ROYAL AIRCRAFT ESTABLISHMENT. 1941

Notes on Examination of German Spirit Level Octant with Averaging Device.

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.

 

Back to Other Sextants