This exceptional piece of equipment was donated to my collection by a befriended collector in 2010. It concerns a military version of a Wild Heerbugg T2 (model T2-56-C-MIL) combined with an "Azimuth-Gyro, Artillery Model C2B ABLE". The abbreviation ABLE stands for Autonetics Base Line Equipment, but in general the combination was either referred to as "ABLE Orienter"¹ or "Gyro-Azimuth Surveying Instrument"².

According to the U.S. army field manual FM 6-2 the "... artillery gyro azimuth surveying instrument (fig. 136) (azimuth gyro) is a portable gyrocompass used to determine a true direction. With this instrument, a direction can be determined under conditions of poor visibility without lengthy computations and with an accuracy that is comparable to that of astronomic observations. Direction is determined by observing the effect of the rotation of the earth on the gyroscope and applying appropriate corrections to the instrument. This instrument is for use in latitudes between 60° north and 60° south of the equator."

The instrument was "...authorized for field artillery howitzer, gun, and missile battalions; for the survey element in the headquarters battery of division artillery; and for the batteries of the target acquisition battalion."³

Originally the set would comprise of a Sensing Element (the gyro), a 0.002-mil T2 (the Wild T2 theodolite) and a Control Indicator, which was "...an electronic package which provides power to the gyro rotor, the heating elements, and the signals for measuring the amount and direction of the gyro misalinement [sic]. The control indicator panel [...] provides the controls and indicators necessary for the operation and the directional alinement [sic] of the instrument.³

The Wild T2 on top is not an ordinary one, but modified at several points for military use:

• The circles are divided in mils (6400 mils makes a full circle);
• The micrometer is divided in 2 milli-mils divisions.
• The reticle contains a solar circle for sun azimuth observations;⁴
• The telescope has five screws around the ocular: two red ones that hold the eyepiece (can be exchanged with an autocollimation-eyepiece) and three for reticle-adjustment;
• The base of the instrument has a special bracket with three contact points for proper alignment with the gyro.

The alignment of the T2 with the gyro can be checked by means of autocollimation measurements on the internal mirror of the latter. This mirror is separated from the outside world by a glass window with a metal screw-on cap (see figure 1 and figure 7.

The MIL-ABLE Wild T2 is as accurate as any period Wild T2, although it has now to be expressed in mills. Both circles can be read down to 0.002 mils and estimated to 0.001 mils. The quality of the azimuth provided by the gyro depended on the latitude and the number of observations. The field manual contains a table that provides a guideline for the number of observations that should be taken to achieve a certain accuracy at a certain latitude (see figure 20).

An accuracy of between 0.057-0.119 mills (0.003-0.007°) could be achieved for any location between the 60° north and south parallels (see figure 20).³

Although the main components are here, the ABLE set did not come complete. It came with its original tripod, but lacks the Control-Indicator box, all the cables, the auto-collimation eyepiece, the circle illumination (mirrors and lamps), all transport cases, and the additional short metal legs the tripod was supplied with (see figure 19). The T2 itself is in good working order, the gyro is in unknown state. The dome of a regular T2 still fits the gyro to protect the theodolite on top of it (see figure 3 where I have placed the dome of my 1969 Wild T2E on top of it).