused by excentricity of the index arm pivoting, errors of graduation ...). The
small mirror errors can be recognized and must be corrected by the navigator himself. The large
errors, however, cannot be recognized and cannot be corrected by the user. If the navigator thinks
that there could be such a large error he should contact a qualified workshop. Reasons for large
errors may be bad workmanship, heavy knocks, shock or drop of the sextant.
When the sextant is in use, the small mirror errors may become apparent in the course of time.
Also the possible reasons here may be knocks or shock.
It is therefore necessary to ensure the following:
1. The index mirror must be at right angles to the plane of the instrument. Any inclination against
the vertical is called tilt error of the index mirror.
2. The horizon mirror must be at right angles to the plane of the instrument. Any inclination
against the vertical is called tilt error of the horizon mirror.
3. The index error must be 0.
Correction screw
of index mirror tilt
error
Tilt errors produce a lateral displacement of the directly seen and doubly reflected image, index
errors produce a vertical displacement. It is very important to correct index errors as they will
cause erroneous observations which will result in wrong positions. Therfore index error should be
checked before each observation. The mirrors fixed within the frames tend to have stable set
points. If these set points are not at positions of zero error it can be advantageous not to adjust the
mirror by the correction screws but to adding or subtracting the error value from the observed
height. This is recommended for smaller errors because this will prevent loosening of the
correction screws within their threads.
Too frequent adjustment of these mirror adjusting screws should be avoided, particularly as
tightening and loosening will work out their threads.
POLARIS Sextant
Correction screw
of horizon mirror
tilt error
... of index error