A Technique for Automatic and Continuous Spatial Calibration and Validation of All-Sky Imagers

A key challenge in the effective use of All-sky Imagers (ASI) in real-time space weather applications such as the detection of the auroral boundary, is the accurate spatial calibration of the imager.
We present a technique for continuous spatial calibration and validation of imagery captured by all-sky imager (ASI) instruments to enable automatic and real-time geo-referencing of the image data. The technique is a two-stage process. The first stage is carried out in a laboratory setting to determine the camera’s intrinsic parameters such as the focal length and image center, as well as the extrinsic parameters that model the camera distortion function. The second step is the continuous alignment of the image such that it is level on the horizontal plane and aligned with True North, by computing on a daily basis the 3-dimensional rotation angles of the instrument when deployed on the field. This is achieved by a fully automated technique based on automatic star detection and expected star trajectories at the location coordinates of the instrument.
This calibration technique is currently operational for CPI’s ASI deployed at Millstone Hill, and is robust regardless of weather conditions. Future plans include extending the calibration technique to a second all-sky imager, and integration of the calibration and validation technique within a continuous data processing framework.