CASSENS & PLATH BOBBY SCHENK Sextant Handhabung Seite 23

BOBBY SCHENK Sextant
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Celestial Observations
Carefully watch not to look into the sun without applying shade glasses of necessary light
reduction!
The sextant has to be held vertically.
1. Prepare observation by choosing the right shade glass if moon or sun shall be measured. This
can be done by direct observation of a celestial body through a shade not using the telescope.
Combine the shades as required. The circular horizon shades are to reduce light reflection on
the sea surface. Even these have to be adjusted to the brightness of the sun if the following
method should be used.
2. Check index and tilt errors (ref. to chapter "Adjustment of Horizon and Index Mirror"). File
index error, if any, for later calculations or correct the mirror position as described in the above
mentioned chapter.
3. Move the index arm to zero and observe the star. You will see the star twice: the doubly
reflected image and the directly seen one through the horizon mirror.
4.
Bring down the star to the horizon. To do this, loosen the lever and move the index arm slowly
forward. The directly seen and the reflected image of the star will separate. Follow the lower
image (reflected image) of the star by moving down the sextant synchronously. When the
horizon appears in the field of view from below engage the lever. Now carry out fine
adjustment by rotating the drum until the star touches the horizon (the star "kisses" the
horizon).
a)
5. The sextant must be exactly vertical when measuring the star altitude. If not, the measured
altitude will be too high. To avoid this swing the sextant around the axle of the telescope. The
star will describe an arc ("Swinging the arc"). The lowest point of it just has to touch the
horizon. Refer to the above illustration: a) traditional split view horizon mirror, b) full view
horizon mirror.
6. Read the whole degrees of star height at the index of the alidade and the minutes and decimal
minutes at the vernier as shown.
b)
Index for degrees
Vernier for decimal minutes
Drum for minutes
Instructions