Micrometer eyepieces
 
By measurement of the physical separation of images in the focal plane of a telescope, it is a simple matter to convert the records into angular measure on the celestial sphere . One way of doing this directly at the telescope was by the use of specially designed eyepieces. Although no longer in commission, such micrometer eyepieces had their main application in the measurement of the separations of visual binary stars.
The micrometer eyepiece would perhaps be of Ramsden design but, in the plane that coincides with the telescope focal plane, a system of adjustable cross-wires was placed. There are many different arrangements in the design of the cross-wires. However, the main feature is that one of them, or sometimes two, could be adjusted relative to the others. The adjustment was made by a micrometer screw and the amount of movement recorded on the scale or drum which was attached to the side of the eyepiece. The eyepiece, together with the cross-wire system, was fitted into a rotatable tube and the orientation of the cross-wires could be altered. The essential components of the micrometer eyepiece are shown schematically in figure 1, which also shows the positions of the cross-wires when they have been adjusted for a determination of the separation of two stars. In a typical arrangement, as shown in figure 1, two wires are fixed exactly at right angles to each other with their intersection at the centre of the field of view. The orientation of the two wires is conveniently displayed by placing a pointer along the direction of one of the wires and allowing this

Figure 1. A micrometer eyepiece set on two stars.
 
pointer to be swept over a circular scale as the eyepiece is rotated. A third wire is set into a yoke so that it is parallel to the wire which is not carrying the pointer. Its position is made adjustable by allowing the yoke to be driven by a micrometer screw. The measurement of a double star could be performed as follows. After the position of the telescope has been adjusted so that one of the stars corresponds to the intersection of the fixed wires, the eyepiece is rotated so that the wire which is attached to the orientation pointer is made to pass through the two stars. In this way, the angular relationship between the stars and the coordinates on the celestial sphere are determined. The adjustable wire is first placed on the star of the centre of the field and then displaced so that it lies on the image of the second star. The difference in these positions of this wire is known in terms of the movement of the micrometer screw and, by means of a calibration, the separation of the two stars is determined in angular measure.