Image tracking observed with a pupil-centring device.

A brief report is given of some observations made during an extended investigation into the subjective image quality of telescopes. For part of the work, a device was required that would indicate directly to an observer the accuracy of eye position, both laterally and longitudinally, with respect to an artificial pupil. This device is described. During use it was observed that the point image produced by the device approximately tracked the eye rotation. This unanticipated property is explained and a different optical arrangement is suggested in order to exploit and enhance this tracking property. The reason for drawing attention to the technique is not only that there may be similar situations in which such devices may be used but also that the tracking property may have other applications, perhaps in a teaching or clinical context.