The astro compass MkII is a simple but very useful instrument. It was developed from a whole series of earlier instruments that can be traced back to the Astrolabe. Again P.F. Everitt was instrumental in its design. It came into use in the RAF sometime in the early thirties, primarily to allow Aircraft Navigation in the Polar regions where Magnetic Compasses are notoriously unreliable because the magnetic field is nearly vertical. Gyro Compasses in the same area tend to be unreliable because of the earths rotation. However,world wide use is obtainable with the Astro Compass and for such an inexpensive device good accuracies can be obtained.
I have had some difficulty finding the history of the Mk II but I know it was designed by P.F. Everitt of the Mk IX sextants and other instrument fame. It was initially made by H. Hughes and Son. The compass was in RAF service some years before the outbreak of the war, certainly being used in bombers and other larger aircraft. P.F. Everitt, and Hughes and Son, took out a patent in 1946 on an improvement to an Astro Compass. When one reads the patent it is obvious that the improvement is the sighting head on the top. I have not seen one of these later designs and it is probable they did not enter into production.
Whilst exactly when it entered RAF service is unclear to me as I state earlier, it was certainly prior to America entering the War. I suspect the design was passed over to the 'States with a lot of other technical information during the period when the USA was not actively involved in the War. Examination of all these devices, whatever the make, shows them to be identical with only minor differences to the Hour Angle Head. The American versions has a distinctive "Skirt" to differentiate between North and South hour angle heads.
The Astro Compass was designed to fit a pre-existing RAF magnetic compass mount. In fact if you look at the O2 compass, or a deivative the SO2, the attachment looks remarkably similar to that on the Astro Compass itself. The mounts on British aircraft had (usually) a magnetic corrector fitted internally. They were called the "Standard 0.5". British Astro Compasses were packed in a plain plywood box which had no space for a mounting or "Standard". The American models that I have seen, admitedly only 2, both have space for a Standard in their transit cases. I only managed to find one Standard still in the box, that was for a Spirti in a rather nice Bakerlite case. This suggests to me the Astro compass may have been an after thought for American aircraft, so when a compass was supplied there was also a standard to mount it on. The cost of a standard possibly discarded could be ignored.
British and American Mountings
A mystery to me so far is the meaning of the suffix "A". It has been suggested that the A indicates American manufacture. I think this is highly unlikely, the product of a fertile brain, one that has never dealt with the British military. Recently I came across some literature which states that there were different sighting heads for use in high or low latitudes. From A.P. 1234D A.L. 43 I extracted this:-
The difference between the two types of astro compasses lies primarily in the shape of the shadow bar. On the astro compass (Stores Ref. 6A/1174) the shadow bar consists of a single vertical bar, whereas in the astro compass (Stores Ref. 6A/11740) the shadow bar is in the shape of a cross, rendering the instrument more sensitive in low latitudes. The quick release on the latitude setting has also been deleted in the astro compass (Stores Ref. 6A/11740).
There is only one version of the American Astro compass as far as I am aware. All the literature I have seen refers to a Mk II only. They are identical and can only be told apart by looking on the True Heading assembly. The Boes version has the profile of an aircraft on it between the two levels. The Spirti is, like the British version, plain.
The Astro Compass has a host of other uses, from use as a simple surveyers level to the solution of astronomical problems. I have seen several examples of the American Mk II that have been modified with a different sighting device, looking like a square telescope. They are obviously a later addition but the fact there are at least two versions looking the same suggests to me there may have been some semi official experiments carried out with them. The sight was in fact polorised to enable the Sun to be found even when behind cloud.
The Russian Air Force had a more advanced clockwork powered version of the Astro Compass. It was a copy of a wartime German design and, like the copied bubble sextant, was used in their long range aircraft well into the 1970s. It appears to have a similar standard.
Mk II and IIA Originally made by H.Hughes and Son but later produced by a variety of makers, Horstman and Venner amongst possibly others. Later, many were made in Canada.
Mk II only, produced by W.W. Boes of Dayton, Ohio.
Mk II only, produced by Spirti Inc. Cincinati, Ohio.
I have also seen a photo of a compass with an Hour Head assembly slightly different from the British version. This had markings D.E.P. Co. I understand these were produced in Canada and again the only distinguishing feature from the others is the Hour Head assembly. Appearance is very similar to the British version but the corners of the Hour Head assembly are of a tighter radius than the British model.
If you can spread any light on the history or origins of the Astro Compass I would be delighted to hear from you.
I have had an eMail from Johannes M. Vogt attached to which were a series of photographs of a DEP made Astro Compass. many Thanks Johannes.