German Description; -
Type; Spirit level octant with averaging device.
Maker: C. Plath, Hamburg, for the observation instrument,
De Te We, Berlin, for the averaging device and spirit level.
Measuring range: Angles of elevation of 0° to 80°.
Weight; 2.7 kg approx. 6 Ibs)
Weight of case: 1.8 kg (approx.4 Ibs)
Order number; F1 83750
Work No 14410
Parts supplied.
1 spirit level octant
2 batteries
2 spare bulbs each for spirit level and averaging device Lighting
1 plug for connecting to aircraft electrical system with Lighting
resistance
1 ground glass disc for use in daylight.
1 telescope
1 spanner
1 screwdriver
1 hair brush
1 wash leather
1 case
Description.
Fig 1 shows a general view of the sextant.
The sextant is a modified form of the British Mk.VIII sextant,
the following being the major differences.
The mirror is semi-silvered to give about equal intensities to
the reflected and transmitted light, which enables all observations
to be made on the reflected image. The silver film is protected
by a cemented on cover-glass. A detachable Galilean telescope
of 1.8 magnification is included, principally for use with star
sights when it is necessary to compensate for the loss of light
at the semi-silvered mirror.
No provision is made for using the natural horizon.
The sextant is provided with one yellow and one red sunshade,
the German instructions stating that the red shade is intended
primarily to increase the contrast of the moon. It was found that
it is necessary to use both shades for observations in bright
sunlight.
Comparison with the Mk IXA Sextant
Accuracy
The Mk.IXA (automatic averaging) sextant is now in production
for the R.A.F. and a comparison of this instrument and the German
octant is given below.
When in good adjustment, the accuracy of the German sextant is
equal to that of the Mk IXA.
Reliability.
The German Integrating device is inherently less robust than the
automatic attachment of the Mk IXA. Trouble with slip in the variable
gear drive is to be anticipated and was experienced during the
tests. It is significant that the German manual states that the
instrument is to be returned to the maker for servicing every
eight weeks.
Ease of Operation.
1. No provision is made for suspending the sextant to take the
weight of the instrument off the operator's arms. The Mk IXA is
suspended from the sextant dome.
2. The instrument suffers from a defect inherent in the Mk VIII
sextant, that the operator's right hand can only support the sextant
by the large adjusting knob. Operation of the adjusting knob tends
to disturb the level of the sextants. In the Mk IXA sextant the
right hand handle is supported in the palm of the hand while the
adjusting knob is operated by the thumb and first finger.
3. The line of Sight in the German instrument is directed downwards,
which is generally considered less convenient than the horizontal
sighting line of the MK IXA.
4. Before the integrator is connected to the operating drum the
altitude setting can be varied continuously from 0° to 80
which is considerably more convenient than the action of the Mk
IXA
5. An extra operation is required with the German instrument,
that of engaging the integrating attachment with the nearest hole
in the measuring drum after the altitude has been set.
6. The method of inserting sunshades is similar to that used in
the Mk VIII sextant, and is less convenient than that of the Mk
IXA
7. It was found far more difficult to obtain and control the bubble
with the German instrument than with the Mk IXA.
8. In reading the German sextant, the degrees and minutes shown
on the integrator dials have to be added to the whole number of
degrees shown on the sextant with the consequent possibility of
error. The Mk IXA is direct reading.
9. No illumination is provided for the sextant
scales; all scales of the Mk IXA are illuminated.
10. The duration of the run of the integrator can be set at will
to 40, 120 or 200 seconds. While 200 seconds would rarely be of
advantage, the 40 second period might be very useful when conditions
were so steady that it yielded adequate accuracy or when sights
were made through broken cloud on a body "visible for brief
periods". The Mk IXA is limited to a run of 120 seconds duration.
11. The integrator may be rewound at any stage of the run,
the Mk IXA can only be rewound at the completion of a run.
12. The termination of a run is indicated by the extinction of
the bubble illumination, which is more satisfactory for star sights
than the scheme of a shutter obscuring the body used in the Mk
IXA.
13. The bubble illumination for star sights is that of the Mk.VIII
sextant with the refinement of a red filter to provide a colour
contrast between the bubble and the star. The illumination of
the Mk IXA giving a darker background is superior.
Miscellaneous
1. Indication of the end of a run by the extinction of the bubble
lighting has great advantage for night use but compels the use
of artificial light for bubble illumination for sun sights. The
latter is not possible with the illumination scheme of the Mk.IXA
because the low level of illumination, of great advantage for
star sights, is insufficient to show the bubble to an eye exposed
to daylight.
2. The bubble is viewed through the mirror while the body is observed
by reflection at the same mirror. This has the serious disadvantage
for star sights that half the light of the star is lost at the
semi-silvered reflecting surface. Consequently much of the advantage
of the telescope is lost at this semi-silvered mirror.
3. It is suggested in the German manual that to assist in "finding"
the star the approximate altitude should be set by viewing the
star directly through the mirror and the bubble by reflection,
i.e. the normal position for star sights with the Mk.VIII sextant.
The position required of the observer is very inconvenient and
the star would be difficult to see through the semi-reflecting
mirror. After the approximate altitude had been set by this method,
the position would have to be changed, to the normal viewing position
with the eye applied to the telescope and the star "found"
for a second time before making the observation.
Conclusion.
It is considered that the German octant is in general inferior
as regards ease of use to the Mk.lXA sextant now in production
for the R.A.F.and that the latter should also prove a more reliable
instrument.