CASSENS & PLATH BOBBY SCHENK Sextant Handhabung Seite 28

Instructions
Page 13 ______________________________________________________________
Tilt Error of the Horizon Mirror
The horizon mirror must be at right angles to the plane of the instrument.
Set the alidade to the zero point of the graduated arc and the drum as well. If the horizon mirror is
parallel to the (corrected) index mirror, then the reflected and the directly seen image of an object
will coincide (if the object distance is over 3 nm). If the images do not coincide the horizon mirror
has to be adjusted. There are the two following methods to do this:
1. Superimposition check (for fullview horizon mirrors only)
The sun (make sure applying the right shade glasses!), a fixed star, or a distant terrestrial
object has to be observed. If the superimposition of reflected and directly seen image is exact,
the horizon and index mirrors are parallel. If the sextant is held vertically in the right hand and
the images appear laterally displaced then the horizon mirror is tilted. Correct this by using the
screw as shown in the illustration on page 12.
2. Visible-horizon check (for traditional split view and fullview horizon mirrors)
To begin, the instrument is held vertically as it is held for celestial observations. Move the
alidade so that the image of the directly seen horizon is made to coincide with its reflected
image. The instrument is then, for example, turned clockwise round the axis of the telescope
through an angle of approximately 45°. If the directly seen horizon and its reflected image still
coincide, the horizon mirror is correctly positioned. If both images move up and down to each
other, the horizon mirror is tilted. The necessary correction can be made by adjusting the
screw as shown in the illustration on page 12.
Index Error
If the index mark and the drum as well of the alidade point to zero of the arc then both mirrors
should be parallel (tilt errors are already removed). This is not a matter of fact in every case so the
angle reading will require correction.
This is known as the index correction. Proceed as follows to correct:
1. Set the alidade (index arm ) to zero. It is necessary to make a fine adjustement using the
drum.
2. Observe a celestial body (make sure applying the right shade glasses!) through the horizon
and index mirrors. Both pictures coincide more or less. In case of the fullview horizon mirror a)
both pictures of the sun are superimposed, in case of traditional split view horizon mirror b)
half of the celestial body is on the right and half of it on the left side of the field of view. Carry
out the correction by adjusting screw as shown on page 12.
a)
BOBBY SCHENK Sextant
b)