The measurement of object distance for projected virtual objects using a scanning pentaprism and digital alignment telescope

A standard method for assessing collimation involves sampling a series of sub-apertures across the collimating optic and evaluating the local wavefront slope at each position. In practice, this is done by employing a sliding pentaprism to scan the smaller pupil of an alignment telescope across the collimator pupil. Variation in angle of incidence, recorded as image translation in the telescope, indicates collimation error. We have increased the utility of this technique by automating the scanning motion, image collection, and tracking of the observed centroid at a sub-pixel level. Automating the measurement process ensures accurate and repeatable results by eliminating reliance on operator judgments of image quality and reticle positions. Furthermore, automation advances the application into the precise measurement of finite conjugate distances. An instrument based on these principles is capable of measuring the apparent distance of a virtual object, making the approach ideal for setting long conjugates in target projectors. An analysis based in geometrical optics is presented which accounts for the variable standoff distance of the sampling aperture from the telescope objective. This analysis parameterizes the scanning pentaprism technique, allowing exploration of how the telescope focal length, scan length, and nominal standoff distance affect performance factors such as sensitivity to telescope focus error. Measured data collected from a portable instrument based on the scanning pentaprism technique is included. The data demonstrates measurement of various conjugate distances ranging from infinity to 1 meter. When setting collimation for a 150mm f/10 off-axis paraboloidal mirror, the instrument enables us to routinely set the projected object to distances greater than 7.5km, corresponding with less than 2/3 λ residual power. A robust mechanical architecture allows the instrument to be hand-carried for characterization of large collimators and target projectors mounted at their point of use.