Re: telescopic gun sights
- From: Alan Dicey <alan@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
- Date: Sat, 01 Mar 2008 00:19:01 +0000
I think some were simple tubes, to help the pilot concentrate on the sight picture, but at least one, the Aldis sight, was an optical system giving 1:1 magnification while projecting the sight picture at infinity - the pilot didn't have to accomodate to keep it in focus and it didn't move around if the pilots head moved (provided his eye stayed within the exit pupil of the sight). Its operation is similar to a modern HUD, except that a HUD projects the sight reticule into the pilot's field of view wheras the Aldis had the sight engraved on the lenses.
A brief google has produced the following quotation, lifted from a web forum: -
From the book by Harry Woodman, Aircraft Armament: The Aeroplane and the Gun up to 1918 (AAP London, 1989) ISBN 0-85368-990-3 (Aldis Sight p232):
_______________________________________
The Aldis Sight
In a roughly printed booklet written in 1916 Maj. L.W.B. Rees advocated types of sights that would be best suited to air fighting. One was for a free gun which involved a rotation arm and there were comments about telescopic sights. He considered that the magnification had to be small, as the vibration of the machine would interfere with the sight, and that the field of vision should be about ten degrees or larger. The eyepiece was to be arranged so that the 'full field of fire is obtained when the eye is held about a foot away from the telescope; this enables one to use goggles or a wind screen. So long as the eye is within the angle shown [in a rough sketch] it need not be in the centre line of the telescope in order to obtain the full field.'
(...) some research involving special optical sights had been undertaken before the war and this exploratory work continued in the workshops of the Aldis Brothers, of Sparkhill, Birmingham. Their first effort was a 32in telescopic tube with a graticule engraved on an internal screen; in aerial fighting magnification was unnecessary and indeed a drawback. After tests at Martlesham in 1915 an improved version of the sight was produced which incorporated all the recommendations (...) it was approved and the Aldis company was then instructed to commence manufacture of the sight against an initial order for 200 items as soon as possible.
The Aldis sight consisted of a metal tube 32in long and 2in in diameter. It embodied the priciple of the ring sight and the sighting system was in the form of two concentric rings engraved on clear glass screens inside the tube which also contained a number of lenses. When the pilot looked through the tube the image was neither enlarged nor diminished and was always seen with it's centre directly on the axis of the sight regardless of the position of the gunner's eye. The rear end was protected by a rubber sleeve and eyepiece while a problem which had arisen during tests, the fouling of the front lens of the sight by oil or smoke, was solved by fitting a protective disc which could be raised or lowered by the pilot.
The secret of the Aldis lay in its series of internal lenses and the company always insisted that sights should not be tampered with and had to be be returned to the factory in the event of damage or malfunction. According to L.W. Sutherland, writing in
Aces and Kings, the reason for this secrecy was that to prevent fogging of the internal lenses certain gases had at the time of manufacture been introduced between the lenses at varying temperatures and if an Aldis were opened up these gases would disperse. It was claimed that this was the reason the Germans never copied the sight dispite the large numbers captured. The German pilots nevertheless liked the Aldis and frequently fitted it to their machines, the range of optical devices produced by German manufacturers during the war notwithstanding.
The first production Aldis sights were issued to operational units in mid-1916 and by the end of the year they were being delivered in large numbers for use with the fixed Vickers gun or the overwing Lewis by the RFC and the RNAS. The Aldis remained in service with the RAF until the late 1930s and the Americans also adopted it, their particular version being known as the "Unit Sight"*
*The Aldis was a collimating sight, that is, it employed lenses which transmitted parallel rays of light. The magnification was correctly expressed as being 1:1, hence 'unit' or one-to-one magnification.
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