Readout is not materially affected by drifts in analog circuitry.
An electronic absolute Cartesian autocollimator performs the same basic optical function as does a conventional all-optical or a conventional electronic autocollimator but differs in the nature of its optical target and the manner in which the position of the image of the target is measured. The term “absolute” in the name of this apparatus reflects the nature of the position measurement, which, unlike in a conventional electronic autocollimator, is based absolutely on the position of the image rather than on an assumed proportionality between the position and the levels of processed analog electronic signals. The term “Cartesian” in the name of this apparatus reflects the nature of its optical target.
Figure 1 depicts the electronic functional blocks of an electronic absolute Cartesian autocollimator along with its basic optical layout, which is the same as that of a conventional autocollimator. Referring first to the optical layout and functions only, this or any autocollimator is used to measure the compound angular deviation of a flat datum mirror with respect to the optical axis of the autocollimator itself. The optical components include an illuminated target, a beam splitter, an objective or collimating lens, and a viewer or detector (described in more detail below) at a viewing plane. The target and the viewing planes are focal planes of the lens. Target light reflected by the datum mirror is imaged on the viewing plane at unit magnification by the collimating lens.
If the normal to the datum mirror is parallel to the optical axis of the autocollimator, then the target image is centered on the viewing plane. Any angular deviation of the normal from the optical axis manifests itself as a lateral displacement of the target image from the center. The magnitude of the displacement is proportional to the focal length and to the magnitude (assumed to be small) of the angular deviation. The direction of the displacement is perpendicular to the axis about which the mirror is slightly tilted. Hence, one can determine the amount and direction of tilt from the coordinates of the target image on the viewing plane. In a conventional all-optical autocollimator, the target is a first reticle, a technician observes the target image through an eyepiece, and a second reticle affixed to the viewing plane is used to measure the coordinates of the displaced image of the first reticle.
In a conventional electronic autocollimator (which could be characterized more accurately as a conventional optoelectronic autocollimator), the target is a pinhole and a position-sensitive photodetector is placed at the viewing plane. The location of the bright pinhole image is measured by use of the position-sensitive photodetector along with analog readout circuits. The net outputs of these circuits are two sets of voltage differences nominally proportional to the displacement of the pinhole image along two coordinate axes (x and y) in the viewing plane. Like all analog devices and circuits, the position-sensitive photodetector and its readout circuits exhibit thermal and spontaneous drifts, which contribute to errors and lack of stability in position measurements. Nonuniformity of the position sensitive photodetector also contributes to readout nonlinearity.