CASSENS & PLATH BOBBY SCHENK Sextant Handhabung Seite 29

Instructions
BOBBY SCHENK Sextant
______________________________________________________________ Page 14
Index Error Correction by the Red Knob
The index error must be checked and removed before each measurement as it will falsify the
result.
In chapter "Adjustment of Horizon and Index Mirror", one will find a detailed description of the
nature of this error. It will also be explained how to correct it your own by adjusting the mirror
position. Alternatively the error can be eliminated by adding or subtracting its value to the
measured sextant altitude.
The position of the mirror in its frame often keeps stable set-points and each time one tries to
adjust it by turning the adjusting screws it will return to the former position and the index error
appears again. Under these circumstances adjusting of mirror is not recommended. The more
frequently one uses the adjusting screws the more probable will be the loosening of the thread.
Therefore it is better to correct the index error with the red knob of the drum. To do this move
alidade and drum to zero. Observe the horizon. Carry out fine adjustment of the drum watching
both pictures of the horizon so that they match each other. Now unscrew the red knob and turn the
drum so that the zero mark of the drum is at the index mark of the vernier. With this the sextant is
ready to carry out the star altitude measurement, the index error does not need any further
consideration.
Loosen red knob
Turn drum
so that
index mark
points to drum zero
Memory scale
Changing the Telescope